Under the Clean Air Act, the USEPA establishes ambient air quality standards to protect public welfare from known or anticipated adverse effects from criteria air pollutants. Although adversity is simply defined (i.e., opposed to one's interest; harmful), its determination is fraught with difficulties. The definition depends on one's viewpoint; society has established that some effects are more serious than others. There is a view that ecological effects are adverse only if they can be expressed in economic terms. This leads to the conclusion that mitigative actions need not be taken unless the costs of the action are offset by sufficient benefits. Adverse effects, however, should be viewed in a broader societal content, i.e., the total benefits from ecological systems. (Copyright (c) 1990, ASA, CSSA, SSSA.)