Is skiing more risky than decorating your house? To understand how to compare them you need to understand how to measure risk, says our resident statistician.

We live in a world of risk. We are told that a good diet and lifestyle cuts the risk of cancer and heart disease, that driving too fast risks lives, and that climate change is a risk to future generations. But how do you weigh up all these apparent threats and decide which ones not to lose sleep over?

Before we begin to answer these questions, we need to establish exactly what we mean by risk. There are two sorts of risks that affect your life: acute risks (which could kill you on the spot), and chronic risks (which accumulate trouble for the future). Of course, the same hazard might have both effects: if you down too much alcohol you might fall over and bang your head, or it might progressively damage your liver and send you to an early grave.

But let us start with acute risks: the sort where you start the day all fine and healthy, but finish the day dead because of an accident or violence. We are all faced with some acute risk in our daily lives, as even if you stay in bed all day an aeroplane might crash into your house. So to compare them we need a nice "friendly" unit of deadly risk, just as we have the second for time and the amp for electrical current.

Life is risky

Thanks to the efforts of researchers at Stanford University in the 1970s, we have just such a unit: the micromort - defined as a one-in-a-million chance of sudden death. This might sound a bit comical, but it is deadly serious as it means that we can translate small risks into whole numbers that can be immediately compared. For example, the risk of death from a general anaesthetic in an emergency operation is quoted as 1 in 100,000, meaning that we can expect 10 deaths in every million operations from the anaesthetic alone. This is described as 10 micromorts per operation, and we can compare this figure with other average risks, such as skydiving (also 10 micromorts), or riding a motorcycle in the UK (yoou have to go around 60 miles to rack up 10 micromorts).

The micromort is also roughly the average acute risk we face each day simply by being alive. To calculate this, we have to count how many people die from external causes – accidents or violence – each year. According to the UK Office of National Statistics, around 17,201 people died of external causes in England and Wales in 2010. This was out of a population of approximately 54 million, so it corresponds to an average risk of 320 micromorts a year for each person, or roughly one a day.

So a micromort can be seen as the average "ration" of lethal risk that people are exposed to daily, and which we should therefore not be unduly worried about. Of course, this average figure disguises huge variability: for every cautious adult and protected child there is a gung-ho teenage boy out for excitement. And we can spend our daily micromort in many different ways, according to the stats: driving 250 miles (400km), cycling 20 miles (32km), going skiing, taking ecstasy or decorating at home on a high ladder.

Bonus life

Comparing chronic risks is a bit trickier, as by definition the consequences do not show up for some time. Think of the increased chances of getting lung cancer or heart disease after a lifetime of smoking, for example. And, of course, some people do not seem to be affected at all – everyone seems to have an elderly relation who drank and smoked heavily all their lives, and “it never did them any harm”.

There is no guarantee that chronic risks will shorten your life, but they will reduce the number of years you can expect to live. Life expectancy for a man aged 22 in the UK is currently 79 years, which is an extra 57 years, 20,800 days, or 500,000 hours. That means that a young man has, on average, one million half-hours ahead of him, the same as a 26 year-old woman. Thirty minutes of your life expectancy is referred to as a microlife. Or, to put it another way, every time that young man or woman watches their favourite soap opera, it equates to one millionth of their life gone.

If you have unhealthy habits, it costs you microlives. If you smoke, each cigarette reduces your life expectancy on average by around 15 minutes, so two cigarettes loses you a microlife. With a 20-a-day habit you are losing 10 microlives, or five hours, so you could say you are accelerating towards your death at 29 hours for each day you live. Fortunately, improvements in healthcare and our environment mean that life expectancy has for decades been increasing by around three months for each year that passes: it is as if each day we all get a bonus of six hours of extra life – or 12 microlives – simply by virtue of living in the modern world. But a young adult who consistently drinks above minimal amounts will pay for it in microlives – around one for each seven units of alcohol. And for every extra 5kg you are overweight, it will cost you around one microlife a day.

There is one major difference between micromorts and microlives, however. If you survive your trip on the motorbike, then your micromort gets forgotten and you start the next day with a clean slate. But if you smoke and drink all day, and live on a fast-food diet, then your microlives will accumulate from day to day. It is like a lottery where the tickets you buy every day are kept in the running and so your chances of winning increase every day – except that this is a contest that you will be very keen not to win.

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