Long-term or lifetime exposure to ambient pollutants may also have pathological effects that eventually result in chronic ailments. The investigation of these effects usually requires large studies and, ideally, follow-up investigations over many years; consequently, fewer studies have investigated these types of effects. However, in the past 10 years, several studies have also confirmed the existence of chronic adverse effects of ambient air pollution.
Mortality and life expectancy
Mortality and life expectancy are important markers of lifetime morbidity and therefore play an important role in air pollution research. Studies conducted in Europe, the USA and Canada have confirmed that the overall effects of pollution on mortality are far larger than the fraction attributed to acute exposures. In general, respiratory disease is less often the cause of death than cardiovascular disease, and the two are often combined in the category of cardiopulmonary mortality. Cardiopulmonary mortality was associated with long-term differences in PM and sulfate concentrations between cities in the Harvard Six Cities Study and in the American Cancer Society (ACS) study. Comparison of community-level concentrations of fine PM with death rates 16 years later among more than 500 000 participants in the ACS study showed a 6% increase in cardiopulmonary deaths per 10 μg·m-3 of PM2.5 in models taking a range of other factors into account. In a further analysis of the data from the Los Angeles (USA) area after 18 years of follow-up, modelled PM2.5 concentrations were assigned to each residence. This more accurate assignment of exposure resulted in larger mortality estimates, with cardiopulmonary mortality increasing by 20% per 10 μg·m-3 increase in concentration of PM2.5. While traffic-related pollutants continue to play a dominant role in Europe, other sources of air pollution – including biomass burning or PM during Saharan dust episodes – also result in adverse effects.
Cohort studies in Europe have been able to confirm the relationship between cardiopulmonary death risk and pollution (figure 1). In three European studies, it was possible to analyse the data for respiratory and cardiovascular mortality separately. The results showed that urban air pollution, assessed individually for all participants by modelling traffic emissions of NOx, was associated with overall mortality, mortality from ischaemic heart diseases, respiratory mortality, lung cancer mortality and (weakly) with cerebrovascular mortality. A Dutch cohort study with 20 years of exposure data observed weak associations of traffic density on the nearest main road with cardiopulmonary death. Respiratory deaths were related to NO2, black smoke, traffic density within a radius of 100 m, and living near a main street.