Climate change and urban air pollution

Air pollution in urban areas is a major concern for environmental health in Europe, especially the effects of particulates (83). The dissemination and concentration of air pollutants (both particles and gases) depend on the prevailing weather conditions - air currents, temperature variation, humidity and precipitation. Large, slowly moving anticyclones may cover an area for several days, or a week or more, and give rise to conditions that readily allow pollutants to accumulate. Predicting the impact of climate change on average local air pollution concentrations is therefore very difficult. However, forecasts of climate change in the United Kingdom indicate an increase in anticyclonic conditions in summer (with a decrease in anti-cyclonic conditions in winter and spring), which would tend to increase air pollution concentrations in cities (43).

Secondary air pollutants, such as ozone, form through photochemical reaction. The rate of reaction therefore increases at higher temperatures and increased levels of sunlight. Other things being equal, climate change is expected to increase the average ambient concentrations of ozone and to increase the frequency of ozone pollution episodes.

The relationship between ambient temperature and ozone concentration is not linear. The US Environmental Protection Agency (14) estimated, based on data from the United States, that a 4 °C rise in mean annual temperature would cause a 10% increase in peak ozone concentrations. This would double the number of cities in the United States that currently exceed the national air quality standards for this pollutant. The model assumed that precursor vehicle emissions and other weather factors (such as the frequency of anticy-clonic conditions) were unchanged. Increased ground-level ultraviolet radiation from stratospheric ozone depletion would also increase the concentration of tropospheric ozone.

Particulate matter and acid aerosols seem to be the main agents of acute effects of air pollutants on daily mortality. Ozone also adversely affects all-cause mortality in European cities (84). High temperatures also have acute effects on mortality, as discussed above. In most epidemiological studies of air pollution effects, temperature is treated as a confounder. Few studies have addressed the need to quantify and describe the separate effects on mortality and morbidity of air pollution and thermal stress. There is some evidence of a physiological synergistic effect between high temperatures and pollutants (61).

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