The 29,000 air pollution deaths that aren't quite what they seem

When is a death not really a death?

It sounds like a morbid riddle, but it's actually part of a serious matter of debate when it comes to measuring the health effects of dangers such as pollution.

That's because long-term exposure to air pollution is only one of several factors that contribute towards a death. Its effects can include - for instance - the exacerbation of asthma, lung cancer and chronic cardiovascular disease.

So simply putting a number on how many people are killed can lead to misunderstanding.

Several papers - including the front page of the Metro today - ran with the story that 29,000 deaths in Britain are attributed to air pollution every year.

As we discussed in more depth last week, this is by and large an accurate description of what the research says, but it isn't actually 29,000 individual people who've been counted as having died. The Committee on the Medical Effects of Air Pollutants said in 2012 that this figure - called the number of 'attributable deaths' - isn't what it looks like:

"it should not be interpreted as the number of individuals whose length of life has been shortened by air pollution, as this would only be true if air pollution were the sole cause of deaths. Rather, it is an estimate of the total mortality effect in the local population"

In other words, air pollution is thought to adversely affect many more than just 29,000 people (researchers have suggested it could make a smaller contribution to the deaths of some 200,000 people, for instance). But 29,000 is the best estimate of the equivalent number of deaths it causes (if you wanted to compare it to, say, the number of people killed in road traffic accidents, you'd use this figure).

The latest figures are being reported after Public Health England recently took the 2010 estimates and calculated how many 'deaths' this means on a local level.