Health effects of acid aerosols

Abstract

Earlier in this century, a number of severe episodes clearly demonstrated that air pollution can affect human health; these included documented increases in mortality and morbidity. Although health was clearly affected during these episodes and acidity is a candidate for the responsible agent, it has been difficult to ascertain which agents were involved. In the past several years extensive research was launched to learn the significance of acidic aerosols on human health. The question of a health threat from acid aerosols was first raised by epidemiology studies, but results of the body of epidemiological evidence collected to date have been mixed. Even when a study finds evidence of a response to exposures involving high ambient acidity levels, it is usually difficult to know which agent or agents are responsible for causing the effects noted. High levels of acidic aerosols are nearly always accompanied by high levels of other air pollutants which may have known or suspected effects on the respiratory tract. For this reason, an understanding of the potential mechanisms of acid aerosol health effects needs supporting evidence from the laboratory, where exposures to various agents can be controlled. To date, this supporting evidence includes demonstration of physiological responses at acidity levels greater than those that exist in the ambient environment. A limited number of studies have considered responses at levels more characteristic of ambient exposures; these studies demonstrate little physiological response, probably due to the airways' ability to buffer acidity at low concentration. Although there is some evidence of impaired mucociliary clearance and modest changes in lung function, there is no evidence of airway inflammation or altered non-specific bronchial responsiveness as a result of acid aerosol exposure. The possibility that acid aerosols may potentiate responses to other pollutants remains a subject of interest. The potential existence of a group of individuals who are exquisitely sensitive to low acid concentrations requires further investigation. Recent epidemiology results are broadening the perspective from a focus on acidity per se to a focus on fine particulate matter, of which particulate acidity is but a subset. These studies find a consistent statistical association between various health responses, including mortality, and ambient measures of particulates, even at locations where levels of acidity are very low and at locations where current U.S. air quality standards are satisfied. There is at present no biological explanation for these associations.