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Build a do-it-yourself home air purifier for about $25 (uofmhealth.org)
801 points by io_io 6 months ago | hide | past | web | favorite | 342 comments

I built my own air purifier in 2016, living in London, I was living in a basement and was having breathing problems due to mould. My landlady was gaslighting me at the time, so I built the air purifier. It worked for a while but mould spores cling to the paper filter and eventually grow into the filter material. I think Dyson uses glass fiber filter to stop this problem. My filter helped a lot, but eventually I had sclerosis infection on my brain and ended up in hospital with lung infection and brain lesion, still recovering after two years, and I will never be my old self again. I was subletting so had no grounds for complaint, I had to choose between homelessness or phoning a distant relative I hadn't spoken to for 24years, I chose the latter and I think it's the reason I'm still here.

I was talking to a buddy that does construction work in a pretty wealthy area (Scottsdale, AZ). They are just finishing up a large luxury apartment.

Apparently, the contractor let the drywall get soaked while sitting on palettes during monsoon season, and since the roofing wasn't done there were inches of standing water that resulted in black mold.

They just drywalled right over it. In the US. In 2018. In a luxury condo.

So, you never know what nonsense you will have to put up with.

"They just drywalled right over it. In the US. In 2018. In a luxury condo."

This happens regularly in the SFBA, in Marin County. Every rainy season I see at least one house being built through Nov-Dec-Jan that is in the bare studs stage and gets soaked with a week or so of constant rain.

And then the sun comes out and they immediately tyvek wrap and seal the whole house up. Sopping wet, soaked studs, entrapped on both sides. I would love to open up the drywall in one of those two years later and see what the inside of the walls look like...

These are very expensive homes ... the four I am thinking of over the past two years were all 2M+.

I'm designing a home fabrication system, so this stuff is very fresh on my mind right now.

This talk really gave me a foundation for understanding how moisture works in a building enclosure through seasons: https://www.youtube.com/watch?v=Ld8pzIu45F8

The speaker is a little hammy, but he's doing (more or less) legitimate science, and he laid out a bunch of principles I didn't understand before.

In particular, the different layers of the wall (ballistic, water, insulation, vapor, structure) really need to go in the right order, and at literally every exterior joint in the house you need to match up all five layers. There are also several layers that it is crucial to vent/drain.

It's extremely complicated, and for the most part just obeying code and convention will only get you 80% of the way there.

It seems to me there are also anti-patterns ensconced in code and convention. The idea that insulation goes inside the structural layer for example. You want your insulation layer to be unbroken.

This exact scenario is going to play out near my house over the next two weeks. SF Bay $2m houses. I should send pictures to the county building inspector.

My brother work in a construction business, mostly for flats. And with everything he's telling me, this does not surprise me

What do you mean "they drywalled right over it"? They installed the damaged drywall and then _another_ layer of drywall on top of it? Sorry if this is a stupid question.

Probably means they put drywall mud on top of the moldy drywall.

The only thing I can think of is possibly a firewall?

I know they're in the desert, but drywall really should happen after the building envelope is sealed.

Wow. I am pretty sure they can not only get sued for this but it may be criminal offence.

I'd imagine that's true on both counts, but with these things it's notoriously difficult to prove. You'd have to prove that the contractor knew about the damage, didn't do anything to mitigate it and then knowingly attempted to hide it. All of that is pretty tricky to pull off, given that a home damaged in this way might not show signs of the damage for a good while.

It's why bad actors (such as the ones described above) keep getting away with what they do.

Those are pretty easy to prove in this case. All you’d need to do is find, remove and test the bad dry wall. There’s no way someone installing it could reasonably claim ignorance of its condition.

how do you prove the timing? that it happened at the time of construction and not after?

You get one of the people who did the installation to admit to it. I'm sure you could find someone who has a conscience or doesn't think they are personally at fault who would be willing to own up to it.

Construction crews in Arizona? They aren’t going to own up to anything because a majority of those workers are in the country illegally. You aren’t going to find them going to depositions or signing affidavits any time soon. That isn’t mutually exclusive with “having a conscience” of course, but that does mean that you aren’t going to have much luck finding anyone who actually worked on that crew and if you did, they aren’t going to talk.

I can only speak for UK contractors/companies, but most of these guys will immediately close ranks on you if you try and pull this. The guys at the lower end of the chain won't want to say anything because it could affect their ability to get work at a later date. A lot of them are on temporary contracts, and these companies talk to each other. If they start making noise about these sorts of mistakes, they could well wind up on a blacklist.

With the chaining of subcontractors, I doubt you'd even find the company that did the work, let alone an employee.

Expert testimony.

Drywalling without a roof is just bad contracting.

Entering a rainy season without a roof being sheathed is also bad contracting.

We live in Hawaii and mold is a huge problem here. There are times during the day when the dewpoint spread is only a few degrees. This means that any object that is cooler than the air temperature will condensate. If you can't control the moisture you can't control the mold. Another major factor is the materials in your house. Things like unfinished birch plywood will absolutely mold if left in open air, where other woods like hardwoods don't have as many problems.

> I was subletting so had no grounds for complaint

How does that work?

You may not be eligible for regular tenant protections, but you're still eligible for recourse through regular courts, no?

This is in the UK. Laws protecting tenants there are, to put it politely, pretty thin.

I'll put it un-politely: they stink!

My first rental in London had bad mould problems. The landlord refused to fix them. We ended up with dehumidifiers throughout the house to keep it under control. When we left after a year, he complained at us for keeping dehumidifiers around when potential tenants came to visit - it raised too many questions! This was a 2200+/mo rental that he had bought for about 100k 15 years ago. What a crook.

There was recently a vote in Parliament to require landlords to keep their rented properties fit for human habitation. It failed: https://fullfact.org/economy/did-mps-vote-against-homes-havi...

What on earth?!? What about hotel rooms? So AirBnB in UK is dangerous?

A lot of MPs are landlords

If you’re subletting a place, you have no legal right to reside there, so one would expect legal recourse to be limited.

Is this about illegal subletting, then? As normal subletting is fully legal in jurisdictions I know about.

Well, the courts may simply tell you that you have no right to live there. So you risk being expelled at the same time you win your trial.

In order to take care of mold, you need to clean up what you can using hydrogen peroxide. It will foam up when it contacts mold. Keep applying until this is no foam up. Incidentally, peroxide in a spray bottle is great for cleaning showers and places that get mildew. Always use fresh peroxide.

For an air purifier, you need one with a UV light.

Different people suggesting different means against mold here, but in the end, you have to find the root of the problem and eliminate it. Otherwise, you are fighting symptoms.

1. Air filters might reduce the airborne spores, but if you have mold already growing behind some wardrobe you are wasting time.

2. Hydrogen peroxide, chloride, and isopropyl alcohol are all good tools to fight it when you find a place where it grows, but ultimately you have to solve the problem at a deeper level as mold has excellent survival skills (you might clean the wall but the spores in the air infect it again right afterward).

3. In order to eliminate mold, you have to change the environmental conditions. As most living organisms mold requires food and water. The water comes either from inside the wall (that is something only your landlord can fix) or from the air. So try to maintain a low humidity level within your apartment (<50% should be okay). The easiest way to achieve that is by using your heating and replacing all air within your apartment by opening all windows for 5 minutes (to let in cold air, no longer, as it will cool down the temperature of the wall otherwise), three times a day. Mold nourishes itself from organic material. Very often that means fabric, wood or wallpapers (yes removing your wallpaper might be a good idea). Since water condenses in cold spots first, those are the place you want to keep looking for (walls next to windows, corners of a room).

Three years ago I had a mold problem in my apartment. Following the above advice, the mold didn't come back (except for a few spots in the shower, but that is kinda difficult to keep dry).

I lived in a small studio in a damp part of SF and we discovered mold growing behind a beureu, I had cold like symptoms for months before which I think was related to a mold allergy. We moved all furniture away from wall, cleaned with mix of bleach and water then invested in a $200 Frigidaire 50 pint dehumidifier (like this one Frigidaire FAD504DWD Energy Star 50-pint Dehumidifier https://www.amazon.com/dp/B00AU7GYXA/ref=cm_sw_r_cp_apa_n8i7...) and it solved the problem for us. It would pull a gallon or more of water out of the air everyday. I tried some smaller/cheaper dehumidifiers but they really didn't compare.

Or a cleaning solution with chloride.

I've heard that vinegar is better for mould, as it leaves the surface rather more "acidy" than aseptic, as chloride does.

In my experience a one-two punch with chloride to kill the mold quickly, followed by a dousing with distilled white vinegar, results in a good short and long term solution for surface mold.

Safety advice: If you decide to try this at home, please keep in mind that mixing chloride based cleaning agents with an acid will release chlorine, which is toxic enough to have been used as a WW1 weapon.

Or similarly toxic chloramine gas, when combined with cleaning agents containing ammonia, such as Mr. Clean.

Chloramine is responsible for the classic "swimming pool smell", as the chlorination in the pool reacts with urine, sweat, and other nitrogen-containing organics. Chloramine is a less effective disinfectant than free chlorine, and more irritating to swimmers, and diffuses out of the water more slowly, so this is why you shouldn't pee in the pool, and why you should rinse off in a shower for a minute before entering. Mainly, just don't pee in the pool.

If you smell "swimming pool" where no pool exists, it might be prudent to leave the area immediately and take a 1000 mg vitamin C tablet, before making any attempts to discover the source of the smell. (Taking vitamin C after the damage is already done won't help.) If the odor is especially strong, or if it causes any irritation to your nose and eyes, evacuate, and do not return. Call your emergency services number and tell them you smelled a strong chlorine odor.

Don't combine bleach cleaners with ammonia cleaners, ever. Don't even store them in the same place. Also avoid mixing bleach with acids, such as vinegar.

My bathtub in my apartment would smell like that after using Tilex. I don't pee in the tub either, so I guess it can only be from sweat.

Chlorine can react with about anything that contains ammonia or amines to form chloramine, so shampoos, lotions, shaving foams, or conditioners could also be responsible. Amino acids are named thus for their amine groups, so anything containing proteins or protein fragments will react. Amine oxides and triethanolamine are used as surfactants in cosmetics. Sweat is unlikely to be the culprit. Check the ingredients in your cosmetics.

In what way does vitamin C help in this case?

I just googled chloramine and found this:


I guess I didn't realize someone would try this without letting the chlorine evaporate first. I should have worded it better: Chloride to kill the mold on contact, let dry, then vinegar to keep the mold away.

Woah, this is pretty scary. Thank you for sharing. How did you know it was mold that was in the air? Is there some sort of test kit that you can buy to test for this. Mold is pretty hard to detect because you can't really see it because it typically hides between the walls.

Any advice appreciated.

I know there is mould in my house. We do three things to deal with it: 1. Air Filter (more for pollution but it helps with mould) 2. Dehumidifier which we leave on 24/7 basically for the mould. This is the big one. 3. Wash any mould with vinegar to change the PH after it is wiped off to prevent it from growing back.

The mould is still there but the dry air stops it from growing and sporing which is what makes you stick. If humidity creeps up the mould will grow again. Be aware that mould is virtually everywhere and is only harmful if there is enough of it around sporing or if you touch it.

Dehumidifiers are expensive in power consumption, I don't know which model is more effectively. I'm living in SEA, and get humidity level to 50% 24/7 in my studio equally to double my monthly electricity bill.

I don't know if these are effective, but in my country we have passive dehumidifiers which are basically containers with a load of salt or something like that which draws the moisture out of the air. Good enough to keep caravans in storage mold free. Probably not good enough for e.g. living rooms or actually moist locations, but maybe it scales up. Of course, you'd then need a lot of salt or whatever the stuff is.

I used the passive variant that you explained in my previous apartment and while it may be effective at keeping humidity down, it's not very effective in bringing a high humidity down. I got an electric dehumidifier and it's drastically more efficient. It pulls over two liters of water per day out of the air if the air is sufficiently humid. I've since moved, so I don't use it routinely anymore, but I still have it and occasionally use it to speed up the drying of clothes on a drying rack.

My dehumidifier draws 4l per 8 hours.

May I ask what model it is, please?

Likely anhydrous calcium chloride.

The problem is, once the calcium chloride has absorbed the moisture from the air and is saturated, you have to take it out and dry it to make it effective again. That might be cheaper (in terms of energy) than refrigerant phase-change dehumidification, but it's a lot more labor intensive.

A house should be able to dehumidify on it's own by natural air flow. Maybe it's different in more humid areas?

Vastly more different. A place like Houston is going to have “sealed” houses far more frequently than Scottsdale for instance. Sealed meaning the air conditioner will keep the indoor environment manageable. I remember as a kid my grandmother would use that god-forsaken attic fan in lieu of her window-unit air-conditioners and it was pretty much the zenith of misery in a Houston summer.

Natural airflow with the outside at 90% relative humidity isn’t going to do a thing other than make the interior occupants ornery.

The natural airflow design of Apple Park for example, that works incredibly well, but that same design in Houston or New Orleans would be catastrophically ridiculous.

The CDC has some recommendations on it (yes you can have mold tested to see if it's dangerous):


I'm not really sure about keeping the humidity below 50% though. The UK and most of Northern Europe has rather high humidity in the winter. It commonly reaches 80-90 RH outside, so you'd have to heat your house to 25c/77F to bring the humidity below 50%.

"The UK and most of Northern Europe has rather high humidity in the winter."

Indoors? No, not at all. In the winter the temperature in my house/office is 21-22C and 17 at the lowest at night, and I have to run a humidifier 24/7 to keep the humidity over 30% (otherwise the wooding floors shrink too much).

(anecdata but...)

Currently in the bedroom (thanks Netatmo) it's 17.8° and 69% humidity (with one of the three windows open about 0.5"). That compares to 14.0° and 78% on the balcony. Looking at the graphs, 50% is the lowest indoor humidity over the last month.

Yes, bedrooms have the windows open more often, and usually less/no heating. How many hours a day do you have the windows open? Is there a heating source on or nearby? If you close the window, you'll see RH drop quite fast (in most houses). I'm going to try this same experiment tomorrow, I'm quite interested to know.

Still - mold in houses is not a problem in houses in Western/Northern Europe, except for the most pathological cases (for example, mold scores you iirc 2 points in Belgium on the 'uninhabitable property' test, where 9 gets your building declared unfit for living in. Having small holes in the roof or small cracks in the walls (still structural defects) gets you 3. That's how unusual mold is around here. OTOH when I lived in New Zealand, mold was met with 'eh, there's bleach in isle 5 at Countdown, just wipe it down.'. I was like 'wut?'.

> If you close the window, you'll see RH drop quite fast (in most houses).

Alas, it does not, but that's possibly because other windows are open around the place and there's relatively good airflow.

> I'm going to try this same experiment tomorrow, I'm quite interested to know.

I'll see how it goes tomorrow with the window closed and various doors closed to minimise the effect of the other windows.

We do have cold winters but that does not mean we have high humidity indoors. Relative humidity means the water contents of the air at 0 degrees is very very low. The higher temperature, the more moisture the air can hold. So when you heat up the air the same amount of water shares a much lower percentage of what the air can hold. In winter the hygrometer is showing 100% moisture outdoors (because ours can't measure below zero degrees C). When that air is heated up indoors, it holds about 20% of what it can hold [0], most likely thanks to warm water taps, us breathing and other moisture sources. This is why many people run their humidifiers so they can at least have 30-40% in winter. Less dry, cracked skin and nose bleed.

[0] https://en.wikipedia.org/wiki/File:Relative_Humidity.png

Yes, but that's my point. Where I'm currently living (UK, south coast) it's 15C at 77% RH outside, so to get below 50% RH inside the temperature needs to be at least 22C (which it isn't). It's not uncommon for the humidity to get up to 80-90% RH when it rains, and the temperature doesn't drop much.

When it gets to the cold part of winter it's obviously easier, but this time of year it's hard to get it below 50% RH inside just by controlling the temperature.

Northern Europe gets really cold, so while RH may be high, it’s not moist at all. People get dry skin in the winter.

Northern Scandinavia and west of Urals (I live here) have moist winters (70+% RH most of the time). Skin would dry anyway because water evaporation rate is proportional to a pressure (hence temperature) difference and convection makes a constant supply of fresh unsaturated air.

Relative Humidity is relative to temperature.

Cold air can simply hold less water overall, at 20C it can easily hold double as much water as it can at 0C.

There being a pressure difference is nonsense, you can get the same air pressure in the winter as in summer (though it tends to go on the Low side more often). What you probably mean is the vapor pressure, ie the natural pressure the liquid wants to be at in a closed container, though while it is largely depdenent on temperature, it does so in a linear/positive way, so at lower temperature, water will evaporate less (otherwise ice would more easily sublimate).

Once you go below zero a water content of a few grams can easily hit 100% RH which would barely manage 5% or less at 30C.

I think the relevant pressures are the vapor pressure of liquid water, the atmospheric pressure, and the partial pressure of gaseous H2O at the gas-liquid interface.

If you can blow dry air over its surface, you can evaporate liquid water (or sublimate water ice) even when the relative humidity is 100%.

You can think of air as a gaseous solution, and its water vapor content like table salt in aqueous solution. Temperature affects the solubility, and if the solution is saturated, no more can dissolve. You can still dissolve salt crystals sitting in a saturated cold saltwater solution by squirting warm freshwater onto the bottom of the container. If that then mixes with the rest of the solution, and everything cools down again, that dissolved salt can then precipitate out somewhere else (like rain), or form a suspension of tiny crystals (like fog or clouds).

The solubility of H2O vapor in atmospheric gas increases with temperature. But you can also do something with a gaseous solvent that you can't easily do with a liquid solvent, which is to change the pressure. Higher pressure lowers the solubility of water in air, but to a far lesser extent than a decrease in temperature. Even though the vapor pressure is dependent only on temperature, evaporation occurs whenever the vapor pressure exceeds the partial pressure of H2O at the interface. Increasing the overall air pressure also increases the partial pressure of H2O vapor by a proportional amount, so inhibits further evaporation. But water vapor is also less dense than N2, O2, and most other atmospheric gases, so the means of measuring pressures gets complicated.

When you involve wind, and stratified airflows, it gets even more complicated. The air blowing across water or water ice could have been previously warmed by the ground just enough to evaporate more water, then get pushed higher by an angled snowbank into colder, saturated air, and it will then dump the excess water as small ice crystals. A solid block of ice can then become "rotten" as a snowdrift forms downwind of it. The overall average temperature may say that ice should stay frozen, but the wind can still carry a tiny bit of water vapor at a time, thanks to local fluctuations, and with enough volume of air to move it, that ice will drift.

Heating your home to like 25 degrees and opening the windows every six hours is actually required in some tenancy agreements.

What? That's an insanely uncomfortable indoor temperature. Anything over 20 and I'm uncomfortable.

I believe the idea is that the landlord is never responsible for mold problems since no sane tenant could comply with those rules.

What is the humidity level in your house? 25ºC should be comfortable for an healthy individuals if the hair is not heavy with humidity.

Bwahahahahaha... air

In Switzerland, they have a tendancy to overheat, now I am used to a constant 25°C over the year. It's actually quiet comfortable.

It's a matter of getting used to it, really. Once you've been at a temperature for a while, it gets easier.

By the time it is a health issue, it is usually visible. There is some air passage from within the walls to the living space, but not nearly as much impact as the visible mold on walls, furniture, etc. Sauce: Get paid to test sometimes.

You can buy test kits that you expose to your rooms’ air for some time, and then a lab will grow the mold inside the Petri dishes to “amplify” what has been collected in your home. That may be expensive, though, and not easily available everywhere, and perhaps even prone to contamination “in transit”?

Mold needs lack of ventilation /dampness. You see the dark spots (maybe that's too late?) and IMO nothing substitutes air flow, when possible. Even in winter, one needs to open windows and let the air flow freely at least for a while.

You can notice black spots in your bathroom or other places where steam doesn't escape properly. I noticed it on my mattress also. I don't think I have any health issues because of that, it was 5 years ago, but it was rough.

Mold is not always black, is it? It can have other colors as well, e.g. yellow.

Wow, what a story. Thanks for sharing it. I feel bad for you, and sincerely hope that you are back on your feet.

A wet paper filter is the perfect place for mold to stick to and send spores all over the place. If you have a mold problem where you live and can't move out of the place, you need an air filter and a dehumidifier. Unfortunately, it's hard to jerry-rig a dehumidifier on the cheap. If you were in London in the winter, it might have been more practical to turn up the heat to keep the room as dry as possible.

Could mold grow inside of a dehumidifier as well? I mean, water is constantly passing the components inside. Or does mold need organic material (such as the paper inside the DIY filter) and thus will not grow on the dehumidifier that consists largely of plastic and metal?

And open your windows a tad. It's the best way to keep indoor humidity under control during winter. Being in a basement room though, I'm not sure this would have helped.

As someone else mentioned downthread, you need to maintain a rather high temperature (around 25C/77F) in order to reduce the relative humidity enough.

This might or might not have been feasible given OP's budget, heating arrangements, and general preference. I for one would not want to live in such a hot room for a whole season even if I could afford to. I keep my room cooler than that even in the summer!

Hmm, my house during winter is usually between 18-20. If I keep the window vents closed (most UK houses have vents above the windows specifically for this reason), I get a lot of condensation around the windows which gets mouldy. Leaving those vents open makes all the difference - no condensation, no mould. Also the house humidity seems to stay under 50% with the vents open.

Yup, that's a normal house. Doesn't work in a basement, though, unless it has a vent or small window above the ground.

Sclerosis from breathing that air? Or did the air only cause the lung infection

Really sorry to hear that.

Sorry to ask, but I’m not really sure I got it right: Do you think the DIY air purifier was the cause of these health problems? Or did it at least contribute by letting the mold come back (inside)?

As others have noted, a dehumidifier could help. But what prevents mold from growing inside these dehumidifiers as well? Sure, they’re mainly built from plastic and metal, so no paper as in your case. Will that be enough to prevent mold (or bacteria)?

Otherwise, a dehumidifier could turn into yet another health hazard over time.

Sorry for you :(

I briefly lived inside an apartment that was previously a cave. There was a plaster wall separating from the outside, and after raining, it ended up covered in mold. We cleaned it with bleach, but moved out soon after. I hope I did not get too much exposure.

Do not forget that black mold release mycotoxins, which are carcinogenic.

That’s devastating. I hope you recover as much as possible.

No grounds for complaint? This is criminal.

In my filter I've UV-C lamp which is turned on and off automatically after some duration.

Important tip: if you set up an air purifier in your home, make sure to plug all drafts around your house: windows, doors, etc. Even a DIY purifier with small airflow can make your indoor air excellent if given enough time to do so and no leaks to fight against.

If you want to get some fresh air in your home, do not leave a window half-open all day long—instead, open all your windows at once to cycle the air as fast as possible, then close them again and let the purifier do its job. If you do that in the morning (ideally just before leaving, if you aren't staying home), you'll have great air quality through the entire evening and night.

Since we're on HN, I'll also mention a toy project I built a while back [0]. It uses a Raspberry Pi to read particulate matter sensor mesurements over serial link from a Dylos 1100 Pro, then pipes it into Redis and InfluxDB and ultimately Grafana for presentation. I've used it to track the air quality in my home in for the past year or so and it's pretty fun. In my case, cooking is usually the biggest contributor to poor indoor air quality!

[0] https://github.com/peferron/air-quality

Very interesting!

How does the Dylos 1100 Pro compare to, for example, the Nova SDS011 sensor? By my cursory review, the SDS011 is much cheaper and appears to offer a more native Raspberry Pi-compatibility experience.

Is the Dylos more precise? Can it produce readings the SDS011 can't? Are the two sensors measuring fundamentally different things?

Finally, if you had to do it all over again, would you choose a different sensor or stick with the Dylos?

Thanks for sharing your project and for your insights!

SDS011 is the basis for luftdaten.info; have a look here for a diy guide (optionally including humidity/temperature as well): https://luftdaten.info/en/construction-manual/

Hi, this is an interesting project. It seems to be targeted for outdoor measurements. Do you know if this also suitable for indoor measurements (meaning if you want something for indoor air quality, would you measure the same things?)

I recently watched a video from dhh (https://youtu.be/MRqh8oLY7Ik) about air quality at home and was thinking if there was a nice diy solution for measuring it.

I bought the Dylos almost 5 years ago. At that time I don't think the SDS011 was available.

A cool feature of the Dylos is monitor mode. It wakes up every hour, spins the fans for 1 minute, and spits out a measurement. It's convenient for 24/7 monitoring because it reduces fan noise and dust accumulation. I don't know if the SDS011 offers something similar, or if you could somehow program that yourself, but that may be something to consider.

I'm very happy with the Dylos overall. In addition to being 5 years old, mine has been running in monitor mode for the past year and a half with no issues so far so I really can't complain about reliability. Issues are size, fan noise, antiquated UI, and also antiquated connectivity, although I personally enjoyed working with serial (threw me back to early Duke Nukem LANs before we moved to IPX). I wouldn't be surprised if there are superior alternatives today, at least regarding usability.

The SDS011 with firmware from luftdaten.info seems to work in this mode by default: sensor is turned on for 20 seconds for measurement at every 150 seconds interval. This should be tunable though. The theoretical lifetime is 8000 hrs of continuous operation, so they estimate 6-7 years longevity in this default mode.

Also open your windows when you vaccuum. Send most of the dust outside instead of just what gets caught in the filter.

Could you explain? I'm curious why it'd be better to have the air filter going rather than having good airflow from outside via open doors and windows? Are we trying to filter dust/ things produced in the house, or pollutants from things outside the house?

Yeah, sorry, just assumed that pollutants were coming from outside. I'm just so used to it after living several years in China and now being affected by California wildfires. If the outside air is better than inside, then yeah you probably should air out your home as much as possible unless it's unbearably hot or cold outside :)

You could also put the filter on the air intake.

I had an energy audit done recently and they said that 50% of the air in most homes comes up from the crawl space (due to the Stack Effect).

"If you want to get some fresh air in your home, do not leave a window half-open all day long—instead, open all your windows at once to cycle the air as fast as possible, then close them again and let the purifier do its job. If you do that in the morning (ideally just before leaving, if you aren't staying home), you'll have great air quality through the entire evening and night."

Why not have a filtered air ingress ?

That is, instead of sealing the house, why not install an air purifier in a window and have constant fresh air inflow ?

Most filters and purifiers rely on the air being recycled through the filter multiple times. They only remove some known percentage of particles with each pass.

How do you know you are measuring bad particles, and not good particles (like salts if you live nearby the sea)?

Why would you want to keep salt in the air? It will promote rusting.

It's healthy apparently. I've been to a health resort in Ciechocinek where they have huge wooden construction through which they flow salted water so it evaporates and makes the air better for people there.

It's called "tężnia", have no idea how to translate that :) Google gives "graduation tower" but I doubt it's correct.


The question was more about whether the measurement tool provides useful feedback in case of "salty" air. Filtering is a lot of work, and might not be necessary.

Some data about effectiveness of air purifiers when leaving windows open: https://smartairfilters.com/en/blog/purifiers-useless-window...

Of course you may also want to be careful doing this, as unless there is a certain amount of airflow/ventilation you may start having other issues, such as a build up of CO2.

This is great! Thanks for sharing :)

Generally there are two classes of objects that you want to filter for.

Particulates and VOC (Volatile Organic Compounds). Think of particulates as specs of dust with varying size and VOC's as random chemical compounds such as spray paint, smoke, oders.

The general go-to clean room/industrial filter stack is Prefilter(F5 Bag)->HEPA->Activated Carbon. The pre-filter catches all the big particulates to prevents clogging the HEPA filter. The HEPA filter catches all of the really small particulates. The activated carbon captures most of the VOC's/oder/smoke.

The official way to monitor when a particulate filter is bad, is to measure the pressure drop across the filter, and compare to manufacture specs. For the carbon filters, you would install a gas sensor measuring whatever chemical your trying to scrub. When there is a spike in that particular gas compound from your sensor. You know the carbon filter is saturated, and should be changed.

It looks like the the filter mentioned in the article video,combines all the filter products into a single product. Nice! If your air handler can manage the increased pressure drops, please buy the most legit filters you can buy. It's literally the cheapest insurance you can buy.

> “The general go-to clean room/industrial filter stack is Prefilter(F5 Bag)->HEPA->Activated Carbon.”

yup, i bought a blueair 211+ (https://www.blueair.com/us/blue/blue-pure-211/1695.html), that has exactly those filter elements in a nice (if bigger than expected) package. this model was recommended by both consumer reports and the wirecutter (until recently, cynically thinking they didn’t pay wirecutter’s “endorsement fees”). it’s effectively eliminated my constant coughing, sneezing and runny noses before i got it.

I just bought the same model. What makes it so effective is that it has a filter on four large sides so it can process a large volume of air very quickly.

Does it spew out ozone? My memory may be off but I remember this model not meeting CA air purifier standards which was why Amazon couldn’t ship it to me

no, there is no ozone generator in this model (usually that's done using electrostatic plates).

I'm still curious then as to why this specific unit doesn't meet California air purifier standards, which is why Amazon won't sell it to me.

i'm not sure. i live in california and bought mine on amazon. wish i could be more helpful!

Wirecutter doesn’t have endorsement fees

they don't, although if they recommend you, they'll ask you for a cut of sales (affiliate program). if you say no, they'll switch their recommendation to your competitor and never review your brand again.


That is an allegation that they strenuously deny. https://thewirecutter.com/our-response-to-nextdesk/

When they link to a product on Amazon they then ask the brand to give them an additional cut of sales? I'm surprised Amazon allows sites to double dip like that.

I called out the inherent affiliate bias issue years ago but HN cult tunnel vision didn’t want to hear it and downvote me to hell. Wirecutter is not to be trusted.

I have a pair of Vornados with the same setup. They're life-changers (no exaggeration).

What model of Vornado do you have? Can you share your experience with them?

PCO350 and PCO500. They're on the loud side, but they work well.

The purifiers I use have the activated charcoal before the HEPA. Probably doesn't matter that much though I sometimes findd a small amount of dust leakage from the pre-filter screen on the carbon filter which would just clog the HEPA.

Many don't have a prefilter and use the charcoal as the prefilter, essentially. I end up vacuuming mine or shaking it out (outside) every couple months to get rid of the build up.

Huh! I independently "invented" this myself a couple of days ago when the Camp Fire blanketed the bay area in smoke. I got a 20x20 filter and some duct tape from Home Depot and a box fan from Target. (After discovering that Home Depot only sells fans in the summer.)

I don't think I was the only one, though. HD was doing a brisk trade in 20x20 filters, and I think I bought the last box fan at any Target in the entire bay area.

My "creation": https://i.imgur.com/jeJfqeW.jpg

When you can't get house parts, try auto and computer parts.

Get some 120mm fans (or an array of 80mm, etc), and some engine air filters.

Don't change the filters too often:

"It is common, however, for servicing to occur when the filter appears dirty. Engine air filters are designed to actually increase their efficiency by using this initial layer of dust as an added filter layer. Initial filter efficiency is usually approximately 98% but increases to more than 99% by the end of the service life of the filter.

Therefore, changing an air filter before the useful service life is achieved can result in premature engine wear."


"Don't change the filters too often"

Thank you for bringing this up ... in industrial settings, filters are "seasoned" for some period before they can be expected to perform at their published ratings.

Many air and water filters' performance continues to rise over their lifetime - the downside being decreased flow rate.

If you are filtering infectious agents with a water filter, I would be reluctant to run them past their lifetime because you could have things living in the filter and filter housing, etc., but if you're just dealing with particles, your filtration should continue rising over time.

I did this a couple years back. 2x 120mm fans and a rolled up filter in between with a cardboard box as the frame. I never thought it could do much, but also never left it running very long.

I've been begging for a good use of old computer fans...

That’s looks as professional as anything I’ve put together!

Is that the spot of use as well? I’d imagine the vicinity of the wall would limit the airflow.

Yes but it’s not as close as it looks. There’s about 2ft behind it.

Don't you want the filter on the incoming side? Air handling systems always put the filter on the inlet as far as I'm aware.

The main reason why furnaces and cars have it on the incoming side is because the dust and particulate can damage the engine/blower. So you want to filter it out before it reaches there. In a system like this, the box fan will not be damaged by the particulate so you can instead push the air through the filter instead of pull it.

The filters are also designed to have air pulled through them.

There's often an electrostatic component that ties into a filter's efficacy, as well.

The fan controls, and the power cable, stick out of the back, so I didn’t have a choice.

There are some cars that have circular air filters which fit almost perfectly over a "universal" radiator fan. IIRC some generations of Toyota Corolla and Tercel which spare parts should still be common enough for. Can be a solution if the more HVAC-type stuff gets sold out. You'll need a pretty beefy 12V DC power supply though.

When you decide to change out the filter I recommend using cheap painters or kraft tape instead of duct tape. I found that painters tape removes easily and tends not to leave adhesive behind.

nice job. can you say anything about the difference it makes?

I can tell you that it doesn't smell of smoke in the house anymore.

I still have a sore throat, but I also have to go outside a lot, so maybe that's not surprising.

The future is already here, it's just not evenly distributed.

Can we talk about Molekule? Their team and some SF tech community folks have been plugging it all over Twitter during this tragic wildfire crisis. But it seems like snake oil to me. It’s $800. Certainly for smoke particles it can’t possibly offer any more protection than a standard HEPA filter. But even for its claimed ability to nuke VOCs, the claim seems ridiculous. While particles can deterioriate under prolonged exposure, the airflow must be too high and the exposure too short to make much of a difference.

What gets me is how these guys misuse “science marketing” to play on people’s fears about germs. Add the shameless exploitation of a real tragedy here.

I agree it seems sketchy. NYTimes writes, “Unlike virtually all other air purifiers—and all we’ve tested—Molekule is not HEPA-rated and does not claim to eliminate micron-scale particulates with a filter. Instead it claims to remove smaller, nanoscale pollutants, including VOCs and viruses, via a chemical rather than a physical process. Out of curiosity, we requested a model for testing for VOC reduction, but Molekule requested conditions around the testing that we could not agree to.”


> NYTimes writes, “Unlike...”

While the New York Times Co. did indeed purchase The Wirecutter/The Sweethome, there’s no reason to start attributing Wirecutter reviews to the parent company.

This is especially true in the settings of reviews, since the NYT performs its own reviews.

I had one. It has multiple issues:

1. CADR is really low to the point where the unit is near useless due to #2

2. In addition to a low CADR, it has the highest noise. It looks and sounds like a jet engine when you have this thing on high, which is needed due to the low CADR. This is the loudest air purifier that I’ve ever owned

3. If you get a defective main filter, the unit will emit an unpleasant metallic smell. I haven’t tested the particles yet but I doubt the air is clean.

4. Unlike other smart air cleaners in the same price range, it’s app and smart features don’t work. You can’t even create a schedule for it

It’s a very flawed device


5. The unit works using some LED light. A. it is bright B. It is not replaceable. Meaning after 2-4 years of operation, the unit becomes a throwaway. This is not a good thing for a unit that costs over $700

What other smart air cleaners do you recommend?

I have a hard time taking them seriously. Some of their "independent" lab results are from some USF lab. The same lab one of the co-founders was a director for... Go figure. Some please correct me if I'm wrong. Read the results in technology, and then read the bios of the people.

Note that restricting airflow on a radial fan can cause the motor to heat up. Make sure you get a 20-inch fan with a fuse. To reduce heat, increase air flow by making a box with three filters and by using 2 inch deep filters. Also, when I built my three filter "box" last fall, I was able to order a set of three MERV-13 20x20x2 filters directly from Nordic Pure for less than what they cost on Amazon. Finally, to make a cube, cut the remaining sides from the filter box. Then roughly tape it together with small pieces of duct tape. When the filters and panels are in the right spots, seal it completely with duct tape. Remember to put the box of filters on the "pull" side of the fan with right sides of the filters facing outwards.

I agree with placing the filters on the pull side, since that is what all hvac units do and what the filters are designed for. To be effective, you are also corrected that a deeper pleated filter with more surface area is the only way to go. But I am unconvinced the motor will heat up. Certainly there will be less air so if it is air cooled it will heat up from that, but at the same time, a fan in a partial vacuum uses less energy and should heat up less. Just slap a meter on it and you will see the draw can be significantly lower, since it is doing less work.

to perhaps reinforce your point: I put a filter on the front (as in this video) and the fan died a few weeks later.

Linked projects put the filters on the opposite side to what you're suggesting. Can you elaborate more on why do you think the filters should act on the air coming into the fan? Is it to increase heat dissipation of the motor?

putting it on the intake keeps the motor cleaner (which will help with motor life), you can vacuum the filter without disassembly, and the air flow will tend to help the filter stay on, instead of pushing it away.

I spent the weekend researching, implementing, etc. air filtration as one of those affected by the CA fires. It was worth it!

The first step for me was getting a device to measure particles in the air. The air quality index (AQI) is an aggregate score that is made up of contributions from several harmful components. Of these, the quantity (weight/volume) of small particles (<2.5um) is often the large contributor. This is called the PM2.5.

Directly measuring PM2.5 is very hard, requiring a device that eliminates the larger particles from the air (typically a cyclonic separator it seems) combined with some way of capturing and weighing of the very small quantity of remaining particles (we're talking ug/m^3).

Many home air quality sensors instead measure the count/volume of particles larger than 2.5 um and call that the "2.5um measurement". You can see the problem. I bought a laser particle counted called Dylos DC1100 Pro. The Dylos measures particles counts larger than 2.5um as is common, but also counts particles larger than 0.5um. Of these two numbers, the 2.5 count is only vaguely correlated with the actual PM2.5 measure, but the 0.5um count is highly correlated.

A bit of internet research (it seems many people have used the Dylos meter in various projects) and I was able to construct a scale to correlate 0.5um+ counts to PM2.5, and therefore AQI. (0.005*count~=ug/m^3)

OK, all that done, to the filtration. First, I installed 3M 1550 MPR furnace filters (~MERV11). The "1550" is a measure invented by 3M 'MicroParticleRating' (or something) and is supposed to speak to how well the filters remove small particles. However these furnace filters don't really knock down very small particle counts in once pass. Sure enough, putting the particle meter at an air duct, the large (2.5+) particle count was way down, but the small particle count was only down a little bit (15-25%?) from the ambient air. Net effect is that after running for several days my house numbers are about 1/3 of the outside air counts.

I also bought a BlueAir HEPA filter for my bedrooms. HEPA is supposed to be 99+% effective for particles 0.3um and up. This filter is in a whole different league. The particle counter placed on top of the filter returns near-zero small particle counts and does a much better job eliminating smells. The room they were in got down to 1/10 of outside levels.

Overall I'm glad I got both types of filters, we are sleeping better, and the meter was very helpful to understand what was going on.

Did you put the filter on the intake or outflow side of the fan?

I have built one of these (20x20 furnace filter on a box fan) and always put the filter on the intake side of the fan. However it seems like people are placing the filters on the outflow side of the fan instead, for reasons that are not clear.

Since it seems like you did some measurements, do you have any idea whether it's preferable to have the filter on the intake or outflow side of the fan? (I note that in furnaces and other air handling equipment, filters are almost universally on the intake, not the outflow, side, presumably because this keeps dust out of the blower/heating mechanicals.)

when you place it on the intake, it is sucked against whatever you are using to stop it pulling into the fan, if it is on the outtake, it can bubble out on the edges and the air can escape rather than pass through the filter. Intake is the only correct solution for these homemade devices without a metal diaphram.

Nifty idea, but I'm not sure I agree with the technique used to prove effectiveness:


The narrator puts a particle counter in front of the fan and notes the 90% reduction. So far so good.

The problem is extrapolating this result to the entire room. Yes, maybe you'd get close to 90% if the device were allowed to run a few days.

On the other hand, it could leave the situation worse (by blowing existing particulates around) or unimproved (by failing to filter room air efficiently enough.

Either way, I'd like to see see a controlled study. Two identical rooms with identical usage. One gets a homemade filter. The other gets just a fan of the same make and model, maybe with a dummy filter to baffle airflow.

What's the average particle count in both rooms after, say one week?

If you check out Matthias wandel's YouTube channel, he does some experiments with hours-long recording of room particle counts. This is in a woodshop (sawdust) setting with a similar diy filter setup.

This DIY version does not have the same pre filter as commercially available ones, and would need to have the filter changed more often at a minimum. There are studies showing the success such as this one. https://www.ncbi.nlm.nih.gov/pubmed/9192919

With pre-registration, while we're at it

I do this and it works wonders if you have pets. Tips: - run this homemade air purifier when you are vacuuming, as that tends to agitate particulates a lot - get 2 filters, a cheap low grade and a premium, high-HEPA-rated filter. Stack them together and you can replace the cheap one frequently and the expensive one less frequently.

Swedish tracing paper works well for a pre-filter. It's inexpensive for a long, seemingly inexhaustible roll, and can be washed numerous times.

It also works well by itself if you just want to cheaply filter large particulates. I used to use it in those cheap $40 Hamilton Beach air purifiers. The OEM filters for those cost almost at much as the new unit and wear out [for me] within a couple of weeks. That setup with Swedish tracing paper works well for dust, as a supplement for my HEPA filter purifier.

I know what I am doing this coming weekend!!

I am so tired of dust in my studio. There is no ventilation other than the small fan in the bathroom or the big sliding door in the front. So I leave the fan turned on overnight but because of pressure difference, very small particles keep accumulating all over my place. I’m really hoping this changes that.

Does it go:

Fan --> Cheap Filter --> Expensive Filter

Fan --> Expensive Filter --> Cheap Filter

Cheap Filter --> Expensive Filter --> Fan

Expensive Filter --> Cheap Filter --> Fan


Cheap Filter --> Expensive Filter --> Fan

You put the filters on the intake, so that the airflow helps making a seal instead of creating leaks.

You put the cheap filter first everything that it catches will not clog the expansive one.

I wonder if you'd put cotton wool, how effective it would be. Here is my set up:

- I bought XIAOMI Mi Air Purifier 2 straight from China for $137.00 and spent 1 euro on a EU plug. It's so quiet that I can sleep next to it, looks nice and it's quite compact, so doesn't take up much space. It has a particle detector to adjust the fan speed. The app is useless, so a cheaper version without WiFi and bluetooth would be great.

- Buy some HEPA filters to put into window ventilation. You may want to remove some plastic inside to increase the flow http://img.archiexpo.com/images_ae/photo-g/69621-5895915.jpg

- Buy roomba like vacuum cleaner https://www.goodcheapandfast.com/articles/best-robot-vacuums

It will cost you less than 500$ to buy these and about 55$/year for new filters. It is worth the money. The air quality is much better (I'm allergic and I haven't had any problem with my sinus since).

What is more, is it worth it. It will save you time cleaning your place (about 10h/year)

Anyone has tried robot vacuums with mop or automatic dirt disposal function?

> I wonder if you'd put cotton wool, how effective it would be.

Not very. Wet wool is better, but only as long as its wet and clean.

> XIAOMI Mi Air Purifier 2 straight from China

Interesting question. Does the removal of particulates offset the health impact of its cheap plastics?

Does anyone know what the actual smell of "chinesium" comes from? The smell of artificial rubber products from china, like remelt and cancer.

I bought a mechanical product like that one time and had to leave it outside for about a month before it off gassed completely

Happened to me, too. Once with a dog house, and again with a set of car floor mats. The floor mats are still out there.

Wool fiber diameters are around 20 micron - HEPA can have glass fibers below 1 micron, and big box retail grades are often 5 - 10 micron. Smaller fibers = better filters.

> health impact of its cheap plastics

Do you mean the health impact of when the device is thrown away?

I think they mean of small plastic particles coming off the cheap device and polluting the air.

More likely they mean volatile organic compounds (VOCs)

Never heard of this. Also, how would it differ from any cheap plastics coming off any cheap fan made partly or wholly out of plastic?

Any source on that?

> The app is useless, so a cheaper version without WiFi and bluetooth would be great.

have you tried it with hass (home assistant)? I've been considering getting one and running it via that instead.

I made a custom wooden computer case with 8 120mm fans and a furnace filter over the intake. The computer side is positive pressure and is as clean as the day I built it 4 years ago.

I buy filters in bulk from McMaster Carr for $5 apiece.

Mcmaster carr[1] is a wonderful commerce website and I am so happy I found them. I buy all of my fittings and hoses and so on from them.

[1] https://www.mcmaster.com/

Check out the blog


The writer has tested a lot of combinations of fans and filters (Simple, HEPA, Activated Charcoal). It's astonishingly easy to build a combination that costs way less than a "professional" Air filter.

The writer has put his findings into a "social startup" that builts these simple, but very effective airfilters in some developing countries:


I use the SmartAir filter setup in Shanghai. It works great and the founder's blog is very useful. I'm glad to support the project.

If anyone is looking for general air purifier recommendations. I'm a huge fan of Coway systems.

I have pretty bad allergies purchased a Coway Airmega 300 for my 850sq ft studio. I've had it for about 6 months and been incredibly happy with it. I wrote some initial thoughts about it here.


It has to be the best looking air filter on the market—I wanted something that would look good in my place when I have people over. Beyond that it is extremely well designed and the pre-filters are super easy to clean. I got mine on sale for about $400.

For smaller rooms the Coway mighty is a better and more affordable pick. It doesn't look as nice, but it is currently at the top of The Wirecutter's air purifier recommendations. You can find it on sale for around $180.


I have 2 of the $180 Coways and so far am pleased with them.

Once in a while, I will light a candle and when I blow it out, will hear the air purifier go on full-blast (Speed 3) to clear the air.

Interestingly, it sometimes turns to medium power (Speed 2) when I'm just shuffling around the room. I imagine its because I have a few rugs and its pushing up a lot of dust.

I wonder: if one is generally healthy and the air quality is unproblematic, should one get an air filter? Does air filtration potentially lower long term immunity and make us less tolerant to particulates?

I worry about the long term effects of being “too clean”, that is, I think there’s something to hygiene theory of disease. The body needs small stressors and perturbations in order to thrive.

I wouldn't worry about this if you live in a city. The amount of vehicle exhaust, brake dust, and just general stuff in the air will make up for and most likely counter the time you spend in a clean breathing environment.

Generally I want to agree with you, but there is also research going the other way...


This article references a more recent London study but mentioned is a more extensive California study of children. TL;DR; One really, really should not live/work any closer to a road with (18 wheelers) traffic than one has to, for children it is paramount - I think all schools in Southern California close to a freeway have filtration systems for this reason.


I built a similar model over the weekend: https://twitter.com/valarauca1/status/1061362369702445056

Used an existing box fan. I had to buy duct tape ($3) and the filter I used one rated for 0.1micron pollen and viruses cost me $17 (rounding up after tax). I cut up a plastic safeway bag to make the seal around it. Seeing as most smoke particles is around 2.5-.5 micron I figured it'd be okay.

I threw this together at about 7am on Saturday after I woke up coughing my lungs out at 6am (I live in the bay area). Just a quick trip down the the Homeless Despot to get the parts and throw it together.

All things consider it works great. The air in my apartment is easy to breath, but if I leave without a respirator (even today) I cough uncontrollably.


As of Tuesday evening it has been running for 4 days straight (no overheating). And has started to become noticeably brown. Needless to say I'm thankful for it.

Where do you get these parts in the UK?

The MERV 13 20" filter is 140GBP on amazon.co.uk and 20" box fans are 40-50GBP.

Is there an advantage to attaching the filter to the front of the fan so air is blown through it (as shown in the video) versus taping it to the back of the fan so air is sucked through it? Just curious if that would make any difference. It might be easier to get the filter to stay if it's getting sucked to the fan.

I would guess that it works similar to a water pump- pushing is always more powerful than sucking - but check this out for some detailed analysis (and more data on DIY air filters) https://smartairfilters.com/en/blog/where-should-an-air-filt...

I never would've guessed turbulent air would be more difficult to push but it makes total sense! Great find.

Excellent page.

I can think/guess two reasons:

1) If the filter is at the back, since the air is pulled through it, the dirty side is outside where people and kids can touch it as it builds up. If the filter is in the front, the dirty side is inside (towards the fan blade) while the clean side is outside.

2) If the filter is at the back, due to air's resistance it is possible, due to nooks/gaps in the fan, that the some of the air may be pulled from elsewhere and not all the air may pass through the filter. If the filter is in the front, it is easier to seal between the fan and the filter to make sure all air is pushed through it.

always wondered this myself... #1 - good point!

I'm curious... doesn't #2 cut both ways? i.e. if you can seal the front, you can seal the back?

I can confirm what others said about air drag. I've done this in the past, assuming I needed the filter on the input side. I had to use two filters, some cardboard, and plenty of duct tape to create a triangular input in order to get a decent amount of airflow.

I wish the guy on the video would take an air reading measurement near the side of the filter. I'm guessing that a lot of unfiltered air is escaping out the sides.

FWIW if you tape the filter to the back then the fan doesn't get dirty as quickly. [Every now and then you should clean the accumulated dust off a fan.]

BTW you need not cover the entire area of the fan with filter material. So, for instance, if your fan's breaker is shutting the motor down b/c of the load of the filter, you can slide the filter up to cover less of the fan with the filter, reducing the load on the fan motor. While not all air passing through the fan will be filtered, over time most air will eventually pass through the filter and the room will be cleared of particulates.

Similarly, it is not necessary that the filter be the same shape or size as the fan, since with each pass at least part of the airstream is cleaned. Eventually all the air in a room will pass through the filter area.

Not sure, but I'd think it would also help to keep your fan clean.

I’ve actually built this fan “mod” and when I change out the filter, I’m going to try attaching it to the back instead.

With the filter thoroughly taped to the front (nearly airtight), there’s a large amount of blowback. I suspect that this is reducing the overall efficiency of air filtration.

With it taped to the back, you don't get blowback, but there is significantly reduced airflow out the front. I used a filter of this type for ~2 weeks during the Thomas fire. Worked very well, though the proper filter I received from Amazon a week after I built this worked better. Two is better than one, though!

Placed in front it would catch particles coming off the motor like lubricant, bits of metal from bearings and carbon particles from the brushes if it's not a brushless motor.

Perhaps strain on the cheap box fan motor as it tries to pull its air through the filter. I've gone through a number of those style of cheap box fans over the years, any kind of long-term resistance very near the intake seems to cause the equally cheap motors harm and shorten their functioning life considerably. I haven't seen the same motor damage problem with resistance added to the exhaust side. In absence of that experience I would have guessed that similar resistance on either side would harm the junk motors all the same.

I'd agree with this.

I used to put the high-efficiency filters on my furnace, my HVAC guy said these filters are so heavy that they put a strain on the blower motor. It's better for the motor to use a plain blue fiberglass filter (MSRP $1). You're not really filtering that much out of a closed house, unless you run your forced-air A/C or Heat with the windows open.

I did the same thing with my furnace. Just to try it I switched to a high merv filter rating and could immediately notice the considerable reduction in air flow that it was causing (and presumably increased strain on the furnace). Seeing as it was a bit more restrictive than I expected, I ended up ditching that and going back to a weaker merv rating filter (haven't gone back to the blue fiberglass style filters, just going with a weaker merv seems to do the trick, haven't had any problems with increased wear on the furnace in the last ~8 years).

We upgraded to a "media filter" when we got new HVAC at my house. IIRC they are 4 inches thick, which gives a lot more surface area due to the deep pleats. You can then run fairly tight filters without putting undue strain on the motor - plus they last quite a bit longer. It made a moderate difference in our house in regards to dust in the air, but we also have 3 pets and live in a very dry, dusty area.

I'd peruse this research on High-MERV Filters and their impact on airflow and pressure instead of taking one HVAC technician's word for it: http://www.homeenergy.org/show/article/nav/issues/page/4/id/...

This graph of airflow vs. MERV rating isn't very conclusive in my mind: http://www.homeenergy.org/UserFiles/file/26-6_p34big(1).jpg

Granted there are about a million variables in all HVAC systems that could affect the results.

You know, I'm going probably believe the guy that replaces blower motors day in and day out for a living as opposed to the group that sets up a "test box". Sorry.

I think the answer is yes, but I'd also like to to know why.

probably makes the fan quieter.

I used a very thick (4") filter rated for smoke particles when the wildfires this year blew their smoke down to Seattle. The filter went from white to a very interesting shade of grey in 3 days. Wife was able to sleep again though, so it does work very well in my experience. (20x20 fan, filter on intake side)

Out of curiosity, where were you able to find the big 4" thick filters in the 20x20 size? I wasn't able to find anything other than the usual 0.75 thick furnace dust filters in 20x20 for a box fan.

I just bought a 4 pack from Amazon Prime Now in SF, YMMV.

Ace hardware. It was not cheap.

I been pondering about this solution for months since my air purifier died back early summer. I also have an additional idea to make it more automatic by using a raspberry pi, hooking it up to a powerswitch tail (essentially cutting power to a fan) and have it automatically turn on and off depending on time of day, etc.

Maybe you could even use it in conjunction with something like a laser particulate sensor, to scope how effective it is and track its progress from day to day. Kick it on when it detects a nasty bunch of filth in the air, and kill it once it reaches a set acceptable level.


This one only goes down to one micron though, so it might not be usable for many allergens.

Skip the pi, just grab an outlet timer from the hardware store for a couple bucks.

I have mine on a homekit enabled outlet ($20) for scheduling and presence-based triggers. Unfortunately I haven't found a homekit enabled air quality sensor to pair with it.

Thanks! Last I checked they were no longer selling version 1 of this product and hadn't released version 2 yet. This might fit the bill.

This looks great but is exclusive to Apple products (requires an iOS device)!

There’s Netatmo Home Coach.

Maybe you could have the input being a air quality sensor or scrape air quality from local government site / sensorup site.

instead of a raspberry pi and all that entails, might I suggest a simple outlet timer

Good time of year to buy them, too, what with the christmas lights and all that. $5 ballpark.

I have seen the mechanical style ones at the dollar store. I wouldn't set my watch by it, but would be fine for this purpose.

Repurpose this device as a jerky dehydrator too: https://altonbrown.com/beef-jerky-recipe/jerky-blowhard-3000...

They used to sell these--box fans with connections for furnace filters. I found it strange that they never caught on, and now you can't find them anywhere.

I was looking up air quality monitors but can't seem to find any on amazon with a calibration certificate that will accurately measure 0.3 micro particles. Am I the only one who thinks this is a great opportunity for a smart product with an app that has the features of a professional air quality monitor and simple UI / UX? If such product exists, does anyone know where I can find it?

https://www.purpleair.com/ seems to work great for me!

Can the sensor be used indoors or does it have to be their "indoor sensor"? The products look interesting but the site does not inspire confidence.

Their site is simple but it works great. I "installed" my outdoor sensor by laying it on my desk inside and plugging it in. Will move outside when fires end.

I'm using their indoor sensor, works well

I bought one of these earlier this year: https://kaiterra.com/products/laser-egg-2-plus/

They are hard to find and buy in the US, but very popular in China where many cities have major air quality issues. I picked one up on ebay.

Most of these consumer-grade PM2.5 monitors seem to use the same basic laser scatter sensor that measures 0.3um+ particles.

With the recent California fires, I've been looking for a decent option. Unfortunately the Elgato Eve sensor is no longer sold, and the few other consumer products still hover in the $100+ range with poor reviews.

PurpleAir sells an indoor sensor, but it appears to be limited release and somewhat 3D printed. https://www.purpleair.com/

The new version of the Eve Room sensor seems to be out: https://www.amazon.com/Elgato-10EAM9901-Generation-Technolog...

How does Eve Room compare to the Netatmo Home Coach?

Having both, I'd say that the indoor sensor is not worth it. It doesn't have both sensors so it isn't equipped to test them against each other. I do like the outdoor sensor though.

What are you using for indoors?

I don't know about 0.3 micron, but all the different sizes tend to come together. Presence of PM2.5 implies PM0.3

I've got a foobot, I like it. It was benchmarked by some gov't agencies and found to track pretty well.

Just got one of these - seems to correlate pretty well with the airnow.gov outdoor measurements https://shop.hellowynd.com/

Why do you need an accurate measurement for consumer air quality monitoring? I believe even a DIY arduino project with cheap optical dust sensor, like PPD42NS or GP2Y1010AU0F, would be enough.

Why use a measurement device at all? Your nose is quite good enough, as long as it's neither clogged nor "calibrated" to city air. Putting a dust mask with a P3+carbon filter on for an hour or so tends to fix the latter.

I've been running a 3M Filtrete MPR 2800 in my furnace filter. https://www.purpleair.com/ confirms it works. There house averages maybe 60 PM 2.5 vs 200 outside. Don't know how good it is for furnace, but it hardly runs otherwise in Northern CA.

Do you have two air sensors, or do you just have one indoors and compare it with weather data?

I am relying on 2 purple air sensors 10 miles away in opposite directions for an outside estimate. They are usually within 10%, and we are in the same valley, so should be close. I also see when I stop the furnace fan, levels spike. Then drops when I turn on again...

Edit: plants do nothing to particle matter in air they can soak up co2, benzene, formaldehyde etc but do ample resesrch before placing plants in your bedroom. Typically snake plants are fine in bedroom as they don't give out CO2 at night.

There is no need for an air purifier if you have ample light in your room. Just plant a few indoor plants

Peace lily

Rubber plant

Snake plant

Spider plant

Aloe vera

Money plant


NASA has done a study in the 80s and these plants have purified a lot of harmful gases including Benzene, formaldehyde, CO etc

Please use more plants!! And on top of that few plants like Snake, Aloe give oxygen during day and night so they are refreshing (as in they don't just clean air they give oxygen as well) and plants are cheaper and require just water and fertilizer every once in a while.

I own 40+ plants in my balcony garden itself. I've experienced these effects first hand!

Your advice is somewhat dangerous if people follow it expecting them to magically work. First, the NASA study is for VOCs, not pm2.5. Second, when I briefly looked into using plants to clear VOCs indoors, I found that the number of plants, lights, and air-circulation required is completely impractical. Plants are awesome, I love them, but they're not enough. I wish it wasn't so.

It would be stupid to expect plants to clean up PM 2.5 material. The best they can do is chemicals. Sure they aren't a perfect solution but then again, anecdotal evidence from my childhood says thatmore plants are better for our environment. Indoor/outdoors doesn't matter.

And yes it isn't a magical solution, we have to be careful while placing plants in our house.

Apparently this NASA study needs a huge asterisk next to it.


What? I have read the report myself. They put in Benzene inside a sealed room and one of these plants. A few hours later the Benzene was gone! Absorbed by the plant.

They did the same with formaldehyde too and other harmful substances.

Isn't that the bar for purification? This is still better than mere air dolter that are costly and requore frequent maintenance

I think the key term here is sealed room.

And this does not invalidate the study, only helps evaluate the results in context of applying at home or office. I.e. it's still gonna help, but unless you restrict your airflow, not as much as you'd like.

Though the same caveat probably applies to all air purifiers too.

>Though the same caveat probably applies to all air purifiers too.

So whatever reservations you have against plants as air purifiers apply verbatim to commercial air purifiers. Then why the bias against plants?

Funny how you contradicted yourself, not to mention those downvoting me for speaking about plants and yet being enthusiastic about air filters!!

I didn't downvote you and I didn't contradict myself, I only posted a link that urges precise understanding of what the study does and does not show, and to what extent. I have no bias against plants or in favor of commercial air purifiers.

You did not downvote me. But you did contradict yourself.

On one hand you say the focus on NASA study is on the word "sealed". That would directly mean that you believe that plants as indoor purification would not be helpful if there was no sealed environment

And then you say that "the same thing would apply to air purifiers"

You clearly have a bias against plants, as do the few who downvoted me. I know you didn't downvote because HN doesn't allow you to do that!

The study which you have linked says that "more research needs to be done to check air purification" and guess what? They are going to do a study in a sealed environment because if a plant can remove Benzene, Formaldehyde and Toulene in a sealed environment then it can do the same when its not in a sealed environment. You cant5 shrug the results of NASA study saying "it was done in a n atypical environment", of course it was done in a sealed room.

The NASA study would have been refuted if no amount of Benzene was removed by plants. That'd mean that it is a waste of time and space for air purification, but they are not. Peace lily and Orchids removed Benzene. Period. Whether it was a sealed environment or not doesn't matter because results matter.

When a plant that costs less than 100$ has proven to remove harmful gases in a sealed room, I can't wrap my head around the logic of "debunking", the myth and going for a commercial air purifier that is costly af.

And why I say you contradicted yourself is that you criticized plants for being in a sealed room for being effective and then at the end you said "the same things apply to a regular sir purifer too"

This is exactly the fallacy Mr Trump had spoken to in a rally. He had said "I'll build a 100foot wall", the Mexicans will climb the wall, sure but they won't jump, unless of course they use ropes"

Yeah, they will use ropes and they will cross if they want to cross. That's not contradicting, it's a stupid logic.

Same issue with your case against house plants. "Keyword is sealed"

Which study has proven that air purifiers available in the market have done well in an aerated environment? The recommended way to use a commercial air purifier is to close doors and windows. Same issue as plants so if you recommend commercial purifiers over plants despite them both facing same shortcomings then you are biased against plants. End of story.

Thank you as much as I love the idea of house plants doing the job alone this has been debunked for some time.

Can you provide proof of the study that debunked this myth?

Orchids and rubber plants from that list, at least, are fairly common allergens themselves.

I didn't know that! That's still fine though, there are a lot of other n9n allergenic plants which one can use. And if nothing can be used then we can use commercial purifiers too

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