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An Air of Uncertainty: Epidemiological Studies of the Impact of Air-Borne Pollutants on Human Health Provide a Foundation for Sound Public Policy

Every day, we inhale about 10,000 liters of air while breathing normally. This air comes from the places where we spend time--homes, vehicles, workplaces, public places, and outdoors. The air contains myriad gaseous and particulate contaminants. The inhaled air passes through the nose and mouth, where larger particles and highly soluble gases are scrubbed, and then through the larynx into the lung. The lung's airways continue to filter pollutants from the inhaled air, and the smaller particles deposit on them. The exchange of gases--oxygen entering the blood stream and carbon dioxide leaving the blood stream--takes place in the lung's alveoli, or air sacs, which have a surface area equivalent to that of a tennis court. Very small particles and gases deposit onto the alveoli, and the diffusible gases, like carbon monoxide, pass through the membrane that separates the air in the alveoli from the blood in the lung's capillary bed. Because of the massive quantities of inhaled air, significant doses of air pollutants may reach the lung, even if concentrations are seemingly low. Many of these inhaled contaminants have the potential to injure the lung, impairing its function or causing or exacerbating disease.' Some contaminants, such as carbon monoxide, have systemic effects. The range of health effects associated with air pollution is broad. High levels of air pollution have caused tragic and dramatic episodes, with obvious increases in the numbers of deaths and hospitalizations. The London Fog of 1952 is a classic and dramatic example. In early December of that year, a bitter cold front moved into London, creating an inversion that held for four days. To keep warm, the residents burned huge amounts of coal, and the smoke coming from the chimneys settled into the tog, creating some of the worst pollution conditions ever recorded. Visibility was so poor, even inside, that theaters had to shut down. An estimated 4,000 people died as a result of the fog. (2) The levels of combustion-related pollutants during the London Fog were approximately 100 times greater than levels in today's major cities in the developed world. Still, high levels of air pollution are being recorded in many of the world's new mega-cities, like Sao Paolo and Delhi, where growing populations, vehicle congestion, industry, and biomass fuel burning combine to create serious air pollution problems that could cause deadly air pollution episodes. Premature mortality still remains a public health concern in cities of the developed world. Time-series studies of air pollution and numbers of deaths continue to show a subtle but positive association, even in today's U.S. cities, as does the health status of people with asthma and chronic obstructive pulmonary disease, also known as emphysema. (3) Studies that track symptoms and lung function in persons with these diseases show that higher levels of pollutants tend to increase symptom rates and lower the level of lung function. Subtle heart rhythm disturbances have also been linked to air pollution. (4) Children, the elderly, and persons with chronic heart and lung diseases are particularly vulnerable to air pollution. Assessing Risk The evidence on the health effects of air pollution comes from multiple lines of scientific investigation. (5) The observations of clinicians have been valuable when levels of air pollution have been extremely high, as with the 1952 London Fog; but at typical concentrations found today, clinicians and their patients cannot readily make links between clinical status and air pollution exposures. Instead, the risks of air pollution for populations are assessed using the methods of epidemiology. Epidemiologic studies of air pollution and health use several research designs. One group of studies, termed ecologic studies, compares the health status of communities with higher and lower exposure to air pollution or tracks the status of the population over time as levels of air pollution change. …