How Much Alcohol Do You Really Need In Hand Sanitizer?
Woman applying hand sanitizer.
The Centres for Disease Control recommends that people use hand sanitizer when soap and water aren’t available.
Washing your hands is considered better for hand hygiene but a sanitizer is more convenient when you’re on the move, and the CDC says sanitizers should reduce the amount of germs in many situations. So do they work in the current situation, in helping stop the spread of Coronavirus?
One way to test whether sanitizers help prevent illness caused by a respiratory virus like SARS-CoV-2 is to record an outcome of infection, like the number of days people are absent from work or school. A trial in Thailand compared the effect of a compulsory hand-hygiene program for three interventions — using an alcohol-based sanitizer every hour, two hours, or before lunch — on absence from kindergarten due to respiratory infections, and found that more frequent sanitizer use meant fewer days off sick. By contrast, a New Zealand study showed that providing sanitizer in classrooms didn’t reduce sick days.
Differences in a sanitizer’s ingredients is one factor that might explain the discrepancy between results. Based on the active agents, there are two main types of sanitizer: alcohol-based hand sanitizers that usually contain ethanol or isopropanol, and non-alcohol-based sanitizers, where the active ingredient is often a disinfectant like benzalkonium chloride. The latter works against some germs, but not viruses like SARS-CoV-2.
The way a sanitizer is delivered also influences whether it’s efficient enough to kill the Coronavirus. Sanitizers typically come in the form of a liquid, foam or gel. Here the results are clear: the modes of delivery aren’t equally effective.
European experiments that compared alcohol-based sanitizers showed that while liquids do the job, gels don’t work quickly enough for healthcare, and concluded that gels “should be considered a retrograde step for hand hygiene because the application time in clinical practice is often shorter than 30 [seconds]; they should not replace alcohol-based liquid hand disinfectants currently used in hospitals or be implemented as first choice agents.”
How you apply sanitizer to the hands affects whether it works properly too. According to a study on the efficacy of ethanol-based hand foams, people don’t consistently cover their hands with a sufficient amount of sanitizer.
Finally, a sanitizer’s efficacy depends on the amount of alcohol it contains. The CDC doesn’t recommend using hand-rub products in which the active agent is less than 60% ethanol or 70% isopropanol, but those recommendations are largely based on how well the sanitizer works against germs with a similar structure to SARS-CoV-2 — viruses surrounded by a fatty outer envelope, such as influenza.
So how much alcohol do you actually need? According to a systematic review of scientific studies by Günter Kampf of the Institute of Hygiene and Environmental Medicine, Germany, ethanol is highly effective at 80% concentration, which is the level where the alcohol will inactivate an enveloped virus within 30 seconds. At 43% ethanol starts working against the original SARS-CoV-1, whose structure is almost identical to the novel coronavirus that causes Covid-19.
The CDC’s recommendation to use hand sanitizer with at least 60% ethanol is wrong with reference to Coronavirus, and its guidelines need updating.
At concentration of 60%, ethanol might work against germs generally, but not coronaviruses specifically. Studies showed that 80% is effective against enveloped viruses within 30 seconds, which means that at lower concentrations (such as 43%) it will take longer — maybe a minute, by which time the alcohol might have evaporated from the skin before it could have the desired effect. People tend to squirt a few drops of sanitizer into the palm of one hand then rub their hands together for a few seconds, and 60% ethanol wouldn’t be effective enough over that time scale.
But it’s ultimately down to the public to weigh-up the factors that affect a sanitizer’s efficacy. In certain settings (a hospital, say) you need the rapid and reliable protection provided by an 80% liquid, for example, whereas a lower concentration may be ‘good enough’ in everyday scenarios.
Washing your hands with soap and water is probably the best way to help prevent viral transmission. Though in many cases, using something is better than nothing.
Update (3 August 2020): I added an extra paragraph to make it clearer why the CDC is wrong to recommend only 60% ethanol.
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