Science: Airborne killer that targets the heart Is the link between asthma attacks and pollution overstated?
Science editor Roger Highfield finds new evidence for the fatal effects of sooty particles.
THIS blistering summer has seen a procession of warnings linking thundery weather and
"poor air quality" with the risk of asthma attacks. This is the disease most of us associate with air pollution, but there is growing evidence that by far the deadliest risks arise from heart attacks and stroke - though no one yet understands why. What is becoming clear is that the killers in the atmosphere may not be noxious gase themselves so much as fine particles. Some experts believe they may be responsible for up to 10,000 deaths each year.
Now the Government has set up a group to investigate the effects on health. Air pollution advisers are to draw up guidelines that will advise on air quality standards for these particles, which are largely produced by diesel engines.
This development marks a new twist to the old story of the health effects of air pollution. In 1661, the diarist John Evelyn linked the rising death rate in London to burning coal. Almost three centuries later, in December 1952, the city was struck by the most lethal smog ever recorded in Britain.
Since then, most attention has been given to the link between respiratory diseases, with health warnings focusing on coughing, wheezing and shortness of breath. But new evidence suggests a link between the levels of particles in the air and cardiovascular disease.The particles of interest are not the black soots generated from burning coal that caused the pea-soupers of the Fifties, but much finer particles responsible for the haze of smogs. They are designated PM10, being less than 10 millionths of a metre across.
With every breath we take, we draw millions of particles deep into our lungs. Owing to their small size, they linger in the atmosphere and can penetrate into the lung's deepest recesses. We take in around 500,000 particles per breath, though this rises as much as 100-fold in polluted conditions.
What is striking is an increasing weight of epidemiological evidence that links not only asthma, but also daily swings in heart disease, with the wax and wane of PM10: as levels of PM10 increase, so the number of heart disease deaths soar a day or so later.
Dramatic evidence linking mortality and PM10 has been found by Dr Douglas Dockery of the Harvard School of Public Health, in a study of six American cities.British research in this field lags behind, though, ironically, some of the key evidence has come by
analysing data from London. Of the 4,000 deaths in the smog of 1952, most were due to heart disease, not respiratory effects.
Dr Joel Schwartz, also at Harvard, said the association is remarkably consistent. A follow-up epidemiological study with Dr Dockery in Philadelphia reveals that there are "more heart disease deaths on high air pollution days than low".
Deaths from respiratory diseases show a larger percentage increase with increasing PM10s; the rise in mortality from heart disease is numerically smaller but it is more significant because heart disease is more common.
Dr Schwartz estimates from annual mortality figures and official recordings of PM10 levels that there were around 10,000 such deaths in Britain last year. "If these associations with mortality prove correct, it has got to be the most serious atmospheric pollutant," he said.Those at greatest risk are the elderly, especially if they are heavy smokers. "They are about to die and their moment of death is being chosen for them," said Prof Anthony Seaton of Aberdeen University, chairman of the Expert Panel on Air Quality Standards. "The evidence is increasingly that particles are the cause."
As recently as 1992, any danger from particulates was discounted by the Department of Health. But the latest research is so striking that the DoH has set up a group to report on the problem to the Committee on Medical Aspects of Air Pollution later this year.
A key witness will be Dr Schwartz. One who is sceptical of his death estimate is Professor Roy Harrison of Birmingham University, chairman of the Quality of Urban Air Review Group. However, though not entirely convinced by Dr Schwartz's assumptions, he is adamant that PM10 is a problem that has not been adequately addressed.
Though we are now exposed to fewer particles than during the height of the smogs, the projected increase in particle emissions from traffic is worrying, Prof Harrison said, given that one third of the PM10 in urban air is from diesel vehicles. "In towns," he said, "diesel is the biggest single source."
The other particulates arise from sources as varied as wind-blown soils and dust, building materials, the nitrogen oxides from petrol vehicles that are oxidised into nitrate particles, and the creation of sulphate particles from sulphur oxide generated by power plants and industry.The evidence against PM10 is epidemiological, that is, the result of statistical studies of health effects on large numbers of people. This is treated with suspicion by toxicologists, who want to understand exactly how the particles can kill: a sceptic may argue, for instance, that the particles could be the by-product of the real culprit.
Dr Schwartz countered this by citing animal studies conducted by the US Environmental Protection Agency. They showed how PM10 particles were more likely to produce inflammation than synthetic particles. In rats with pulmonary hypertension, a heart -lung disease, the PM10 particles proved deadly.
Though it is hard to understand the link with heart disease, it is not so surprising given that blood has to pass through the lungs, Dr Dockery said. There are two fundamentally different possible mechanisms:
Physical - The particles penetrate so deep into the lung that they remain there as a constant irritant, causing inflammation and the production of sticky mucus. When breathing becomes difficult, individuals with heart problems suffer most, sometimes with fatal consequences.While sifting death certificates, Dr Schwartz was struck by how respiratory conditions were more likely to be contributory causes on high pollution days, as one would expect in a smoker or sufferer of heart disease.
Chemical - The particles act as a vector that carries chemical pollutants deep into the lung. Pollutants of concern include acids and metals such as iron, which can catalyse the production of a highly reactive - and thus damaging - form of oxygen called a free radical.
Without a precise mechanism, and thus proof that PM10 is the cause, many will argue against expensive precautionary measures. Adding to this resistance is the prevailing orthodoxy that one of the main sources of the particulates, diesel vehicles, are "green". However, limits are being drawn up by the DoE's Expert Panel on Air Quality Standards. "The target for the next few years is to be the diesel engine," Prof Seaton said. "We are reviewing the problem of small particles and will recommend to government an air quality standard by early next year."
