High doses were known to cause cataracts, burns and temporary sterility, but low doses were considered relatively benign unless they caused subtle changes by heating biological tissues. In recent years, however, the consensus has swung toward recognizing more low-level effects, even at doses too slight to cause measurable heating. However, much of the information is still in dispute. The gradually shifting consensus led the American National Standards Institute to issue a new safety standard for radio-frequency exposures last September. The standard sharply reduced permissible exposures in the range of frequencies from which human beings absorb the most energy.
Some scientists consider the new standard highly conservative.
But Nicholas H. Steneck, a professor of history at the University of Michigan who has been studying microwave developments, told a microwave power symposium in Philadelphia last month that the values underlying the standards reflect military-industrial interests rather than the interests of people exposed to the radiation. But Przemyslaw Czerski, a Polish expert now working at the Food and Drug Administration, told a conference in June at Boulder, Colo., that microwave radiation at relatively low levels can cause chromosome damage and abortions in mice. And he told the Philadelphia symposium that some recent data are disturbing enough to justify still further reassessment of permissible exposure levels. Concern over ultrasound has been voiced by Alice Stewart, a British epidemiologist who heads the Oxford Survey of Childhood Cancers. She said in an interview last month that there are some ''very suspicious'' hints that children exposed in the womb to sonograms appear to be developing leukemia and other cancers in higher numbers than unexposed children.