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Smoke and mirrors?

Polluted air is a major contributor to the global burden of ill-health. A comprehensive WHO study (Global Burden of Disease) attributes nearly 6 million deaths to it in 2010, second only to overall diet and high blood pressure. It kills more people than smoking, alcohol or drugs. So, when we see headlines such as UK air pollution ‘linked to 40,000 early deaths a year’, it is not something we should dismiss lightly.

This headline comes from a report about a new study – Every breath we take: the lifelong impact of air pollution – recently published jointly by the Royal College of Physicians and the Royal College of Paediatrics and Child Health. These are highly respected professional bodies, but they base their findings on published literature and the figures quoted could themselves be subjected to bias.

The figure reported is for deaths attributed to outdoor pollution in the form of particulates and nitrogen dioxide (NO2). But the study also deals with indoor pollution, saying "There is now good awareness of the risks from badly maintained gas appliances, radioactive radon gas and second-hand tobacco smoke, but indoors we can also be exposed to NO2 from gas cooking and solvents that slowly seep from plastics, paints and furnishings. The lemon-and-pine scents that we use to make our homes smell fresh can react chemically to generate air pollutants, and ozone-based air fresheners can also cause indoor air pollution."

Radon is a very specific problem, which affects certain parts of the country because of the rock on which houses are built; the greatest problems are in the far South West. Being a radioactive gas, radon can cause cancer when breathed in over a period of time, and is believed to result in about 1,100 lung cancer deaths each year. Second-hand smoke has been a much greater problem, and was still estimated to cause 2,800 deaths annually in 2010. However, these two indoor pollutants – the only ones for which mortality figures are quoted – appear to cause only about 10% of the number of deaths as outdoor air pollution.

To look at the figures in a different way, if we take the current world population to be about 7.4 billion and that of the UK roughly 64 million, we can estimate an annual death rate of around 60,000 in the UK on a pro rata basis. But this assumes that the risk from air pollution is the same in a prosperous western European country as the average for the entire world, the great majority of whose people live in considerably worse conditions than ours. Even so, the 40,000 headline figure seems to suggest that this island is still a pretty polluted place in global terms.

This seems surprising, given the end of coal fires brought about by the Clean Air Act, for example. During the lifetimes of many of us, we have seen London and other major cities reveal their true colours once the layer of black soot had been removed. Also, despite the justified bad publicity about diesel engine emissions recently, both petrol and diesel engines have become much cleaner and more efficient and the removal of lead from petrol has removed a major source of one very serious pollutant.

The other anomaly is that the problems from outdoor air pollution appear to be much greater than for indoor pollution in the UK, in contrast to the global situation.

To go back to the report, “A WHO global burden of disease analysis identified household air pollution as an extremely important risk factor accounting for an estimated 4.3 million deaths worldwide in 2012, mostly in low- and middle-income countries and including some 99,000 in Europe. Around 60% of these deaths are due to stroke (34%) and ischaemic heart disease (26%), with the remainder accounted for by COPD (22%), acute lower respiratory disease (12%) and lung cancer (6%)”.

That’s about 70% (4.3 million out of a total of 6 million) due to indoor air pollution. If we consider simply the deaths due to outdoor pollution – estimated at 1.7 million globally – the UK’s pro rata share would be less than 15,000.  Knowing that urbanisation in developing countries is proceeding rapidly and that Beijing and Delhi, to name but two, suffer from horrendous pollution at present, it seems highly likely that the UK figure would be well below that. So, what is the reality? How much of a problem is external air pollution in a rich modern society?

Given these contradictory figures, we really can’t say, but there are two main possible answers: either that there are many more deaths globally from air pollution than is recognised or that the risks in Western Europe are overstated. There again, another possible explanation is that life expectancy in developing countries is significantly lower, so that many people die from other causes before the heart or lung diseases brought on or exacerbated by pollution (and that typically kill older people) can take their toll.

Whatever the reason, we can see the need to take a broader view and put the findings in perspective. The report from the Royal Colleges reports, for example, the total estimated deaths due to PM2.5 (very fine particulates) as 28,861, or 75 per 100,000 people over 30. But to put it another way, the number of days of life lost is, on average, 194 for women and 182 for men. Would people be less worried about dying six months earlier than statistics would suggest towards the end of a long life than about the headline figure of total deaths?

These discrepancies also point towards a real problem with epidemiological studies: attribution of death or disease to a single cause when there may be a number of contributory factors. In our safe, prosperous European countries we worry increasingly about risks which we once took for granted. That doesn’t mean that we should ignore urban air quality, neither should we stop opening windows to get fresh air into houses, but the real problems globally are in the developing world: indoor cooking fires and highly polluted cities.

Finally, the report inevitably includes a reference to the modelled impacts of climate change on health: “If we act now to reduce greenhouse gas emissions to target levels by 2050, we can have a real impact. An analysis for the European Commission suggests that, each year in the UK, this would prevent the following impacts related to local and regional air pollutant exposure: 5,700 deaths, 1,600 hospital admissions for lung and heart problems and 2,400 new cases of bronchitis.”

Reliance on the output of models which have failed to predict the pattern of global temperatures over the past twenty years doesn’t exactly inspire confidence. Modelling of all sorts should be seen as giving a picture of what might happen given a particular set of circumstances, and should not be confused with evidence.