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.
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”.