Chinese doctors have found no ill effects from long-term exposure to low-level radiation nearly twice as intense as most Americans get.

The finding is the latest -- and one of the most striking -- in which medical scientists have detected no increases in cancers, birth defects or other health problems in areas where everyday natural radiation is far greater than normal.

In all cases, this inescapable radiation is many times greater than the extra radiation received by the general population in the area of Pennsylvania's Three Mile Island nuclear accident.

The Chinese Ministry of Public Health's High-Background Radiation Research Group reported its findings yesterday in Science, an American Journal.

Everyone in the world is exposed to so-called background radiation -- from cosmic rays, from space, from rocks and soils and from a small amount of radioactivity in everyone's body.

The Chinese compared the health of 73,000 residents of Guangdong province -- where natural radiation averages 196 millirems a year -- with the health of 77,000 residents of nearby areas, who are subjected to 72 millirems yearly.

Over five years -- 1970 to 1975 -- the Chinese researchers found no significant differences in cancer rates or death, spontaneous abortions, hereditary diseases, congenital deformities, growth rates of children or changes in chromosomes, the body's carriers of genetic information.

No scientists in the world have demonstrated any greater incidences of diseases or defects in higher radiation areas, Dr. Jack Fabrikant, University of California at Berkeley radiation biologist, said yesterday.

This is true of the United States, where average exposure is between 100 and 130 millirems a year. Exposureaverages 100 in the Washington D.C. area, 116 in the Three Mile Island area and 197 in "mile-high" Denver.

It is even true, Fabrikant said, of some areas of India and Brazil with even higher average exposures. There are some high rates of genetic defects there, but they can be accounted for by high rates of in-breeding, he said.

By latest estimates, persons near Three Mile Island received an extra 1.5 millirems during and after the accident there. A federal advisory panel estimated this might produce one extra cancer in the next 20 years in people living within 50 miles of the accident site.

Some scientists -- but only a relative few -- ascribe greater dangers to even low-level radiation. All scientists agree that everyone should keep exposure to all radiation as low as practically possible.

The Chinese scientists said their results may show there is a practical cut-off beneath which low-level radiation does no harm. Or, they said, it may take scrutiny of a far larger population to detect any effects.