I am very suspicious with regards to air purifiers. Have you ever seen real HEPA filter? It is thick and it is quite difficult to push air through it. I can't imagine small PC fan that is designed to push air through UNOBSTRUCTED space will make any significant flow of air. I have spent some time with the guys who build PCs and who know great deal about fans and one thing I learned even an object that the air has to pass around or any kink in the path of air can severly impact performance of PC fan. Because they are designed to be quiet and still provide good flow of air they produce almost no pressure differential to speak of and the flow can be halved by a wire mesh that is a bit too dense.
You need to cycle the air in your room through the filter couple of times to clean it, assuming your filter is any good.
The cheap fan in the picture would barely be able to cycle the air in a small room maybe once an hour, if it was completely unobstructed. It is very likely that with any kind of HEPA filter the flow will drop by an order of magnitude.
Go with suggestion of other readers. If you want good, cheap filter, use a box fan and especially one that was designed to give good pressure difference. Take large cardboard or plastic box and mount large HEPA filter in it so that the air has to pass through. The larger the surface of the filter the easier time the fan will have to push the air through it and more efficient it will be.
There are high static pressure fans which address this problem, specifically designed to work with pressure differential, usually used to blow air through radiators for water cooling. The fan linked in the github page (Noctua NF-F12, the "cheap fan" in the picture) is such a fan, it provides a static pressure of 7.63 mmH2O, which is very high.
Edit: you can get regular-sized 12cm pc fans that need 48 watts and rotate at 7160rpm, move 7.160 m^3/min air at 35.877 mmH2O static pressure:
Actually, it depends on the specific HEPA filter if that is enough or not. If it's not enough, the air will find the path of less resistance and circulate in and out thru the fan blades, but not thru the filter.
For some HEPA filters you can find specs. These (https://www.airclean.co.uk/download/4724) are spec to 250 Pa which would be about 25 mm H2O if I am correct.
If anyone else is wondering, that's about 0.8 mbar (roughly 8 ten-thousands of an atmosphere), or 80 Pa of over-pressure.
To put this into perspective, my local atmospheric pressure varied about 2000 Pa in the last 48h due to weather changes.
Car tire pressure is about 1.8-2.5 Bar.
Your 2000 Pa atmospheric pressure change is about 20 mBar.
Pressure in an inflated balloon is about 2 mBar (0.002 Bar).
Noctua's static pressure is 0.8 mBar. Let's call it 0.5 mBar dynamic working pressure.
Not as low as I expected. Not sure if it will have a semi-reasonable speed, but that will definitely push some air.
Another comparison, 0.5 mBar is 50 Pa = 50 N/m². That gives about 50 * 0.3 * 0.3 / 10 ~ 0.5 kg force on a 30x30 cm² surface.
Finally, as the article video and data shows, it does actually work.
That said, your time machine idea could be simply implemented with a large, airtight box. Simply put the filter over it and weather changes will push and pull air through the filter with two orders of magnitude more pressure than this little fan. Volume of filtered air will depend on the volume of the box though, so better get a large one.
Static pressure 2,61 mm H₂O <-- according to the vendor
Or if you just follow the link on the linked page:
This is quite true. Assuming the media in your particle filter is of sufficient quality, the most important metric to look at is the aggregate surface area. The larger the surface area, the more air you will be able to move through it and the more particulate matter it will hold before requiring replacement.
Particle filters use pleated media for this reason. It's not uncommon to see a filter with 1 square foot face having 50 square feet or more of media. Without specs, the thickness of the filter and the density of the pleating will give you a decent estimate of the filter area. To improve performance and longevity further, it's also typical to have a coarser, pre-filter.
The machine itself is just a fan to push air through the filter and as noted, any reasonably powerful fan will do a sufficiently good job. High quality machines are only really required where you're filtering air before it enters a given area (e.g. LCD/semiconductor plants), and thus cannot afford to have any unfiltered air leak past the filter.
Even good filters do not typically remove all particles, only particles above certain size. If you have cheap filter it may not even capture a portion of the particles you are interested to remove, it may remove none no matter how many times you cycle the air.
Is that good?
I mean, of course it's good that PM doesn't just pass straight into our bloodstream, but isn't our lungs doing the purifying exactly the thing that people try to avoid with mechanical air purifiers?
I was just talking about the test setup. A test of any purifier should be compared to a test with a person in a room without purifier. Otherwise those results are not significant.
just make the analogy pressure ~ voltage, flux ~ current, filter ~ resistance.
A doubling in flux can be achieved either by doubling the voltage across the resistor ( ~ doubling pressure differential across the filter), OR by halving the resistance of the filter ( ~ placing 2 filters in parallel, without halving the filter thickness) so technically a PC fan could blow at a typical PC fan pressure differential and flow rate purified air across an arbitrary number of layers of HEPA, as long as enough of those [N layers of HEPA] in parallel...
In China there is a whole company founded on doing just this, although they've since pivoted into selling specific air purification hardware at a significant discount. See: https://smartairfilters.com/cn/en/product/diy-1-1-air-purifi...
Cardboard is the vastly more cost-effective option. And taping a HEPA filter directly to the back of the fan effective and proven.
The net filtration rate per filter will fall as that's a function of flow rate * filtration size.
Yes, the four- or five-filter version will probably have a slightly better overall filtration rate ... but that's really not the principle constraint here. A single-filter design will drop particle density by ~90% in about 30 minutes. That's sufficient. It's doubtful the final particle density measurement will be much reduced (though if someone's got the data I'd be interested in seeing it), or that this reduction is clinically significant from a health perspective. Remember that you're likely going outside the filtered area, so achieving perfection inside but spending a substantial fraction of your time in unfiltered or far-more-poorly-filtered air, isn't much of a win.
Meantime, you're tying up 3-4 filters that you don't really need, and which others could make effective use of, for a very low marginal imrovement in your own experience.
Where the filters themselves are in low supply -- typical for a region where a wildfire has errupted, and particularly true where filters aren't a high-volume item as with Australia presently -- designs which economise on filter use are a net social benefit.
I used a cheap box fan for some DIY ventilation and the loss in airflow is massive after filter and ducting.
Flow drops a lot, but it still works. Filter was dirty after some time. It was hilariously undersized for my purpose (serious belt-sander action leveling sub-floors), but air does move.
I went for ventilation systems that take air from outside, filter it and then bring it inside the room. Namely Xiaomi MJXFJ-300-G1: https://www.xiaomitoday.com/xiaomi-mijia-air-purifier-mjxfj-...
I've installed one in every bedroom.
My "measurement" was my lungs when working from home: coughing before, no coughing after. Filters turned black.
For extra flow, set up a triangle with the fan on one side and 2 of these filters on the intake end as the other two sides, and duct tape some cardboard to seal the top and bottom.
I don't know if "Lung India" is a great journal, but the "Efficacy of indoor air filters" section of this paper  doesn't offer much support. All studies are pretty small and focused on people with asthma or pet allergies. The few studies of populations without such conditions show what look to me like minor improvements, e.g. a ~5 mmHg drop in blood pressure. These aren't super strong studies though.
The argument that pops up in pro-air-filter pieces draws a line through "air pollution can be a big health hazard" (true), "air filters reduce some particulate content in the air" (true), and "air filters improve health" (unclear). But the "big health hazard" conclusion seems to come from intensely polluted environments, like poorly-ventilated homes that use kerosene, or areas with lots of smoke. For a generic first-world home I can't find much evidence.
Similarly they make some very dense HVAC filters if standard ones aren't catching the level of particles you need, so I'm not sure what the purpose of these filter-fan things are for most people.
Could moving to a coarser filter but adding room filters be useful in that context?
These are a scam, especially all those 3M filtrete ones. Air handlers in homes and most other HVAC applications are not made to also serve as tiny particulate filters, so it will make the whole HVAC system slow or even stop since it won't be able to get the air flow it needs to function.
All the commercial HVAC technicians I speak to say to use the cheapest non fancy filters if your goal is to maximize life of the HVAC system, but I'm sure it all depends on everyone's specific system and how the return is and whatnot.
We have a filter that runs constantly at low speed because we have pets and without the filter there would be issues for members of my family.
You should be aiming for something larger that would work for an apartment.
There is thousand vidoes on Youtube on how to DYI one.
So alternate designs using available materials are required.
(Or someone might drop ship a few containers worth of box fans and HEPA furnace filters down under.)
You can also make DYI air-quality instrument on a cheap using SDS011 sensor and Raspberry Pi. 
i don't think they make much difference for air quality, but i bought some "air purifying" plants and cross-checked them against the aspca list for safety:
(my cat likes to munch on my spider plants, even with cat grass available)
With a few adaptations, however, it might be good as a soldering station fume extractor. These already exist and are little more than a tube, a fan, and a filter (usually with activated charcoal).
The nice thing about this one is that it's circular and could easily adapt to a flexible tube and you could perhaps stack fans for greater sucking power.
A circular exit mask might help slightly, but you're talking a few percent efficiency improvements, not a quantum leap in efficacy.
Simple, cheap, available, and effective are the goals here. Diminishing returns set in early.
Just did this recently actually here's a pic https://mobile.twitter.com/jonathanfly/status/12155378275654...
So unless the author buys a real PM2.5 sensor and does a test in a true room and not a box of the size of a Desktop PC, with the sensor on the opposite side of the room, and proves that the pollution drops to below 8micrograms/m3, in a reasonable time frame, than we can say that it is working.
Until than we can only say that the purifier is only able to purify a desktop PC.
Seriously? 600mA? Air purifiers are rated like 30W for a reason.
You can do this by putting the box fan in a window and sealing up any gaps with something like cardboard or thick plastic sheeting. On the outdoor end of the fan, put your filter. Close all other windows & doors in the house.
Keeping a positive indoor air pressure this way makes sure all the air in your house is filtered. Obviously you'll want to make sure your box fan's filter is rated for smoke / particulate matter.
A decent filter's throughput is going to be multiples of your structure's air-exchange rate.
What you want to do is to filter the interior air, by passing it, multiple times, through the filter. If you put the fan and filter in a window, you're hugely multiplying the air-exchange rate (to no net benefit), but are filtering that air only once.
The net result is a much higher indoor particulates level.
So no, don't do this.
I mean in labs and manufacturing facilities that need ultra pure air, I’m pretty sure they maintain a positive pressure inside the sensitive areas. Otherwise you’ll be sucking in bad crap from every nook and cranny...
For biocontainment you generally want negative pressure, such that all exhaust is filtered.
For clean rooms, positive pressure, to ensure filtration and minimum particle reduction levels within the conditioned space. These also have staged zones, much like a cascade refrigerator, with finer levels of purity as you approach the core. Humans themselves are excluded from the higher-purity zones (we're leaky bags of flakey skin, hair, dander, sweat, and mucus). Most especially in chip-fab, but also I suspect various nanofab facilities.
All of which is pretty much entirely outside my paygrade, so I can't really comment intelligently.
I believe such air purifiers already exist.
The main problem of air washers is that it is not enough to change the water.
Fungus and spiders love the wet environment. Algae start growing if there is sunlight using just the dust particles on air as food. Also very bad bacteria like salmonella and others could grow if the water gets warm.
They do an excellent job of humidifying the treated space, which may or may not be ideal.
Materials for DIY Air Purifiers are:
- Floor Fan of any variety (BYO or we will be providing options for large and small fans)
- PM2.5-suitable Air filter (we will be bulk ordering the best type of Ryco MicroShield Cabin Filters for this purpose)
- Carboard & tape (we will provide these materials)
Efficiency of the US-based version seems to be quite good:
Here is one from Phillips: https://www.youtube.com/watch?v=n3xC3qxVphw
It does not "wash" the air. It has a HEPA-filter on the intake side before the humidifying cycle. So it is basically the same thing as all other examples on this comment page.
You can actually run it without water (which we do), then it does only filtration.
Don't buy this particular product. It is overprized and has dramatic production quality issues (we sent two back, amazon reviews indicate larger issues).
I have a pair of Electrolux EAP300s, in small rooms.
Unfortunately on the lowest manual setting ("Quiet") they hardly do anything for air quality. And they're still louder than I like. I think they really optimized for cost; these devices feel very light and plasticy, the fan sounds cheap and has an uneven tone to it, and its speed keeps oscillating back and forth. The purifier itself is also not stable and it can make an annoying noise if placed on a surface that ends up vibrating along.
I also thought about DIYing one with noctua fans (which I know are quiet, and I have plenty of them in my PCs), but I suspect they're not going to be very effective.
Out of 30 or so air purifiers available in shops here, of all the ones that are in the $100-500 category, the Winix P/U450 are able to push the largest amount of air under the constraint of <40db.
HEPA filters are rated at which air speed they are most effective and how much pressure is needed. The ammount of pressure will only rise as the filter is saturated with dirt. You need a sufficiently powerful fan, and fans that are both powerful enough and quiet enough are very expensive.
I don't mind expensive, I'm tired of buying poor quality products.
I can vouch for their ability to detect impure air. If I start cooking anything, it switches on within seconds. It’ll turn on within about three minutes of sweeping the floor. And if the detectors can turn on when the air is dirty, it stands to reason that they can tell when the air is clean and they turn off as well.
Shortly after getting this, some intriguing news surfaced regarding the installation of air filters in a California school (due to a gas scare) which resulted in a jump in test scores vs other schools in the same district.
I am not a scientist, but I’m feeling pretty convinced about the efficacy of these things.
We have several IQAir units, of at least a couple different models. None of them have sensors to automatically ramp up the filtering when they detect pollution, that would be nice. But according to the air quality sensors I have in the house, they do an amazingly good job of filtering the air, at least if you turn them up to high enough levels.
I have yet to see demonstrated clear benefit of the units with activated charcoal filters for removing VOCs from the air, but my gut feeling is they do help in those areas where VOCs are a concern. Not all parts of our house needs that, however.
Having four furballs does greatly increase the dander and other allergens we have to deal with, but we wouldn’t give them up for anything, so the IQAir units at least make our lives a bit more livable. Especially during “Cedar Fever” season.
If you're referring to some quality of the air beyond particulates… well that's not what a HEPA is even for, is it? I could be mistaken.
Eco-rated houses have a circulation of about 2-3 times per hour. Older houses that are well insulated are about 5-6 times per hour. The purifier needs to be powerfull enough to spin the entire air volume couple of times faster than the natural circulation.
If your windows and doors leak too much, or if you open them too much, or if you burn oil / light up cigars too much and produce smoke then it might be pointless.
I live in one of the cities where the pollution is regularly at the top 10 in the world, and the unit I have is able to reduce the pollution inside to a level that is 5-6 times lower than outside if it runs constantly in a mode that is like 30% of the power. It could do better, but it would be too loud for me.
You're not talking about HVAC, are you?
Nevertheless, I do know people living in leaky homes/building, and they do get >1 exchange/hour.
In general, there is no escaping physics: you need to move a large quantity of air through something that traps dust particles. The low-tech solution of a fan and a HEPA filter is a very good one, but I would suggest getting a filter that traps PM2.5 particles as well.
Any sources on that? I have one and I swear by it. They have a glass HEPA filter and a carbon filter. I'm not quite sure how much air it moves but it's enough to scrub the air of my two floors apartment when someone smokes indoor. It's not really fanless per-se, it's a turbine in the base. You are simply moving the blades down into the base and away from the output area.
Or read the specs and compare with norm
In my bedroom I could see and taste the smoke. The air purifier struggled, but it cleared the air around my computer quite well. I have dividing curtains in my room that I was able to close, which made it easier on the purifier.
All in all I think it was definitely worth the buy. FWIW I picked up a secondhand Kogan SmarterHome air purifier for AUD$60.
First days I started coughing a lot when staying the room for a bit, and I woke up quite stuffy in the morning. Got me an air purifier with HEPA filter and let it run at max setting while at work. After running it a few days like this, I didn't have any more issues.
For some strange reasons it immediately reminded me of the scene in Apollo 13:
The net effective increased filtration rate is marginal.
If you can afford this without concern, it's fine, but it's not necessary.
You can use lower-efficiency (higher effective particle-size) filters ... but you'll just be blowing the bad stuff through them rather than removing it.
The net metric is how fast the room particulate level drops, which is the treatment goal. Flow rate is a red herring.
I’ve got a Honeywell that’s $150 and it must have 20-40x the throughput of this little thing.
These do filter out particulates, but also add enough humidity to the air that this is billed as a feature of the system. In high-humidity conditions this is less than ideal.
With something as simple and effective as HEPA-filters, it seems unreasonable to try other technologies.
Also, ozone air purifier make wonders to keep at bay smells and fats (wonderful to have on the kitchen), and kills bacterias, spores and fungus.
Also, ozone purifiers not need to keep working all the time. A usual usage it's have a burst of ozone to sanitize a place, and wait a few minutes to decompose to O2. The result is that any smell is removed, and the ambient is sanitized. I saw this a lot on hotels where they use "ozone cannons" where they are cleaning rooms.
If not, then cycling the vacuum on and off would actually Dave energy compared to the small fan setup.
If you own a Shop-Vac (a wet-dry vacuum, generically), could you just turn it on without the hose? It would draw much more power, but presumably would work much quicker, and so may end up being more efficient.
It's just airflow + filter, which is why DIYing them is so popular.
I don't really think airfilters in that size so much. They need to push a serious amount of air through the filters to be able to accommodate for natural air circulation. Some pc fans don't do the trick I'd suspect
The dual-ruled calipers used in the next photo would be an acceptable substitute for just using a plain old ruler.
For a quick scale judgment, a smartphone is perfect. It's not supposed to be precise.
A "for scale" comparison is most useful with a metric-scale ruler. Metric lengths are standard worldwide. Including an inch-scale ruler may help US-based persons. The calipers were useful inasmuch as they had a metric length scale etched into them--a ruler with extra bits.
By your argument, it is acceptable to report large lengths or areas in multiples of US football fields, volumes as multiples of Olympic-size swimming pools, or database sizes in multiples of the entire literary content of the US Library of Congress. Journalists do this frequently, and every time, it drives me absolutely bonkers (an aggravation level equivalent to 3 rush-hour traffic jams).