Rub-a-dub-scrub

On 9 March, my local NHS hospital put out a furious tweet regarding a hand sanitiser cartridge stolen from a wall dispenser. This incident was symptomatic of a much wider issue, growing public anxiety regarding the spread of Covid-19. Panic-buyers had depleted shops of hand sanitisers globally. Some online suppliers then stoked the fires of fear using unscrupulous marketing tactics and price gouging.

This scramble to stockpile hand sanitiser was (accidently) triggered by the issuing of official guidance to boost hand cleaning frequency to help slow the spread of Covid-19. But why are soap and (some) hand sanitisers such effective tools against these viruses? The answer lies in the structure of their particles.

Virus make-up

Covid-19 is the seventh coronavirus to be identified as able to infect humans. These viruses vary in seriousness from the fairly harmless common cold to Severe Acute Respiratory Syndrome (SARS). But their particle designs are all the same. Genetic material in the form of RNA is encapsulated in a spherical lipid bilayer with proteins projecting out of it.

The lipids are amphiphilic – they have a hydrophilic and a lipophilic end – and spontaneously self-assemble into bilayers. The role of this lipid bilayer is to protect the RNA when the viruses are outside a host cell. It enables viruses to survive on surfaces for quite some time. How long exactly is unknown for Covid-19, and varies depending on surface type and other factors such as room temperature anyway, but official estimates of its survival time on surfaces range from hours to days.

Coronavirus breakdown

That said, compared to other virus types the coatings of the coronaviruses aren’t actually very robust. Soap molecules, which are also amphiphilic, can compete with the lipids in the bilayer. This disrupts the structure of the coating, breaking it apart, exposing the RNA and rendering the virus particle inactive.

This scramble to stockpile hand sanitiser was (accidently) triggered by the issuing of official guidance

Alcohol also damages the lipid bilayer coating of the coronaviruses. This is why the official advice is to use alcohol-based hand sanitisers when handwashing isn’t logistically possible. To be effective against Covid-19, your hand sanitiser must contain at least 60% alcohol. You also need to use a big blob of it. The virus particle needs to be momentarily soaked in alcohol to ensure coating damage occurs. Length of exposure is also important when using soap – and that’s the reason for the 20-second handwashing rule. (Which isn’t a new rule by the way, that has always been how long we’ve been advised to wash our hands for!)