Recent scientific evidence has made it clear that air pollution emissions may have environmental effects in regions much more distant from the source than previously believed possible. Oxides of sulfur and nitrogen produced in the burning of fossil fuels and in smelting operations may be carried hundreds or even thousands of miles through the atmosphere, chemically transformed in the process, and eventually returned to earth as sulfuric and nitric acids, often in rain or snow. As a result, many areas of the United States and eastern Canada are now experiencing precipitation 25 to 40 times more acidic than "natural" rainfall.¹ Though the problem has been developing for decades, monitoring has been sporadic, and only in the past two or three years have scientists begun to focus study on the phenomenon and its associated environmental effects. It is now clear that in receiving areas, which are especially vulnerable because of soil naturally low in the capacity to neutralize acids, acid rain and snow can have devastating consequences.

The acid precipitation problem is particularly difficult to deal with through traditional approaches to air pollution control. Acid-forming compounds are emitted as conventional air pollutants, but they return to earth either in precipitation, where arguably they are not an air quality problem, or as chemically altered compounds (sulfates and nitrates) which are not yet the target of direct regulation. The pollution routinely crosses internal political boundaries (state and provincial borders) and international boundaries, making existing locally oriented approaches to control inappropriate. The difficulties are exacerbated by the fact that low levels of acid pollution deposited over many years can accumulate to cause serious environmental damage.