THE PROBLEM is not merely acid rain; it is acid rain, sleet, fog, snow, mist, dew and particulates. The question is whether enough is known about the various effects of all this to warrant action now to reduce its incidence--as environmentalists, the Canadian government and the National Academy of Sciences believe. Or are years more of study required first--as the Reagan administration, the coal companies and the utilities argue? The choice is one of the more important ones Congress will make in its reauthorization of the Clean Air Act.

The dispute is likely to be profoundly affected by a recent report of the Committee on the Biological Consequences of Fossil Fuel Combustion of the National Academy of Sciences. The study found that acid rain does come from man-made sources, largely power plants; that the acidity is causing well-documented "widespread damage to aquatic ecosystems," increased levels of a number of poisonous metals in ground water supplies, and the "elimination" of "several important species of fish and invertebrates over substantial parts of their natural ranges." Other effects include damage to human health, to forest and food crops and, over the long run, leaching of vital nutrients from soil.

In uncharacteristically blunt terms, the academy's report concludes that continued emissions at current rates "in the face of clear evidence of serious hazard to human health and to the biosphere, will be extremely risky from a long-term economic standpoint as well as from the standpoint of biosphere protection." The committee believes the situation "is disturbing enough to merit prompt tightening," by as much as 50 percent in badly affected areas, of allowed emissions standards, particularly for power plants.

The scientists' sense of urgency was recently seconded by Canadian officials in an unusual appearance by foreign representatives before a congressional committee. After 10 years of research, the Canadian government believes that indisputable evidence of acid rain damage warrants prompt action by the United States, which is the source, it believes, of more than half of its acid rain. Acid rain has become the hottest issue on the U.S.-Canadian agenda, and a growing source of conflict.

In the face of all this, claims by coal and utility companies, such as that made by a spokesman for Consolidation Coal Co., that "there is no good data or evidence linking sulfur emissions to alleged increases in acidity of rainfall," begin to sound more and more like the claims of tobacco company spokesmen on the subject of the causal relationship between smoking and cancer.

Enough is known about acid rain to put an end to debates over whether the phenomenon is real, man- made and damaging. It is all three. The important area for discussion now is how best to go about reducing sulfur and nitrogen oxide emissions, and how fast.