Which is better: Soap or hand sanitizer? - Alex Rosenthal and Pall Thordarson

Your hands, up close, are
anything but smooth.

With peaks and valleys, folds and rifts,

there are plenty of hiding places
for a virus to stick.

If you then touch your face,
the virus can infect you.

But there are two extraordinarily simple
ways you can keep that from happening:

soap and water, and hand sanitizer.
So which is better?

The coronavirus that causes COVID-19

is one of many viruses whose protective
outer surface is made of a lipid bilayer.

These lipids are pin shaped molecules
whose heads are attracted to water,

and tails are repulsed by it.

So in water-rich environments, lipids
naturally form a shell like this,

with the heads outside
and the tails inside.

Their shared reaction to water makes
the lipids stick loosely together—

this is called the hydrophobic effect.

This outer structure helps the molecular
machinery of the virus

break through cellular membranes
and hijack our cells.

But it has thousands upon thousands
of weak points

where the right molecules
could pry it apart.

And this is where soap comes in.

A single drop of any brand of soap
contains quadrillions of molecules

called amphiphiles, which resemble
biological lipids.

Their tails, which are similarly repulsed
by water,

compete for space with the lipids that
make up the virus’s shell.

But they’re just different enough to break
up the regularity of the virus’s membrane,

making the whole thing come crashing down.

Those amphiphiles then form bubbles of
their own around particles

including the virus’s RNA and proteins.

Apply water, and you’ll wash that
whole bubble away.

Hand sanitizers work less like a crowbar,
and more like an earthquake.

When you surround a coronavirus with
water,

the hydrophobic effect gives the bonds
within the membrane their strength.

That same effect also holds the big
proteins

that form coronavirus’s spikes in place

and in the shape that enables
them to infect your cells.

If you dry the virus out in air,
it keeps its stability.

But now surround it with a high
concentration of an alcohol,

like the ethanol or isopropanol found in
most hand-sanitizers.

This makes the hydrophobic
effect disappear,

and gives the molecules room to
move around.

The overall effect is like removing all of
the nails and mortar from a house

and then hitting it with an earthquake.

The cell’s membrane collapses
and those spike proteins crumble.

In either method, the actual process of
destroying the virus

happens in just a second or two.

But doctors recommend at least 20
seconds of hand-washing

because of the intricate landscape
that is your hand.

Soap and sanitizer need to get everywhere,
including your palms, fingertips,

the outsides of your hands, and between
your fingers, to protect you properly.

And when it comes to a
coronavirus outbreak,

doctors recommend washing your hands
with soap and water whenever possible.

Even though both approaches are similarly
effective at killing the virus,

soap and water has two benefits:

first it washes away any dirt which
could otherwise hide virus particles.

But more importantly,

it’s simply easier to fully cover your
hands with soap and water for 20 seconds.

Of course, hand sanitizer is more
convenient to use on the go.

In the absence of a sink, use the
sanitizer as thoroughly as possible

and rub your hands together
until they’re dry.

Unfortunately, there are billions of
people

who don’t have access to clean
drinking water,

which is a huge problem at any time
but especially during an outbreak.

Researchers and aid groups are working to
provide solutions for these communities.

One example is a device that uses salt,
water, and a car battery

to make chlorinated water that kills
harmful pathogens

and is safe for hand-washing.

So wherever possible, soap and water are
recommended for a coronavirus,

but does that mean it's best for every
viral outbreak?

Not necessarily.

Many common colds are caused by
rhinoviruses

that have a geometric protein structure
called a capsid

instead of a lipid membrane.

The capsid doesn't have nearly as many
weak points

where soap amphiphiles can pry it apart,

so it takes longer for
soap to be effective.

However some of its surface
proteins are still vulnerable

to the destabilizing effect
of hand sanitizer.

In this and similar cases, hand sanitizer
may be more effective,

especially if you then wash your
hands to remove residual particles.

The best way to know which to
use for any given outbreak

is to do what's best for all
things illness-related:

follow the advice of accredited
medical professionals.