Exposure to ambient air pollution is increasingly recognised as a risk factor for cardiovascular morbidity and mortality. Associations between exposure to combustion-derived fine particulate air pollution and cardiovascular mortality are consistently demonstrated in a number of large-scale epidemiological studies.1 These reported associations have even withstood legal challenge in the USA, where industry representatives attempted to sue the US Environment Protection Agency after this evidence was used to impose strict air quality standards. An independent review of the evidence by the Health Effects Institute reaffirmed these associations and the case was lost.2 3

The World Health Organization estimates that indoor air pollution from the combustion of solid fuel may be responsible for up to 2.4 million deaths worldwide each year, representing the fourth leading cause of mortality in developing countries. In addition, urban air pollution may be responsible for a further 800 000 premature deaths each year.4 Taken together, air pollution exposure is thought to be responsible for approximately 5% of deaths worldwide each year and therefore represents a significant public health concern.

Isolating the risk attributable to a single factor in epidemiological studies is challenging due to the myriad potential mediators and confounders. The effect of air pollution exposure can be modified by many factors, including temperature, geographical area and social deprivation. Similarly, investigations of the effects attributable to meteorological variables are also susceptible to confounding by exposure to ambient air pollutants, seasonal effects and infectious disease epidemics. Effects of temperature may be further modified in areas of extreme weather whereby inhabitants modify their behaviour by, for example, wearing warm clothing.

In order to tease out the effects attributable to …