THE TOUGHEST PROBLEM for health and environmental regulators is what to do about a particular risk when there are suggestions or theories of danger, but little proof is available. The risks posed by human meddling with the earth's atmosphere are among the hardest to judge and the most important to understand.

Relatively little is known about the atmosphere, which sets the biological and chemical conditions for life on this planet, though not for any lack of interest. The reasons for the modest knowledge are that anything so all-encompassing is exceptionally hard and expensive to study. The five thousand trillion tons of air that surround the earth represent not only a complex thing in itself; they also interact continuously with the oceans. Small changes in such huge realms are obviously difficult to predict or document.

Despite the difficulties, a great deal is being learned about one threat to the atmosphere--depletion of the stratospheric ozone layer caused by a class of chemicals called chlorofluorocarbons (CFCs). Depletion of this shielding layer would mean more skin cancers from higher levels of ultraviolet radiation and possibly also damage to vital crops and fish species. It might also affect the worldwide climate.

The hypothesis that CFCs could damage the ozone layer is very young: it was first proposed in 1974. Despite the lack of conclusive evidence that damage was actually occurring, the United States banned CFC use in aerosols just four years later. Canada, Norway and Sweden soon did the same. Last year the 10 members of the European Economic Community bound themselves to reduce CFC use in aerosols, and went a step further by agreeing not to increase production capacity for CFCs' many other uses. An international commission, under United Nations auspices, will meet in a few months to take the first steps toward a global convention to protect the ozone layer.

But this is only part of the picture. Concerted international action is still years in the future, and while scientific knowledge is growing much faster than most people had expected, the findings are still patchy. Recently the first direct evidence of depletion was gathered from satellite measurements, but a good deal of work remains before these findings can be confirmed and the extent of the damage calculated.

Meanwhile, American CFC producers, the world's largest, reportedly have persuaded the Environmental Protection Agency to abandon plans to limit future production levels. It would be profoundly wrong to break the momentum that has accomplished so much in so short a time. Here, as in all the unresolved questions of health and the environment, the only defensible rule is to err on the side of caution until the risks are more precisely known.