What Constitutes an Adverse Health Effect of Air Pollution?: This Official Statement of the American Thoracic Society Was Adopted by the ATS Board of Directors, July 1999

As the twentieth century ends, the health effects of outdoor air pollution remain a public health concern in developing and developed countries alike.In the United States, the principal pollutants monitored for regulatory purposes (carbon monoxide, nitrogen dioxide, sulfur dioxide, particles, ozone, and lead; see Table 1) show general trends of declining concentrations, although ozone pollution now affects many regions of the country besides southern California (1).Yet, even at levels of air pollution now measured in many cities of the United States, associations between air pollution levels and health indicators are being demonstrated at concentrations around those set by standards of the U.S. Environmental Protection Agency (2,3).In many countries of the developing world, concentrations of air pollutants are rising with industrialization and the increasing numbers of motor vehicles (4, 5).Extremely large and densely populated urban areas, often referred to as "megacities," have the potential to generate unprecedented air quality problems.There are common principles to air quality management throughout the world.Public health protection unifies all approaches, whether based on voluntary guidelines, mandated standards for concentrations, or source control.The intent is to limit or to avoid any impact of air pollution on the public's health.Air quality management is thus based on a scientific foundation built from the epidemiologic, toxicologic, and clinical evidence on health effects of air pollution.In interpreting this evidence for public health protection, there is a need to identify those effects that are considered "adverse" and to separate them from those effects not considered adverse.The American Thoracic Society has previously provided guidance on the distinction between adverse and nonadverse health effects of air pollution in its 1985 statement, "Guidelines as to What Constitutes an Adverse Respiratory Health Effect" (6).Definitions of adverse effects have also been offered by the World Health Organization (7-10) but the guidance of the American Thoracic Society has received particular emphasis in the United States.Preparation of the original statement was intended to coincide with consideration of the passage of an amended Clean Air Act and to provide a framework for interpreting scientific evidence relevant to the mandate of the act.In particular, the Clean Air Act requires that the Administrator of the Environmental Protection Agency promulgate, for certain pollutants, standards that will be sufficient to protect against adverse effects of the air pollutants on health.The act is silent on the definition of "adverse effect" and, at the time of the 1985 statement, there was considerable controversy around the interpretation of this language as revi-