(Probably a lot?)

Disease experts are constantly telling us how important it is to wash our hands. But how much does it actually matter? This is surprisingly hard to answer.

In handwashing’s favor: We have very good reasons to believe it should work. Soap destroys the integrity of cell membranes, killing them. So regular soap kills bacteria and fungi. It doesn’t kill viruses but helps wash them off your hands.

Against handwashing: There are quite few randomized controlled trials that actually test the hypothesis of “if ordinary people washing their hands more they get sick less”. And those trials that exist are tricky to interpret.

What seems to be the most ambitious trial by far took place outside Karachi in 2002-2003. They assigned 300 households each as controls, to regular soap and to antibacterial soap. They visited the soap households weekly to give out soap, and to encourage people to wash their hands.

They found that in soap households, children younger than 5 were ~50% less likely to get pneumonia or diarrhoea, and ~33% less likely to get impetigo (a skin infection). Here’s a chart for children under 15:

There are various other reviews of different studies. These typically find reductions in illness between 20% and 33%, depending on the disease and if they are studying high or low-income countries.

At first glance, 33% seems significant, but not overwhelming But I think two factors are missing from these studies:

Limited effectiveness of the intervention . These studies basically give people soap and encourage them to use it. How well does this work? I don’t know, but I bet a study that forced participants to wash their hands at gunpoint would have larger effect sizes. All the studies are really measuring is “how effective is repeatedly asking people to wash their hands”? But you aren’t deciding if you should be exposed in such a campaign– you are actually deciding if you should wash your hands.

. These studies basically give people soap and encourage them to use it. How well does this work? I don’t know, but I bet a study that forced participants to wash their hands at gunpoint would have larger effect sizes. All the studies are really measuring is “how effective is repeatedly asking people to wash their hands”? But you aren’t deciding if you should be exposed in such a campaign– you are actually deciding if you should wash your hands. Hand washing in the baseline. Just as these studies surely don’t take people to 100% handwashing, many in the control populations are already washing their hands. If we really wanted to check how important it is to wash hands (and we were horrible) we would force a group to avoid all hand washing. This would surely make hand washing look even better.

Then there’s another issue:

Externalities. When you wash your hands, you don’t just prevent yourself from getting sick, you prevent others from getting sick. Some studies try to account for this. The Karachi study made sure that the “no soap” and “soap” groups were all clustered in different neighborhoods. But, still, they were still only reaching a subset of the population. Other studies don’t deal with this at all. If you’re altruistic, this is probably another major factor. (It’s possible in principle that certain diseases could be eradicated just by handwashing if that drove transmission rates low enough.)

How much should we increase our estimates based on these ideas? It’s not totally clear, but it seems like handwashing would look a lot better if it were possible to fully account for them.

In terms of personal choices, it probably doesn’t matter. I’d say they push careful handwashing into the territory of something you’d be crazy not to do if you might be exposed to pathogens.