In recent years, a large body of epidemiological evidence has linked air pollution and cardiovascular disease. Studies have focused on the acute effects of pollution episodes and on the long-term association between air pollution and mortality.
The latest study, by researchers at the University of Southern California, breaks new ground by exploring the role of pollution in the pre-clinical development of heart disease. It focuses on atherosclerosis - a condition in which fatty deposits cause artery walls to thicken and harden.
The researchers examined data from two clinical trials covering 798 individuals aged over 40. Data on the thickness of the two inner layers of the carotid artery, which feeds the head and neck, had already been gathered for other purposes.
The scientists investigated the association with levels of fine particles (PM2.5) using data from 23 monitoring stations in the Los Angeles basin. Long-term mean concentrations at each participant's home address were estimated using a geographic information system. The data were adjusted to account for other factors such as diet, income, medication, blood pressure and physical activity.
Overall, the most exposed individuals experienced about 8% more thickening of the arteries than the least exposed. The annual mean concentration of PM2.5 ranged from 5 to 27µm3.
Each 10µg/m3 increase in PM2.5 levels was associated with a 3.9-4.3% thickening of the artery walls. However, the association was stronger in older subjects, women and those who had never smoked - rising to a 15.7% increase in arterial thickening in women aged over 60.
The findings are supported by animal studies showing that inhalation of fine particles promotes oxidative lung damage, and inflammatory responses and lesions to artery walls.
The researchers acknowledge limitations to the study, including the relatively low number of participants, the focus on a relatively healthy population and the adequacy of home addresses as a proxy for exposure to pollution. They call for further investigations - but stress that their finding is "of high public health relevance" given "the leading role of heart disease as a cause of death in most westernised countries and the growing contribution in developing countries."
In the UK, the Department of Health's Committee on the Medical Effects of Air Pollutants (COMEAP) is expected to publish a definitive report on air pollution and heart disease in the spring.
The report examined the residence and migration of children who died of fatal tumours between 1966 and 1980. He compared the postcodes with maps made available in the national atmospheric emissions inventory for 2001 to identify "hotspots" - particularly large combustion plant and sources of industrial chemicals.
Professor Knox claims to have demonstrated associations - which are "probably causal" - between cancer rates within 1 kilometre of a hotspot and emissions of 1,3-butadiene, dioxins and the PAH benz[a]pyrene. Other elevated risks were found for carbon monoxide, fine particles, nitrogen oxides, benzene and volatile organic compounds (VOCs).
Professor Knox's paper will be considered as part of a review of childhood cancer, set up last November by the Department of Health's Committee on Carcinogenicity. The Committee has already considered an initial review of the literature on childhood leukaemia which found a "suggestive" association with exposure to benzene from petrol stations, garages and roads.
In a parliamentary answer on 24 January, however, Environment Minister Alun Michael said the Committee would need to consider "the validity of the assessment of exposure for children who died in 1966-80 based on emission maps for the year 2001. The magnitude and location of pollution sources is likely to have changed dramatically between these two periods."