Figuring out how many people die because of pollution is a tricky business. Widely quoted numbers vary enormously and mask a great deal of complexity, uncertainty and misunderstanding (see “How many deaths“).
First, it’s important to realise that nobody drops dead from walking down a polluted street. Rather, air pollution aggravates other things that are likely to kill you, cutting months off your life. The UK Committee on the Medical Effects of Air Pollutants (COMEAP) estimates that anthropogenic PM2.5, released at 2008 levels, would shorten the average person’s lifespan by six months. By totting up all this lost life, the group worked out that outdoor air pollution would cause the equivalent of almost 29,000 deaths. COMEAP stressed that PM2.5 was shortening the lives of many more people rather than causing that number of deaths. Yet the media often reports that air pollution kills 29,000 a year in the UK.
COMEAP’s calculations were based on a number of assumptions. One question is whether all PM2.5 has the same health effects regardless of its source. “I think everybody who studies this believes there are differences,” says Michael Brauer of the University of British Columbia. “But it’s been hard to consistently demonstrate what they are.”
Advertisement
A further complication comes from the probable overlap between the effects of PM2.5 and NO2 – the two most harmful pollutants. “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 Jon Ayres at the University of Birmingham, UK, who was chair of COMEAP until 2011.
International estimates of mortality due to air pollution have also varied dramatically, but for Brauer, quibbling over 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.”
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”