
Why is 70% isopropyl alcohol a better disinfectant than 99% isopropanol? - montalbano
https://blog.gotopac.com/2017/05/15/why-is-70-isopropyl-alcohol-ipa-a-better-disinfectant-than-99-isopropanol-and-what-is-ipa-used-for/
======
mthoodlum
"The presence of water is a crucial factor in destroying or inhibiting the
growth of pathogenic microorganisms with isopropyl alcohol. Water acts as a
catalyst and plays a key role in denaturing the proteins of vegetative cell
membranes. 70% IPA solutions penetrate the cell wall more completely which
permeates the entire cell, coagulates all proteins, and therefore the
microorganism dies. Extra water content slows evaporation, therefore
increasing surface contact time and enhancing effectiveness. Isopropyl alcohol
concentrations over 91% coagulate proteins instantly. Consequently, a
protective layer is created which protects other proteins from further
coagulation."

Many viruses have a hydrophilic outer membrane and the water helps the alcohol
to penetrate the virus.

~~~
nodesocket
Thank you, I immediately hit back when I saw how long that article was.

~~~
penguin_booze
I relate with you. I wish articles also published a TLDR version.

Personally, I've trouble focusing if the content is less engaging, or am there
just to know the gist. I suspect I've ADD-like traits - when I see an article
beginning like "it was a warm sunny day in...", I say, "OK, maybe later". I
however do appreciate long read articles have their place.

So far I've noticed only Business Insider does this on their articles.

~~~
debian3
« It was a warm sunny day in... » and i saw your comment on ADD, i was
thinking it’s not really how or it works. It’s that you can’t focus on what
your are reading. You start thinking about something else while reading and
you don’t even notice it. « ...the end. » damn it. Let’s read it again « It
was a warm sunny day in... » and I’m telling you it’s the worst thing ever
because you can’t control it, no matter how hard you tell yourself I need to
focus on it. It’s not a conscious decision to procrastinate. « ... the end. »
Shit, again?

At least that’s how it is for me.

------
jszymborski
The explanation I was always given when taking my biohazard/biosafety courses
was that concentrations of EtOH or IPA greater than 70% evaporated too quickly
to have the contact time you needed to sterilize e.g. work surfaces or (even
worse) your gloved hand.

Apparently here they mention that higher conc. of water make it a better
solvent... which I buy, but alcohols still aren't the worst solvent for e.g.
hydrophillic membranes, so I think other factors like contact time are likely
in play.

~~~
lifeisstillgood
The interesting one is solutions >91% - they kill cells immediately- but this
coagulates the cell proteins forming a protective layer - dropping time to
disinfect from ten seconds to over two hours.

It's fascinating how deep one can dive in any subject and still produce useful
actionable insights :-)

~~~
thatcat
Yea, I mean that is crucial information for using the product. You would think
they would explain some of this on the bottle.

------
elsjaako
Can someone link to a study that shows this is the case for virusses also?

The example listed in the article, Staphylococcus Aureus, is a species of
bacteria.

Last time I had this discussion they linked to a study that said that for
viruses, higher concentrations are better.

(Epidemiologic Background of Hand Hygiene and Evaluation of the Most Important
Agents for Scrubs and Rubs Günter Kampf and Axel Kramer)

~~~
giarc
Likely depends on the type of virus. There are two types, enveloped and non
enveloped. The enveloped would likely act similar to bacteria whereas the non-
enveloped would likely suffer more from higher concentrations of alcohol.

~~~
EForEndeavour
And the coronavirus is enveloped. [https://theconversation.com/vodka-wont-
protect-you-from-coro...](https://theconversation.com/vodka-wont-protect-you-
from-coronavirus-and-4-other-things-to-know-about-hand-sanitizer-133593)

~~~
draugadrotten
and soap is both sufficient and great at destroying the coronavirus.
[https://www.engineering.com/DesignerEdge/DesignerEdgeArticle...](https://www.engineering.com/DesignerEdge/DesignerEdgeArticles/ArticleID/20146/A-History-
of-ViolenceHow-Soap-Attacks-a-Coronavirus.aspx)

------
alister
Since we're talking about counterintuitive things related to the coronavirus
crisis, I was surprised to learn that N95 masks, that remove 95% of particles
of size 0.3 μm, are _even more_ efficient with smaller particle sizes. In
turns out that 0.3 μm is the hardest particle size to filter--they call it the
most penetrating particle size--hence filter testing is done at this worst-
case size. The reason is that filters work due to several mechanisms which
dominate at different particle sizes according to a curve:

"For very small particles, less than 0.1 μm in diameter, the primary
filtration mechanism is diffusion and the filter is very efficient. For
particles between approximately 0.1 and 0.4 μm the filter is less efficient as
the particles are too large for a great diffusion effect and too small for a
large interception effect. Particles above approximately 0.4 μm enter the
region where interception along with inertial impaction are predominant and
the filter is very efficient again."[1]

So if your filter is 95% efficient at 0.3 μm, it's even better at bigger _and
smaller_ sizes. The SARS-CoV-2 virus is 0.05 to 0.2 μm in diameter, so an N95
mask will be substantially better than 95% for coronavirus.

[1]
[https://www.tsi.com/getmedia/4982cf03-ea99-4d0f-a660-42b24ae...](https://www.tsi.com/getmedia/4982cf03-ea99-4d0f-a660-42b24aedba14/ITI-041-A4?ext=.pdf)

~~~
whatever1
Same reason that we care about the concentration of PM2.5 particles in the
atmosphere. Once we inhale them they manage to reach the very end of our
lungs.

~~~
EForEndeavour
Is it? That's different from the counterintuitive N95 filtration efficacy.
PM2.5 designates particles of diameter 2.5 microns _and smaller_ , and I've
read nothing that implies particles smaller than 2.5 microns are somehow
easier for the respiratory tract to keep out of the lungs. The graph of
filtration efficacy as a function of particle size is U-shaped for N95 masks,
but not for the human respiratory tract.

------
azinman2
If I have a 91% currently, can I just add water to dilute it? Do I need some
kind of special water vs what’s from my tap?

~~~
p1mrx
If you mix cheap booze (e.g. 40% vodka) with 91% isopropyl, you can make a
larger quantity of 70% sanitizer than you could with pure water.

Mixing on a scale can be pretty confusing, though, because the percentages are
(I think) by volume, and water and alcohol have different densities.

~~~
hutzlibu
Any booze you buy has a huge tax on it, so you would be better of, with
cleaning alcohol.

~~~
mokus
Normally, yes. Today, I’m not even able to find isopropyl at many stores. I’m
actually thinking about ordering flux cleaner for my soldering, for the first
time in my life.

~~~
ficklepickle
You can get methanol pure and cheap from auto stores (in regions that get
cold, anyway). Just try not to get it on your skin or breath the fumes too
much. It's quite a bit more toxic. Definitely don't use it to make hand
sanitizer.

------
noncoml
Not being able to clean my electronics from flux has been a collateral damage
of Covid-19.

I wish this article was more popular so people would stop hoarding 99%
isopropyl.

~~~
coronadisaster
I had more luck going to a small rural town

~~~
chasd00
that's what I have been doing. I live in Dallas and head an hour or so East
for items I know are going to be out of stock locally.

~~~
coronadisaster
One downside is that they dont take as much precautions in my experience....
Almost none had masks for example.

------
cashsterling
At work, we already use technical grade 99.8+% IPA for various things and have
a bunch on hand. so we have been prep'ing 70% with RO water and wiping down
high touch surfaces.

70% IPA in water is the go-to cleaning/sanitizing agent for most
pharmaceutical cleanrooms. They also use other agents, 0.2-0.5% bleach, low pH
cleaners, peroxides, etc... but IPa is the mainstay.

------
senectus1
Why water is so "sticky" is a very interesting subject
[https://www.exploratorium.edu/ronh/bubbles/sticky_water.html](https://www.exploratorium.edu/ronh/bubbles/sticky_water.html)

~~~
jcims
I think our personal every day experience with water dulls the intrigue around
one of the more remarkable and unique substances on the planet.

~~~
dylan604
My favorite is Ultrapure Water (UPW) [0]. It's so pure, that it is used a a
stringent. I watched video about one of those underground subatomic particle
experiments where they use UPW. One of the technicians accidentally let his
hair get wet, and it immediately "bleached" his hair.

[0][https://en.wikipedia.org/wiki/Ultrapure_water](https://en.wikipedia.org/wiki/Ultrapure_water)

~~~
ivalm
Stringent just means something is strict, as in the standards for making UPW.
Nothing is “used as a stringent.”

UPW also doesn’t immediately bleach hair, it would have to be a pretty
prolonged heavy stream. Mostly UPW is pain in the ass because it corrodes
metallic pipes, but is pretty harmless to people unless you really have
massive amounts. One of the reasons it’s harmless is because the moment it
touches you it isn’t really ultra pure anymore, it really takes parts per
million to become not UPW.

~~~
wnoise
I think he meant an
[https://en.wikipedia.org/wiki/Astringent](https://en.wikipedia.org/wiki/Astringent)
.

~~~
ivalm
Didn't know the word, thanks!!

------
StavrosK
Also, why does alcohol easily kill microorganisms but doesn't destroy skin
cells?

~~~
compsciphd
outer skins cells are already dead....

~~~
oliveshell
Also, alcohol will certainly leave your hands dry and cracked given prolonged
contact.

(I used to refurbish old PCs and we used isopropyl alcohol liberally as a
cleaner.)

~~~
masklinn
> Also, alcohol will certainly leave your hands dry and cracked given
> prolonged contact.

Indeed, alcohol will dry the hell out of the skin, which is why alcoholic hand
sanitiser have some sort of moisturiser included (e.g. glycerol).

~~~
Wistar
Even with the moisturizer, hand sanitizer will cause my knuckles to become so
chapped they cracked and bled within a few days with just a few applications
per day. I had to stop using it as I began to fear infection.

~~~
masklinn
Yes, the moisturiser mitigates the issue but will not solve it. If you need to
sanitise your hands a lot and they’re sensitive you may want to use or switch
to disposable gloves.

~~~
AstralStorm
Or a well chosen brand of non-disposable gloves that survive disinfection with
it well. Nitrile and butyl don't and are good for incidental contact only,
they will swell; latex, vinyl, thick PET and norprene do work for more
extended time. You have to be thorough in rubbing so mechanical resistance to
that could be important too, making norprene the material of choice closely
followed by PET.

(The latter three depend on the brand, some might start to slowly dissolve or
harden after many applications due to plasticizer leaching. Always check with
manufacturer.)

------
mslack616
Wow... I used to watch a youtuber clean his tools with a bottle labeled as
"Isopropyl Alcohol, AKA IPA" I always thought he was making fun of IPAs as in
the beer style. I feel so dumb hahaha x_x

~~~
kens
My theory is that three-letter acronyms are so popular because they are strike
a balance between length and ambiguity. I.e. two-letter acronyms are too
ambiguous, but four-letter acronyms are too long for the uniqueness benefit.

Most of the remaining ambiguity of three-letter acronyms could be avoided if
people didn't get upset about redundant acronym expansion: "I cleaned with IPA
alcohol after having an IPA ale."

[1] RAS syndrome:
[https://en.wikipedia.org/wiki/RAS_syndrome](https://en.wikipedia.org/wiki/RAS_syndrome)

~~~
lonelappde
Most people don't care about RAS syndrome, except semi-intelligent people who
think grasping at (incorrect) pedantry proves they are smart.

~~~
samatman
Depends on the acronym for me.

ATM Machine grates on my sensibilities, since we all know what an ATM is and
calling it a "machine" is both redundant and low-information. It's like
calling an ATV an ATV vehicle (which I have never heard): would you call a
truck a truck vehicle?

PIN number on the other hand, I'm fine with. A pin is a small piece of steel
used in sewing, a PIN number is a number you type into an ATM.

~~~
vikramkr
I think people stop parsing ATM as an acronym and start parsing it as just the
name of a thing. Which is understandable. When I see ATM I dont think ... I
just realized I forgot what the A and T stand for. Apparently automated
teller? Perfect example I guess. As far as I'm concerned, ATM is just the name
of the thing. And I parse it as such, so ATM machine isn't redundant since ATM
doesnt point to "automated teller machine" in my mind. It just points to
"ATM."

I also find misuses of RAS syndrome fun. AAV vector isn't RAS syndrome. It
stands for adeno associated virus vector. But people are eager to jump on
whatever pedantry they think they can find, makes them feel smart I guess.

~~~
samatman
I don't know that it's even RAS syndrome.

Some machines have 'machine' in their name, most don't. A milling machine,
yes, but who says lathe machine?

ATM machine just sounds weird to me. The fact that the "M" stands for machine
doesn't _help_ but it's not decisive.

That's what I mean by redundant: it's like saying spoon utensil.

~~~
vikramkr
But for me ATM is like milling, it's a machine that has machine in its name.
It's not like lathe, probably because I've grown up surrounded by people that
call it an atm machine. You cant fight language after a point - there's a
point where double negatives are grammatically incorrect, and there's a point
where we are at now where wikipedia just classifies some dialects of english
as having negative concord, where multiple negatives affirm (instead of
contradict) eachother. Same way, at a point it probably makes more sense to
talk about redundant acronyms as just a thing english does than try and say
that a construction that a lot of people use and that everyone can understand
is a grammatically incorrect thing. So ATM vs ATM machine becomes a dialect
choice rather than some grammatical discussion, like British vs English
spelling

------
jcims
I wonder if they call this passivation like they do in chemistry -
[https://youtu.be/2yE7v4wkuZU?t=394](https://youtu.be/2yE7v4wkuZU?t=394)

~~~
selimthegrim
And here I thought the 'acid to water' rule was only so you would have water
splashing instead of acid. TIL (and I was a chemistry undergrad)

~~~
lonelappde
What do you mean? How does the order of adding ingredients influence
passivation?

You should create an acid water mixture before adding the mixture to another
chemical.

~~~
selimthegrim
"Do what you oughta, alway add acid to water" instead of water to acid is a
safety mantra taught in high school and undergrad chemistry labs. You are
correct but if you add the water to acid in the situation they'd shown the
acid would be too concentrated as well.

------
tgsovlerkhgsel
For me, the site only shows a multi-step hCaptcha. I solved it the first time,
then the page reloaded and showed me the second one, which after two puzzles
told me to try again.

------
robbrown451
My understanding is that soap and water is even better for coronavirus. Not
just to wash it away, but to actually destroy is because it has a very fragile
lipid layer.

~~~
remote_phone
It’s not even better. Washing with soap and water is effective but rubbing
alcohol (Both 70% and 99%) takes seconds to kill coronavirus. It is almost
instantaneous in terms of effectiveness. Soap and water take a few mins to
reach the same level of effect as rubbing alcohol.

There was a recent study by Stanford that measured the effectiveness of
various cleaning methods.

~~~
robbrown451
That's not what I get from these articles, which say it takes 30 seconds for
alcohol to kill it (on surfaces), while soap and water (with scrubbing) will
do the trick with 20 seconds.

[https://www.uchealth.org/today/why-soap-and-water-work-
bette...](https://www.uchealth.org/today/why-soap-and-water-work-better-than-
hand-sanitizer-to-remove-the-coronavirus/)

[https://www.uchealth.org/today/ins-and-outs-of-
disinfecting-...](https://www.uchealth.org/today/ins-and-outs-of-disinfecting-
coronavirus/)

[https://www.marketwatch.com/story/deadly-viruses-are-no-
matc...](https://www.marketwatch.com/story/deadly-viruses-are-no-match-for-
plain-old-soap-heres-the-science-behind-it-2020-03-08)

If you've got a better reference I'd be interested in seeing it, but this is
what I've found.

~~~
remote_phone
[https://www.thelancet.com/cms/10.1016/S2666-5247(20)30003-3/...](https://www.thelancet.com/cms/10.1016/S2666-5247\(20\)30003-3/attachment/bfda5654-ca06-42d3-8beb-5f3aa5fc2df0/mmc1.pdf)

Table C lists the various disinfectants they tried including ethanol at 70%.
All of them found undetectable levels of SARS-CoV-2 after 5 mins except hand
soap, which took between 5 mins and 15 mins.

I was off in my recollection that the study showed that alcohol killed the
virus immediately. I may have confused that with a press conference several
weeks ago but that data is probably suspect. All we know is that within 5 mins
alcohol killed it to undetectable levels.

Also note that the 20 second rule has never been tested. There is some value
in washing in a sink with running water since it is physically removing the
virus off your hands. And it’s been socialized that is will work in general
for bacteria but no one has actually tested to see if 20 seconds of hand
washing with soap will clean you of coronavirus specifically. The study above
actually tested for coronavirus although I don’t think the soap test was for
running water.

~~~
mtrower
This link is not at all working for me (brings up the Lancet site with no
content). Is it possible to get a link to the actual file?

------
aaron695
What does it mean for viruses?

Covid-19 is number one issue...then let's work down.

This matters because we need to keep it simple. If you run around telling
people 99% is bad for C19 when it isn't, you are ruining peoples solutions
when you don't have to.

Sorry but these factoids are the fking issue. Telling people soap is better
when hand sanitizer is clearly a better real world solution has caused lots of
issues.

Does 99% work worse on a virus like C19 by a significant amount?

------
achillesheels
I’m working on a nebulized ethyl alcohol biocidal therapy for SARS-CoV-2 and
had to research this (plz see my Twitter for a chronology @jhazani). I’m
disappointed the link abstracts the actual mechanism of action, other than a
hypothesis that evaporation time and contact time makes a difference. It still
does not explain distinctly the _effects_ of contact time, which principally
involves the polarity of alcohol which disrupts the viral envelope and
“denatures” proteins according to the CDC [1] and their arcane reference text
you can only find in a medical school library [2].

In other words, the hypothesis is there is electrostatic attractive forces
that rip the buggers apart, in which this “pressure” is quantitatively
stronger in the presence of molecular hydrogen bonding, ie greater dipole
moments between the aqueous solution and the virus.

(PS our study design will rely on 200 proof absolute ethanol for go-to market
simplicity and for lowest surface tension deposition into the alveolar
pulmonary region to promote alveolar stability)

[1]
[https://www.cdc.gov/infectioncontrol/guidelines/disinfection...](https://www.cdc.gov/infectioncontrol/guidelines/disinfection/disinfection-
methods/chemical.html)

[2] Ali Y, Dolan MJ, Fendler EJ, Larson EL. Alcohols. In: Block SS, ed.
Disinfection, sterilization, and preservation. Philadelphia: Lippincott
Williams & Wilkins, 2001:229-54.

------
godelmachine
While we are discussing this, please don’t mind if I ask which is more
effective - 70% ethyl alcohol vs 70% isopropyl alcohol?

I am using the hand sanitizer by 3M which has 70% ethyl alcohol.

------
diebeforei485
At first glance, I thought they were comparing two different chemicals.

To minimize confusion, the title could be changed to: Why Is 70% Isopropyl
Alcohol (IPA) a Better Disinfectant Than 99% IPA?

------
TomMarius
I am not sure if this is the case, but disinfectant that is too aggressive
will slowly but surely destroy your skin's top layers.

~~~
prmph
I fear I may have done this to my hands out of ignorance; after frequently
assaulting them with alcohol-based sanitizer and dishwashing soap while on
lockdown, they now get pruney very quickly when they come in contact with
water.

Is this situation irreversible?

~~~
adrianN
No, skin is regenerated regularly. You need to destroy it pretty deeply to
permanently disrupt this mechanism.

