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Do 24-hour hand sanitizers actually work?

by gilch2 min read1st Mar 20204 comments



With the recent virus scare, I've become more interested in hand sanitizers because even for "airborne" viruses, most transmission happens through contact with surfaces, so hand hygiene is very important. I came across one called Armor Mist that claims to work for 24 hours with a single application. (The label only says "12+ hours", the 24-hour "or ten hand washings" figure is from the FAQ on their website.) This sounds too good to be true (but so was the smallpox vaccine), so I'm not sure.

I'm rational enough to spot and avoid obvious woo with no plausible mechanism of action (like homeopathy), but once it passes that bar, I become really uncertain. The sonic mosquito repellers, for example, had me going for a while. The claim was that it mimicked the sound of a predator, like the wing-beats of a dragonfly. In principle, it could have worked. There's a plausible mechanism of action compatible with both biology and physics. There are also examples of using fake predators to control other pests. However, it turns out that mosquitoes are pretty much deaf and these are completely ineffective. That it seems to work at all is pure confirmation bias for those times that you happen to not encounter mosquitoes, and placebo because you can "hear it working". (In my case, during a camping trip, I was lucky to hear a report in-person from someone who tried using one instead of spray and got bites, so I never bought one.) Not only is this a waste of money, it's dangerous to rely on a defense that doesn't work if that means not using one that does (like bug spray).

Armor Mist does have a plausible mechanism of action. The active ingredient on the label is benzalkonium chloride 0.1% (BZK), which is a surfactant used as a preservative. It probably does kill bacteria. But how effectively and for how long? And does it work on coronavirus or influenza? They claim it works like a catalyst and is not consumed in the process--you only have to reapply because your skin naturally sloughs off over time. They also mentioned something about a positively-charged polymer. I have found some similar-looking BZK-based hand sanitizers, like Zylast and Germfree24. Zylast only claims to last for 6 hours though.

On the other hand, the FDA recently banned triclosan in soap due to questionable safety and effectiveness, along with a number of other antiseptics. BZK was also up for review around the same time, but was not banned with the others (yet). Like the sonic repeller, a defense that doesn't work is a danger if that means not using one that does (like alcohol-based sanitizer).

How should a rationalist go about evaluating claims like this? Does it work? Should we use it?


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A brief Google suggests BZK is effective against SARS-Cov-2 on surfaces. There is no research on it on hands that I found specifically for coronaviruses. Hands are generally more difficult to disinfect.


https://www.ncbi.nlm.nih.gov/pubmed/15923059 (About SARS-CoV, but similar enough virology that it probably applies)

I'd be most skeptical of the claim it is effective for 24 hours on hands, so if you did get it, I would re-apply whenever you would normally use alcohol-based sanitiser and not rely on this claim.

This paper suggests BZK is more persistent on hands than alcohol based sanitiser for a bacterium (MRSA), which makes sense a priori, but only up to 4 hours.

Update with some more info:

https://www.compoundchem.com/2020/03/04/hand-sanitisers/ has some general information about hand sanitisers and includes info about BZK - they caution that it works less quickly than alcohol based ones, so perhaps that's useful to take into account.

Also, it may interest some people that BZK is the active ingredient in Lysol spray (US) and Dettol spray (UK). (Do not use them as hand sanitiser as they have other ingredients and are not formulated for hands!)

For the object-level question, Wei Dai linked to this study showing benzalkonium chloride (and a few related chemicals) ineffective against enveloped human coronavirus (although this was one of the common cold variants).