Risk: micromorts, microCOVIDs, and skydiving

12.42, Tuesday 1 Sep 2020

There’s a standard way to understand the relative danger of any activity. A micromort is a unit of risk defined as one-in-a-million chance of death (Wikipedia). For example:

  • skydiving is 8 micromorts per jump
  • running a marathon: 26 micromorts
  • 1 micromort: walking 17 miles, or driving 230 miles

Generally being alive averages out at 24 micromorts/day.

Assuming a 1% mortality risk, being infected with Covid-19 is 10,000 micromorts.

But what about the risk of catching Covid in the first place?

The microCOVID project: 1 microCOVID = a one-in-a-million chance of getting COVID.

From the white paper:

For example, if you live in a region where about 1 in 1,000 people currently has COVID, then you could calculate based on studies of other indoor interactions … that meeting a friend for coffee indoors has about a 1 in 17,000 chance of giving you COVID. Such small numbers are hard to think about, so we can use microCOVIDs instead. Your coffee date would be about 60 microCOVIDs. …

One benefit of using microCOVIDs is that you can straightforwardly add up microCOVIDs to estimate your risk over longer periods of time.

There’s a calculator for regular activities (try it!) from which I can see that

  • going out to buy groceries is 20 microCOVIDs
  • having a small party, indoors, with no masks is 3,000 microCOVIDs
  • a 30 minute commute on the train is 100-200 microCOVIDs

The calculator takes into account the virus prevalence where you live.

So I might decide that I have a risk-tolerance of 10,000 microCOVIDs per year (i.e. a 1% chance of contracting Covid per year). That is, I really don’t want to get Covid, but I’m also not prepared to never, ever leave the house.

That gives me a budget of a little under 200 microCOVIDs per week. And I can measure my activities against that.

(I’m not sure, from the calculator, how to account for household risk: do we have this budget between us, or each?)

I find these kind of calculators useful to educate my intuition.

For example, an outdoor restaurant is only 30 microCOVIDs vs 500 indoors. A significant difference! Especially against my weekly budget of 200. Commuting via public transport is out if I want to do anything else. Useful to know.

Back in May, I was speculating about realtime, hyperlocal pandemic forecasts:

Maybe your phone could track your location and give you a live exposure number over the day, like a badge? It’s 2pm and you’re at 40 co-rads today. We recommend you leave before rush hour and take this 20 co-rad route home, also WASH YOUR HANDS.

And this microCOVID calculator is the foundation of this. If you could automatically plug in realtime regional prevalence figures, you’d be able to make a risk assessment like short journey on the bus vs slow journey walking.

The framing of the microCOVID project gives me pause: it’s about personal risk.

But there are three distinct reasons why I follow the government lockdown advice:

  • risk to my personal/household, which is the focus of the microCOVID project
  • risk to others I might meet. I don’t want to accidentally infect my mum, for example
  • society a.k.a. public health – we beat this pandemic through collective action, by bringing down Re, the effective reproduction number.

Re isn’t a measure of prevalence. It’s a measure of how easily the virus spreads. It spreads more easily when people are meeting lots of other people without masks; it spreads less easily when social contact is reduced.

If Re is below 1, prevalence decreases; above 1, and it goes up.

I think of society as a whole having an Re budget. The figure I heard, at the beginning of lockdown, was that we needed to reduce in-person social interactions by 75%. I assume that social interactions are the key factor in Re (or at least, were believed to be at the time). Other factors might be: % people wearing masks; proportion of unique vs repeat people encountered.

There are some people we need to spend against the Re budget: health workers, anyone involved in the grocery supply chain, and other key workers. I am happy to reduce my in-person interactions by, say, 90% if that means that key workers need to reduce by only 60%.

Is there a translation between microCOVIDs and Re? I don’t know. Maybe +100 microCOVIDs/week/person in a region with a population density of such-and-such contributes +0.1 to Re.

I’d love to have that connection between personal activity and social good.

This pandemic has given us a whole new vocabulary around virality that wasn’t commonplace before. I wonder how we’ll use it in the future?

How many micro-RTs does one of my tweets have, where 1 micro-RT is a one in a million chance of it going viral?

Can we measure the effective reproduction rate of a given social media influencer?

And so on.

I mentioned skydiving at the top of this post (8 micromorts). Of course, there are also externalities. And that reminds me of something else I read:

In the UK, skydiving is a common way to raise money for charity.


The injury rate in charity-parachutists was 11% at an average cost of 3751 Pounds per casualty. Sixty-three percent of casualties who were charity-parachutists required hospital admission, representing a serious injury rate of 7%, at an average cost of 5781 Pounds per patient. The amount raised per person for charity was 30 Pounds. Each pound raised for charity cost the NHS 13.75 Pounds in return.

Conclusion: Parachuting for charity costs more money than it raises.

Here’s the paper:

Lee CT, Williams P, Hadden WA. Parachuting for charity: is it worth the money? A 5-year audit of parachute injuries in Tayside and the cost to the NHS. Injury. 1999;30(4):283-287.

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