IN A RECENT letter to the editors of Science magazine, a noted drug researcher pleaded against classifying chemicals as either positive or negative carcinogens. Clear and simple distinctions between chemicals that do and don't cause cancer, he suggested, are useful, but they also distort reality.

A messier, but more accurate, picture would identify at least three types of carcinogens: a chemical that, by itself, causes tumors in a healthy individual; a chemical that can cause cancer only in an animal whose resistance has been lowered by other circumstances, like stress or illness; and third, a "promoter" chemical -- a substance that increases the effects of other carcinogens that may be everywhere in the environment. This classification approximates reality more closely. But it also poses nearly insurmountable problems for regulators whose job is to set safe standards of exposure to particular substances for millions of individuals who are differently exposed to dozens, if not hundreds, of different cancer risks.

A perfect example of the kind of controversy that can ensue is the case of saccharin and other artificial sweeteners. Last week, the results of a study of the linkage between artificial sweeteners and bladder cancer were announced. The study was commissioned because earlier studies had produced contradictory results: some showed an 80 percent increased risk for male users, while others showed no linkage at all. If the measure of success was to determine once and for all whether saccharin is or is not a carcinogen, it was hardly successful. But it did produce some important insights.

The results showed a 60 percent increased risk of bladder cancer for heavy users of artificial sweeteners -- defined as those who have six or more servings a day of a sugar substitute or two or more eight-ounce glasses of diet soda. It confirmed that men are more susceptible than women, but showed a lower risk than earlier studies. It also revealed a connection between use of the sweeteners and cigarette smoking: heavy smokers who also used the sweeteners heavily had a significantly higher risk of bladder cancer than heavy smokers who did not use them. While the results are still preliminary, the study's authors believe it shows that saccharin and cyclamates acting alone increase the risk of bladder cancer, and are apparently also promoters of other carcinogens.

While these results clarify the situation somewhat, they are unlikely to ease substantially the confusion in the minds of most consumers. Nor do they simplify the regulators' problem: what levels of exposure to aritificial sweeteners pose an acceptable level of risk? It appears that until the biological events underlying cancer are understood -- and that is likely to lie many years away -- confusion and controversy over the proper regulation of carcinogens will be a constant and continued presence.