British scientists say that the ultra-fine particles - measured in billionths of a metre - which are too small to settle or be dispersed by rain, may be the most important and dangerous aspect of pollution.
The particles can drift for miles, and accumulate inside most buildings. Vehicles are the major source of the particles in urban air, particularly diesel engines.
During a period of high air pollution, people breathe in millions of these acidic particles which penetrate into the microscopic air sacs of the lungs. Scavenging white blood cells, known as macrophages, are "overwhelmed" by the particles. They release astream of chemicals that set off an inflammatory action in the lungs and increase the stickiness of the blood so it is more likely to clot.
Professor Anthony Seaton, of Aberdeen University, and colleagues at Edinburgh University suggest that the exacerbation of lung disease after an episode of pollution, and the rise in deaths from lung and heart problems - an unexplained phenomenon associated with pollution - is due to the tiny particles.
Unconfirmed reports suggest that 160 excess deaths were due to the severe freezing smog of December 1991; the London smogs of the 1940s and 1950s were also linked with excess deaths from heart and lung disease.
According to the report in tomorrow's edition of the Lancet, studies in 14 different locations show that overall daily deaths, particularly from heart and lung disease, increase as the concentration of small particles in the air rises. Older people and smokers are most susceptible.
Corresponding studies in industrial centres do not show a similar increase, even in workers exposed to high concentrations of dust, the scientists say. The particles present in industrial dust and pollution tend to be larger and settle within hours. In addition, macrophages can deal with these particles and limit the damage to lung tissue. Further research is necessary to test the hypothesis, the scientists conclude.Reuse content