An evaluation of trends in air quality is a means to assess the net result of numerous interrelated factors that affect the quality of our air resource. Factors such as population growth, industrial activity, energy consumption, rural-urban distribution of our population, the shape and size of our metropolitan areas, various social and economic changes, and the effectiveness of our efforts to control pollution at its source all play an important role in determining the resulting air quality. It must be noted that the trends in ambient levels of pollutants at any one place is a complex function of some or all of these factors and not merely the direct assessment of the degree of control applied to the various sources of pollution.
We assess trends presumably because we would like to anticipate what is happening to us now, and we inject into this picture projections of potential pollution increases related to our nation's goals in population growth and standard of living, in order to anticipate some of the future consequences of trends in human activity. This also allows us to assess the need to control pollution and to undertake now the research and development required to bring such control about. It is to be noted that the subject of this article is by no means merely of scientific curiosity; rather, it is the basic building block on which our nation's need and program, to restore and protect our environment, is founded.