ONE OF the hottest debates in the field of public health these days is over the question of whether the cancer rate is increasing or not. For years most experts believed that it was not, and many argued that if the cancers caused by smoking were discounted, the rate was actually falling noticeably. But a few months ago, National Cancer Institute scientists published new evidence from cancer surveys that they believe demonstrates a dramatic rise in the rate during the past decade.

Through the NCI's data are open to criticism on many grounds, they are convincing enough to have reversed the general consensus. For example, the American Cancer Society's 1979 edition of Cancer Facts and Figures contained the statement, as had previous editions: "The overall incidence of cancer decreased slightly in the past 25 years." The 1980 version reads: "The overall incidence of cancer decreased slightly form 1947 to 1970, but has increased between 5 to 10 percent since 1970."

This apparent -- though still uncertain -- reversal in the cancer trend has major implications for public health, for government regulatory policies and for the national economy, because many believe that much of the increase has been caused by exposure to chemicals. The chemical industry began a period of explosive growth in the '40s and early '50s. Since most cancers have a latent period of between 20 and 30 years, the timing is highly suggestive. And since the industry has continued to grow, this theory -- it is still only that -- suggests that the cancer rate will continue to climb in the years ahead.

The worry over chemical effects is compounded by the accumulating evidence that it isn't only industrial workers and people unlucky enough to live near a chemical waste dump who are being exposed to these substances, but probably everyone. In the rash of legal battles over Love Canal and other hazardous waste dumps, measurements are being taken for the first time to determine the "background" levels of chemicals in the environment. What these measurements are showing, to the consternation of the Environmental Protection Agency, is that the air and water in Texas or New Jersey or around most major U.S. cities may have levels of chemical contamination equivalent to those found near major waste dumps.

The problem with all these data is that no one knows what they mean. The fact that a chemical can be shown to be present does not mean that it is a health threat. Conversely, with so many chemicals distributed throughout the environment, tracing a particular health threat to the guilty chemical(s) is nearly impossible.And as the cancer debate shows, even dsicovering that a health threat exists is no easy task. The optimists in that debate believe that the facts will become clear in five to 10 years -- the pessimists believe that the answers may never be clear.

As two pieces on the opposite page today dramatically illustrate, the regulatory struggle over just one chemical -- benzene -- quickly becomes lengthy and complex. The important thing to remember when all the absolutes are being thrown around with passion and certainty is that what is known -- really known -- about chemical dangers at the moment is a tiny fraction of what remains to be discovered.