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.
