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What’s most likely to kill you: understanding the micromort

It’s not a pleasant thing to admit, but the world is full of death. According to estimates, around 150,000 people die each day around the world, and from a huge variety of causes – natural, accidental, malicious. The likelihood of certain forms of death is far higher than others, but how can we communicate this? How can science assign values to risk and likelihood of death, and what do they even mean? Let’s dig into the micromort.

Most people are quite bad at estimating death risk probabilities, and thinking about death in terms of percentages is the wrong way to go in most occasions. Imagine you were undergoing an operation with an 85% survival rate – that seems a good number, but a doctor performing just that operation would have more than 5,000 deaths a year to contend with. Routine surgical procedures with risks greater than 5% are classified high risk, and if you did something with a 99% chance of survival every day, the maths suggest you’d be dead before the year was out.

In 1979, Ronald A. Howard realised that this wasn’t the right way to think about risk, and so he drew on the microprobability – a one-in-a-million chance of some event – to create the micromort. A micromort is a unit of risk, which denotes a one-in-a-million chance of death, and it can be used to estimate the riskiness of various day-to-day activities. This is such a small number, roughly akin to tossing a coin 20 times and getting heads on all 20 throws. Not impossible, but not likely.

A micromort is a unit of risk, which denotes a one-in-a-million chance of death

In many cases, micromorts for individual activities are defined through analysing existing data, seeing how many people died of a particular cause and extrapolating that out. And this can be done with more than just activities – everything carries a risk of death. Because it’s unlikely that people our age are going to die, being alive for a day at age 20 carries a risk of one micromort – if you make it to 90, that risk factor increases to 400 micromorts. Grim, perhaps, but clearly intuitive. Simply living in England or Wales increases your risk of dying from any cause by 24 micromorts per day. Even getting out of bed in the morning carries a weight of 2.4 micromorts, while sitting on a chair is worth 1.3 micromorts.

Picturing micromorts helps us dispel some of our natural concerns about danger. It’s often said that the drive to the airport is more dangerous than the flight itself, and we can use the data to confirm that – driving a car for 248.5 miles increases your risk by a micromort, whereas you’d have to travel more than 6,200 miles by plane to have the same effect. It also indicates how risky some activities are – skydiving increases your risk of dying by eight to nine micromorts, scuba diving increases the total by around ten micromorts per dive, and ascending Mt Everest carries a phenomenal risk factor of 40,000 micromorts per ascent.

We should differentiate between the two types of risks, because micromorts are best used to measure the size of acute risk – things that kill you immediately. A micromort is a good measurement for, say, the risk of dying in a car crash. But there are also chronic risks – lifestyle habits, exposure to pollution, etc. – which don’t kill straight away, but certainly reduce your life expectancy. Howard tried to quantify these chronic risks in his 1979 work, attributing an additional one micromort to half a litre of wine or smoking 1.4 cigarettes. But, in many cases, it’s better to use the microlife, a unit discussed by David Spiegelhalter and Alejandro Leiva which represents a half-hour change to life expectancy.

This may seem a little terrifying, but it’s simply how life works – everything we do comes with some form of risk, and many of these risks as so small as to barely ever matter. And it’s important to stress that micromorts are simply a measure of the magnitude of a risk based on population-level data, not a measure of your own personal risk factor, so be weary of applying any of these numbers directly. I anticipate that my personal risk factor for Everest, say, is the best part of zero – I can’t picture myself ever climbing it.

The important takeaway is that everything carries risk, and we need to figure out which risks are worth taking in order to make life worth living. We add to the micromort tally every day doing things, so make sure to spend your time well!

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