Because of the potential public health implications, the importance of toxic air pollutants in ambient air has been recognized to some degree for many years. Efforts to “regulate” human activities resulting in the production of ambient air pollutants probably date back many centuries, even as the combustion of fossil fuels and air pollution from other organized human activities began having a noticeable impact on the environment. Schemes to classify ambient air pollutants by their human health impacts have evolved with increasing sophistication in this century, culminating in the past 25 years or so with the identification of


as a key parameter in identifying ambient air pollutants with serious adverse human health effects, thus warranting their mitigation in some manner.