Weather conditions influence air pollution via pollutant (or pollutant precursor) transport and/or formation. Weather conditions also can influence biogenic (e.g., pollen production) and anthropogenic (e.g., as a result of increased energy demand) air pollutant emissions. Exposure to air pollutants can have many serious health effects, especially following severe pollution episodes. Studies that are relevant to climate change and air pollution can be divided into two categories: those that estimate the combined impact of weather and air pollutants on health outcomes and those that estimate future air pollution levels. Climate change may increase the concentration of ground-level ozone, but the magnitude of the effect is uncertain (Patz et al., 2000). For other pollutants, the effects of climate change and/or weather are less well studied.

Current air pollution problems are greatest in developing country cities. For example, nearly 40,000 people die prematurely every year in India because of outdoor air pollution (World Bank, 1997). Air quality also is one of the main concerns for environmental health in developed countries (Bertollini et al., 1996; COMEAP, 1998).

Radon is an inert radioactive gas. The rate at which it is emitted from the ground is sensitive to temperature (United Nations, 1982). High indoor exposures are associated with an increased risk of lung cancer (IARC, 1988). There is some evidence from modeling experiments that climate warming may increase radon concentrations in the lower atmosphere (Cuculeanu and Iorgulescu, 1994).

9.6.1.1. Effects of Air Pollution, Season, and Weather on Health

The six standard air pollutants that have been extensively studied in urban populations are sulfur dioxide (SO₂), ozone (O₃), nitrogen dioxide (NO₂), carbon monoxide (CO), lead, and particulates. The impact of some air pollutants on health is more evident during the summer or during high temperatures (Bates and Sizto, 1987; Bates et al., 1990; Castellsague et al., 1995; Bobak and Roberts, 1997; Katsouyanni et al., 1997; Spix et al., 1998; de Diego Damia et al., 1999; Hajat et al., 1999). For example, the relationship between SO2 and total and cardiovascular mortality in Valencia (Ballester et al., 1996) and Barcelona, Spain (Sunyer et al., 1996), and Rome, Italy (Michelozzi et al., 1998), was found to be stronger during hot periods than during winter. However, Moolgavkar et al. (1995) conclude that, in Philadelphia, SO2 had the strongest health effects in spring, autumn, and winter. Increases in daily mortality and morbidity (indicated by hospital admissions) are associated with high ozone levels on hot days in many cities (e.g., Moolgavkar et al., 1995; Sunyer et al., 1996; Touloumi et al., 1997).

High temperatures also have acute effects on mortality (see Section 9.4.1). Some studies have found evidence of an interaction between the effects of ozone and the effects of higher temperatures (e.g., Katsouyanni et al., 1993; Sartor et al., 1995). Other studies addressing the combined effects of weather and particulate air pollution have not found evidence of such an interaction (e.g., Samet et al., 1998). Correlations between climate and site-specific air quality variables must be further evaluated and, in some instances, need to include temperature, pollution, and interaction terms in regression models.

Climate change is expected to increase the risk of forest and rangeland fires (see Section 5.6.2.2.1). Haze-type air pollution therefore is a potential impact of climate change on health. Majors fires in 1997 in southeast Asia and the Americas were associated with increases in respiratory and eye symptoms (Brauer, 1999; WHO, 1999b). In Malaysia, a two- to three-fold increase in outpatient visits for respiratory disease and a 14% decrease in lung function in school children were reported. In Alta Floresta, Brazil, there was a 20-fold increase in outpatient visits for respiratory disease. In 1998, fires in Florida were linked to significant increases in emergency department visits for asthma (91%), bronchitis (132%), and chest pain (37%) (CDC, 1999). However, a study of 1994 bushfires in western Sydney showed no increase in asthma admissions to emergency departments (Smith et al., 1996).

9.6.1.2. Future Changes in Air Quality

Weather has a major influence on the dispersal and ambient concentrations of air pollutants. Large high-pressure systems often create an inversion of the normal temperature profile, trapping pollutants in the shallow boundary layer at the Earth's surface. It is difficult to predict the impact of climate change on local urban climatology and, therefore, on average local air pollution concentrations. However, any increase in anticyclonic conditions in summer would tend to increase air pollution concentrations in cities (Hulme and Jenkins, 1998).

Formation and destruction of ozone is accelerated by increases in temperature and ultraviolet radiation. Existing air quality models have been used to examine the effect of climate change on ozone concentrations (e.g., Morris et al., 1989; Penner et al., 1989; Morris et al., 1995; Sillman and Samson, 1995). The models indicate that decreases in stratospheric ozone and elevated temperature increase ground-level ozone concentration. An increase in occurrence of hot days could increase biogenic and anthropogenic emissions of volatile organic compounds (e.g., from increased evaporative emissions from fuel-injected automobiles) (Sillman and Samson, 1995). These studies of the impact of climate change on air quality must be considered indicative but by no means definitive. Important local weather factors may not be adequately represented in these models.

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