An Epidemiological Appraisal of the Effects of Ambient Air on Health: Particulates and Oxides of Sulfur

This review has attempted to evaluate the present state of our knowledge of the effects on health in man of environmental exposure to oxides of sulfur, sulfates, and particulate matter. There has been a great deal of activity in this field over the last 15 years, and therefore any collation of this material will represent the selected biases of the reviewer. The conclusions reached can be summarized as follows: (1) These pollutants, as they have been measured in epidemiological investigations, can only be considered as indirect indices of general air pollution and in many cases cannot be separated from each other. Therefore, we cannot incriminate a specific source of any one pollutant as the producer of the most harmful substance to reach the ambient air. Conversely, we cannot excuse any specific source of one pollutant because that specific pollutant has not been found to cause disease at a given concentration. The measurements in ambient air are the net results from all sources of pollution in combination with factors influenced by weather and meteorological considerations. (2) Direct effects from acute, high ambient air pollution disasters have been adequately demonstrated. Significant excess mortality has occurred in association with particular air pollution episodes. All of these episodes have occurred during cold weather, and the effects of temperature must also be considered along with elevated levels of smoke and sulfur oxides. (3) Specific working groups exposed to unusually high levels of these pollutants do not demonstrate dramatic effects. This is presumably related to the fact that susceptible people are self-selected out of these environments. (4) Associations between the prevalence of chronic respiratory disease in the general population and specific levels of these air pollutants have been demonstrated. The major thrusts of epidemiological investigations have been to study the effects of chronic exposure to ambient levels of smoke and sulfur dioxide. The studies to date have collected and analyzed point-prevalence data and information obtained from retrospective investigations. Although epidemiological investigations cannot prove a cause-and-effect relationship, the consistency of the results is such that one must conclude that a causal association is likely. In this reviewer’s opinion we have reached the stage at which we no longer need to demonstrate the effect of past exposure to these pollutants. What is needed now is to demonstrate the effects of current and continued exposure. This will require a better understanding of the natural history and pathophysiology of the diseases thought to be associated with chronic exposure to smoke and sulfur dioxide. Because of the nature of chronic respiratory disease, groups of subjects for whom exposure is known, must be followed over extended periods of time. The logical extension of these observations will be the follow-up of large populations for whom exposure has been reduced. Only by studies of this kind may we be able to prove the cause-and-effect relationship which most likely exists.