It’s around every corner and in every backpack and purse. And – after the initial buying panic this spring – it’s even back on your local grocery store shelf! With flu & cold season around the corner and the Covid pandemic continuing you’re likely to use these gels this fall. So why not learn a little about their biology and chemistry?
What’s in it?
The major and only active ingredient in most hand sanitizers is alcohol. Either isopropyl alcohol, ethanol, n-propanol, or some combination of the three. (There are also non-alcohol versions that contain antibiotics like benzalkonium chloride or triclosan but these are less effective and less popular.)
Hand sanitizers ideally have a high, but not too high, alcohol content, so water is also present as a diluting agent. This is because concentrations lower than 60% lack the potency needed to kill the majority of germs on a surface but concentrations higher than 80% evaporate too quickly.
Most store-bought sanitizers also have a gelling agent. This serves two purposes. The first is to further slow evaporation and thus maximize the time the alcohol has to work against any microbes. The second is convenience – gels are easier to dispense on the go.
Moisturizing compounds such as glycerol are also added to prevent dry skin. This is more than a comfort issue, the cracks that form in dry hands can create especially good habitats for microorganisms.
How does it work?
Alcohol “kills” germs by attacking their outer defenses. In the case of bacteria, this is the cell membrane which is made out of a lipid bilayer. In the case of many viruses, this an outer envelope that is made from the phospholipids and proteins of its host’s cell membrane but which also include special viral glycoproteins.
Alcohol molecules have both polar and nonpolar regions. This structure, and the resulting mixture of charges, allows alcohol molecules to bond with lipid molecules in a cell’s membrane or in a virus’ envelope. This has a powerful and cascading destabilization effect. As these lipids and proteins jostle around in an attempt to create a new stable structure, the alcohol molecules can further “invade” the membrane.
When this happens, alcohol molecules further interact with protein molecules in the membrane/envelope. In particular, new hydrogen bonds form between the alcohol molecules and certain protein side chains which disrupts the protein’s intramolecular bonds. This destroys or “denatures” these essential molecules which in turn kills the bacteria cell or virus particular.
This two-step process takes between 10-30 seconds which is why timing and slowness are such an important part of successfully using a hand sanitizer.
TEDed has a great video that goes into even more detail about the chemistry behind both hand sanitizer and soap which I highly recommend.
What does (and doesn’t) it work on?
Alcohol can eliminate many bacteria, include such Escherichia coli, Salmonella sp., and Staphylococcus aureus. However, it is much less effective at getting rid of spore-forming bacteria such as the bacteria responsible for botulism (Clostridium botulinum), tetanus (Clostridium tetani), and anthrax (Bacillus anthracis).
Alcohol has also been shown to kill many viruses including those that cause herpes, hepatitis B, HIV, influenza, rhinoviruses, and COVID. However, some viruses lack an envelope and just contain a protein covering. The virus that causes norovirus, polio, and hepatitis A are all non-enveloped viruses. Alcohol-based sanitizers are not as effective against these.
Viruses and bacteria can also be protected by dirt, oils, or mucus. This is why hand sanitizers are most effective in situations where the surface/hands are already clean (such as a hospital setting). It’s also why hand sanitizers have a mixed record when it comes to preventing the spread of the common cold.
How is it made?
Mass producing commercial hand sanitizer is hard! That’s because most gelling agents like gelatin and agar can’t form intermolecular bonds in a high alcohol environment. Instead, producers need to use high-molecular-weight cross-linked polymers like acrylic acid. Finding this ingredient as well as finding ways to make the alcohol in less potable were just two challenges that many producers faced this spring when demand for the hand sanitizer spiked.
The WHO offers two simple formulations for making your own hand-sanitizing liquids. These recipes are intended to be used in resource-limited or remote areas where people don’t have access to sinks or other hand-cleaning facilities.
Any tips or tricks?
Using hand sanitizer is as simple as it can get but many people negate this product’s effectiveness by wiping it off too quickly. It’s also a good idea to check that the alcohol content is between 60 and 80% and when possible to rinse your hands so that they are free of dirt and grease.