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Note: for a less technical review see Indoor Air Quality and Ozone Generators
Several major epidemiological studies (supported by the NIEHS and EPA) have had a major impact on our understanding of the adverse health effects associated with exposure to ambient concentrations of SO2, particulate matter, ozone and acid aerosols. These studies have all been conducted by the Harvard School of Public Health and are referred to as the Six Cities Study (Dockery; R01ES01108), the Twenty-Four Cities Study (Speizer; R01ES04595), and the Five Cities Study (Dockery; R01ES06239).
The Six Cities Study explored the associations between air pollution produced by fossil fuel and adverse respiratory health in adults and children between 1974 and 1989. These studies were initiated after the Middle East oil embargo in the early 1970s forced power plants in the U.S.A. to burn more high sulfur coal. Particulate air pollution was associated with increased cough, bronchitis and chest illness among the sampled children. There was some suggestion that lung function was lower in children living in communities with higher particulate levels. The longitudinal follow-up of these children and adults over 12 years showed that defects in lung function persisted into adulthood and potentially put these children at increased risk of developing chronic obstructive pulmonary disease later in life. A recent analysis of the follow-up of the Six Cities Study adult sample showed that the mortality rate in adults increases by about 26% with exposure to inhalable particulate concentrations, especially the fine particulate fraction, such as smoke and soot (produced by industrial and automobile emissions). In a comparison of individuals living in the six U.S. communities, those in the most polluted cities had a 26% greater mortality rate than individuals living in the least polluted cities. This translates into a two-year shorter life for an individual living in the most polluted cities, a significant public health finding.
The findings of increased bronchitic illness in children with fine particles were followed by the "Twenty-Four Cities Study," of 15,000 children in suburban and rural communities in the U.S.A. and Canada. This study was initiated in response to an RFA focused on Acid Aerosols (1987) and designed to test the hypothesis that acid particulate matter produced from fossil- fueled power plants was responsible for the increased respiratory illness and lower lung function in children, which showed the highest acid particle concentration in the U.S. downwind of the major sulfur emissions in the Midwest. Children living in the areas with high aerosol acidity demonstrated more bronchitis and chest illness and lower lung function than children in clean areas. While asthma prevalence has been increasing in the past two decades, both the Six Cities and the Twenty-Four Cities Studies have shown no association between air pollution concentrations and the prevalence of asthma. On the other hand, in both studies, episodes of high air pollution have been associated with exacerbation of existing asthma.
An ongoing study (Five Cities Study) is assessing the degree to which minority and/or economically disadvantaged children are at increased risk for adverse respiratory health effects of ozone, acid aerosols and particulate air pollution. Inner-city poor and minority children appear to be at greater risk of respiratory illness, especially asthma. Previous studies (the Six and Twenty-Four Cities Studies) have demonstrated increased respiratory illness and lower lung function associated with air pollution among white children in suburban and rural communities. This study was designed to evaluate the risks among inner-city minority children for increased exposure to air pollutants (fine particles, acid aerosols and ozone) and the potential for increased sensitivity of inner-city children to air pollution as compared to white suburban children. Preliminary data from this study indicate that there were differences in sensitivity among the specific groups examined. Children living in the inner-city demonstrated a greater response to the same exposure than children in the suburbs. Black children showed a greater response than whites; girls more than boys, and children with hay fever more than those without allergies. All these differences suggest that genetic, socioeconomic, or housing characteristics may contribute to increased sensitivity to air pollution.
In the Spring of 1997, the US EPA proposed, then initiated changes in the National Ambient Air Quality Standards for particulate matter and ozone. The Harvard School of Public Health studies (along with a companion study by Pope, et al, entitled 'Particulate Air Pollution as a Predictor of Mortality in a Prospective Study of U.S. Adults') were the most significant among many that showed the association between particulate air pollution and adverse human health effects, such as mortality, and the aggravation of asthma and other chronic breathing disorders. While these studies are not without controversy, independent scientists from many different institutions have compiled a large body of epidemiological data showing increased mortality and morbidity associated with particulate air pollution in a wide range of cities around the world.