I was in a large supermarket the other day and out of interest, decided to have a look for hand sanitiser. Sure enough, the shelves were empty. It was the same in my local shop, where a member of staff told me that new stock couldn’t be had ‘for love nor money’. It didn’t bother me though, because what I was actually looking for was soap.
In the last few days, we’ve seen a lot of ministers and health professionals doing their best to persuade us that soap and water is actually better than alcohol-based hand sanitisers for cleaning and sterilising our hands. In this post, I will explain why they are right, using the diagram below which shows the structure of a coronavirus.
All viruses have the same basic structure: a strand of DNA or RNA encased within a protective envelope. There are various proteins on the surface of the envelope, and it may contain enzymes as well as DNA or RNA. It’s the envelope we are concerned with here, because it protects the genetic material. Destroy the envelope and you destroy the virus.
The envelope is made up of molecules called phospholipids. These are a type of fat, and are the same thing that makes up cell membranes. The structure of a phospholipid, shown in the diagram on the right, can be thought of as being a bit like a tadpole. A strange, mutated tadpole that has two tails… The head contains a phosphate ion, and the two tails are made up of fatty acids. The phosphate ion in the head has an electrical charge (it’s polar if we want to get technical), which means it can interact with water – it is hydrophilic. The fatty acid tails are not charged (they are non-polar), which means they cannot interact with water.
In a cell membrane, or the lipid envelope of a virus, phospholipids line up tail to tail, as seen on the left, forming a double layer – the phospholipid bilayer. Disrupting this bilayer will cause the envelope to break down, destroying the virus.
So, why is soap and water better than alcohol-based hand sanitiser? The answer is that soap and water can interact with both parts of the phospholipid. Water is a polar molecule, meaning that it is attracted to anything with an electrical charge. So, water can get in amongst the phosphate heads, causing them to separate. Hydrophobic substances, such as the tails of the phospholipid, do not interact with water, but they do interact with detergents such as soap. So, while the water separates the heads, the soap separates the tails; the envelope breaks up and bingo – no more virus!
Why are alcohol-based hand sanitisers not as good? Basically because they do not include a detergent. Alcohols are polar molecules, meaning that they behave in the same way that water does. Alcohols can interact with the hydrophilic head of a phospholipid, but not with the hydrophobic tail. So, they will destroy the phospholipid envelope of a virus by interacting with the heads, just not as efficiently as soap and water which interacts with both heads and tails. What this means is that in order to be effective, the concentration of alcohol in the gel must be very high. Sanitisers on general sale are typically 60 – 80% alcohol, while those used in healthcare settings may be as high as 95%. This is not very good for the skin; frequent use of alcohol-based hand sanitisers will quickly dry out the skin of your hands, causing cracking and soreness. In addition, frequent use of alcohol-based hand sanitisers in hospitals has been linked to antibiotic resistance in faecal bacteria such as E. coli.
What about alcohol-free hand sanitisers? They are also less efficient than soap and water. They rely on antiseptic agents; two of the most common are benzalkonium chloride and triclosan. Benzalkonium chloride is widely used as a disinfectant in hospitals, but its use has been linked to the development of antibiotic resistance in MRSA. Triclosan breaks down into various compounds that accumulate in the environment and cause significant problems. Alcohol-free hand gels also have a shorter shelf-life and are more easily contaminated, so it’s important to keep an eye on the use-by date. Anyone who has been panic-buying alcohol-free sanitisers may well find that they end up having to throw them out if they can’t use them all by the use-by date.
So when are hand-gels useful? Basically for anyone who is in contact with people or animals, and for whom hand-washing is not always possible. This may include retail workers; veterinary workers; those in healthcare and those working with young children. Even in healthcare settings, though, staff are advised to wash their hands with soap and water whenever possible.
In conclusion, soap and water is more effective and better for your skin. It’s also better for your pocket – just compare the price of a bar of soap with that of a bottle of hand sanitiser.