There is urgent need to discover the combined effects on humans of man-made radiation and thousands of other potentially harmful agents, including chemicals and fossil fuels, a research scientist said yesterday.

"I don't want to sound alarmist," said Dr. Mortimer Elkind of the Argonne National Laboratory near Chicago, "but there is increasing evidence" that two harmful agents may act together to cause more harm then the sum of the harm each does.

Today, he said, "We are adding hundreds of new chemicals" every year to our personal environments and burning more fuel with complicated combustion products, and what effect the million of combinations may have in causing disease and death is not known.

But "we do know from some epidemiological studies" that enhanced effects can occur when harmful agents interact, Elkind said. For example, he said, cigarette smoking greatly increases the number of lung cancers in uranium miners who also breathe radioactive gases and silicon dust.

In the 1950s the scalps of many children were X-rayed to treat ringworm, and some developed skin cancers. But he said this number was greatly increased when children were also exposed to the ultraviolet light of the sun.

Elkind, senior biophysicist at the federal laboratory that built the first nuclear reactor nearly 40 years ago, was one of many speakers at a National Institutes of Health radiation conference who urged more research on man-made radiation and associated effects.

Man-made radiation is probably better understood than any other potentially harmful physical agent, said Dr. James V. Neel, a University of Michigan geneticist. This radiation is produced by atomic tests, nuclear reactors and wastes, but in far greater amounts by medical and dental X-raying.

The best evidence, Neel said, is that man-made radiation has probably added "a very little," 1 or 2 percent, to the number of genetic mutations -- mostly harmful changes in heredity -- that have been caused every year for thousands of years by radioactivity in all earth and rocks and a constant rain of cosmic rays from the sun and outer space.

Dr. Edward Radford of the University of Pittsburgh put the estimate of man-made genetic mutations slightly higher -- an addition of 1 to 6 percent to natural background radiation.

In addition, Radford and others pointed out, man-made radiation also causes some cancers and other diseases -- probably 1 or 2 percent of all cancers. Here too, he said almost all of these man-made cancers are the products of medical radiation.

Many authorities have called much medical and dental radiation unneeded. "In the light of all we know" about the unnecessary harm still caused by such radiation -- and "the fact that we have known about it for so many years" -- it is time for some federal regulation or control, said Dr. Sidney Wolfe, director of the Public Citizen Health Research Group. d