WASHINGTON (AP) _ Scientists are warning Congress that although there is no scientific certainty that chemicals are destroying the Earth's vital ozone layer, there is far too much at risk to wait for absolute proof.

''We have to take action sometime soon,'' said Dr. Donald Heath of the National Aeronautics and Space Administration's Goddard Space Flight Center. ''The problem is, there are serious changes taking place we don't understand.''

Dr. Robert T. Watson, another NASA scientist, said ''prudence'' should dictate worldwide controls on the suspect chemicals, chlorofluorocarbons.

''Even in the face of scientific uncertainty, now is the time to act,'' Watson said Monday as the House Energy and Commerce health and environment subcommittee held a hearing on depletion of stratospheric ozone.

Ozone is a natural screen for the sun's ultraviolet rays, which doctors say are the principal cause of skin cancer in humans. Scientists say the ozone layer is thinning and suspect that chlorofluorocarbons are the culprit.

These chemicals, first theorized as ozone-destroyers in 1974, are in increasing use around the world as aerosol spray propellants, refrigerants and industrial solvents and in some foam packaging materials.

The United States banned them in spray cans about 10 years ago, but only Canada and Sweden have followed suit. The Reagan administration is pushing international negotiations aimed at a 95 percent cut worldwide in chlorofluorocarbon production levels in 10 to 15 years.

The scientists say prompt action is necessary because some chlorofluorocarbon s retain their destructive properties for decades and that even an immediate halt to their use would not prevent future ozone loss.

As chlorofluorocarbons use has been growing, there has been a rapid surge in skin cancer cases in the United States, with all forms reaching near- epidemic rates and the most serious, often fatal type, malignant melanoma, rising eightfold in the last seven years, according to Dr. Darrel Rigel.

''At the current rate, about one in seven Americans will develop (some form of) this disease during their lifetime,'' Rigel, a research physician from New York University Medical Center, told the hearing.

He said that although many factors have been linked to skin cancer, ''the generally accepted most important factor is exposure to ultraviolet light. ... One would expect skin cancer rates to increase as the ozone thins.''

And that, the other scientists say, is exactly what is happening, not only in the much-publicized ''ozone hole'' that appears several months a year over the Antarctic but to a lesser extent in warmer latitudes.

Heath said that in the 1978-86 period, scientists recording a reduction in stratospheric ozone ranging from 4.4 percent to 7.4 percent in non-polar areas.

''There's no question in my mind that (ozone) decreases in the mid- latitudes are real,'' said Heath, suggesting that these could be linked to the more dramatic seasonal decreases seen at the South Pole.

''I believe that the observed long-term changes in stratospheric ozone do indicate a potentially serious problem based on predicted consequences to the biosphere,'' he said.

''The satellite observations do not contradict model predictions of ozone depletion by chlorofluorocarbons but instead show losses which are much larger than predicted,'' Heath said.

Dr. Susan Solomon of the National Oceanic and Atmospheric Administration headed last fall's U.S. team to study ozone depletion in the Antarctic.

''I think we will eventually see large depletion of the (ozone) layer at all latitudes,'' she testified. ''It's a question of when and how much.''

Watson said that whatever the cause, ''There is now compelling observational evidence that the chemical composition of the atmosphere is changing at a rapid rate on a global scale. ... Protecting the ozone layer is one of the most important environmental issues of our time.''