RESEARCHERS HAVE finally uncovered what it is that makes onion-slicing humans weep. The stuff (called propanethial S-oxide, if you care), dissolves in any available water (tear drops), thereby producing...sulfuric acid. Meanwhile, other researchers have been documenting that sulfur and nitrogen oxide emissions from the burning of fossil fuels - coal especially - combine with moisture in the atmosphere to produce another new phenomenon of the industrialized world...acid rain. The rain contains both sulfuric and nitric acids.

Like the buildup of carbon dioxide in the atmosphere that is causing a warming of the earth's climate, the increasing acidity of rainfall is a worldwide problem. Policy-makers and regulators have just recently responded to the fact that pollution from power plants and smelters, for instance, can affect rainfall hundreds of miles away. Now, research still in progress over the Arctic Ocean is turning up evidence that such pollution can travel not hundreds, but thousands of miles.

Information about acid rain is sparse. It is known, though, that rainfall over the northeastern United States is almost 100 times more acid than normal rainwater, and 50 times more acid than it was 25 years ago. Research programs from the Adirondacks to southern Sweden have shown that increased acidity causes decreased agricultural and forest yields and sharply lower fish populations in freshwater rivers and lakes. The acid rain also leads to the loss of important nutrients from the soil. Microorganisms that are at the bottom of freshwater food chains are also apparently affected, as are the vital nitrogen-fixing bacteria that live in the roots of certain crops and enable them to grow without the addition of artificial fertilizer. The rain also corrodes buildings, works of art and other structures. Possible health effects are many and varied. The annual costs of all this are still anybody's wildest guess.

The eventual consequences from increasingly acid rainfall are obviously immense - both in dollar costs and in possibly irreversible damage to the various forms of life. The implications, especially for energy plans based on greatly increased burning of coal, are also serious. In his recent environmental message to Congress, the president announced a new federal effort to assess the magnitude of this problem - but the announced duration is long (10 years) and funding low. Since this air pollution doesn't respect national boundaries - as Canada's recent demand for an air-pollution treaty attests - a much more intense international effort is clearly called for.