Figuring out how many deaths air pollution causes is tricky business. The numbers repeatedly quoted in the media mask a great deal of complexity, uncertainty and misunderstanding.
First, it’s important to understand that nobody drops dead from walking down a polluted street. Rather, air pollution aggravates other things that are likely to kill you (see “How does pollution affect my health?”). In other words, it won’t kill you but it could cut months off your life. The UK Committee on the Medical Effects of Air Pollutants (COMEAP) estimates that PM2.5 from industrial sources, released at 2008 levels, would shorten the average person’s lifespan by six months. By adding this lost life together, the committee worked out that outdoor air pollution would cause the equivalent of almost 29,000 deaths. COMEAP stressed that PM2.5 was not killing that specific number of people but was instead shortening the lives of many more. Yet even reputable media outlets frequently report that air pollution kills 29,000 per year in the UK.
Read more: Cutting through the smog on air pollution
The bad news on bad air seems to get worse by the day, but all is not as it seems over Western cities. We take a look at the numbers
The problems with the statistics go deeper still. COMEAP’s calculations were based on a number of assumptions, about the relationship between exposure and early death and whether the health effects of PM2.5 are the same regardless of where the particles come from. “I think everybody who studies this believes there are differences,” says Michael Brauer of the University of British Columbia, who contributed to the HIE/IHME report. “But it’s been hard to consistently demonstrate what they are.”
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As a result of these and other uncertainties, COMEAP says the number of attributable deaths for 2008 was probably between 5000 and 55,000, adding that there was still a one in four chance this estimate was wrong. The range is wide, admits Jon Ayres, emeritus professor at the University of Birmingham, UK, and chair of COMEAP until 2011. “It reflects different views of the data.”
Then there are disagreements over what constitutes a safe level of exposure. COMEAP assumes no safe level for PM2.5, but the WHO assumes that being exposed to around 7 µg per cubic metre or less is acceptable. As a result, they put the number of attributable deaths in the UK at 16,400, based on 2012 levels.
A further complication comes from the fact that the health effects of PM2.5 and nitrogen dioxide – the two most harmful pollutants – probably overlap, but how is still not clear. Two pollutants might not have cumulative effects, says Frank Kelly of King’s College London, the current chair of COMEAP. “If you get a biological response to one pollutant, the pathway it triggers may not respond in the same way when a second pollutant arrives.”
“My personal belief is that the NO2 epidemiology is largely a signal due to ultra-fine particles, which would already have been largely counted within PM2.5,” says Ayres. Accounting for these overlaps has led to new estimates of deaths attributable to the combined effects of PM2.5 and NO2. The Royal College of Physicians and the Royal College of Paediatrics and Child Health put the number at 30,000 to 50,000 (hence, the oft-quoted “40,000 deaths per year”); the UK government says the numbers are in the range of 44,750 to 52,300.
For Brauer, quibbling about specific numbers misses the point. “The actual number makes for a nice headline but it’s probably not that important,” he says. “What is important is that we can see how air pollution compares to other major risks such as smoking, and so can prioritise policy and funding.” (see graphic, above)
Q. How does pollution affect my health?
We still have a lot to learn about how outdoor air pollution causes ill health, not least because its effects on our bodies are likely to be multiple, complex and interdependent. Studies suggest PM2.5, NO2 and ozone mess with oxidation reactions in the lungs and elsewhere in the body. This triggers inflammation and can cause tissue damage.
Most studies look for correlations between increased exposure to pollution and the prevalence of diseases. For instance, a 2014 study that followed some 100,000 people in five European countries for more than 11 years found that a 5µg/m3 increase in annual average PM2.5 exposure was associated with a 13 per cent increase in either heart attacks or unstable angina. Another study found the same increases in PM2.5 were associated with an 18 per cent increase in the risk of developing lung cancer.
There is also a well-established association between pollution and respiratory and pulmonary diseases, and stroke. A study published in January found that people living within 50 metres of a major road were 7 per cent more likely to develop dementia than those who lived 300 metres or more away. Other research has linked air pollution with diabetes, kidney diseases, Alzheimer’s, premature births and mental illness.
There is also growing evidence of effects on child development. A 2004 study found that 18-year-old Californians who had been exposed to 28µg/m3 of PM2.5 per year for eight years, on average, were 4.9 times more likely to have reduced lung function than those exposed to an average of 5µg/m3. Researchers found delayed cognitive development in children in Barcelona who went to school in polluted areas.
A new US-UK-Chinese collaboration, led by Frank Kelly of King’s College London, should offer a more precise understanding of the links between pollution and ill health. The study will give 120 Beijing residents and 120 people living in an outlying village portable pollutant monitors. It will then compare exposures with health data taken from urine and blood samples to help understand what pollution does to our bodies.
“Moving from just estimating people’s exposure to actually measuring it and linking that to biological response markers is a major step forward,” says Kelly.
How many UK deaths?
• Estimates of the annual deaths attributable to air pollution vary wildly
In 2010, the Committee on the Medical Effects of Air Pollutants estimated 29,000 annual UK deaths were attributable to particulates less than 2.5 µm across (PM2.5), assuming no safe limit
• Uncertainties meant there was a 75 per cent chance the number could be anything between 5000 and 55,000 deaths
• In 2012, the World Health Organization put the figure at 16,400, assuming a safe limit of 7 µg/m3
• 44,750-52,500 is the UK government estimate of deaths from PM2.5 and NO2
• 40,000 is often quoted in the press as the number of deaths from air pollution. It comes from the UK Royal College of Physicians and Royal College of Paediatrics and Child Health, which estimate that PM2.5 and NO2 cause between 30,000 and 50,000 deaths a year
This article appeared in print under the headline “Cutting through the smog”
