In 1896, less than a year after the discovery of X-rays, Sir Joseph Lister complained after having his hand X-rayed that "if the skin is long exposed to their action, it becomes very much irritated, affected with a sort of aggravating sunburning. This suggests the idea that the transmission of rays through the human body may not be altogether a matter of indifference to internal organs . . ."

The famous surgeon made an insightful observation, but no one paid much attention.

"The early attitude was that X-rays were not dangerous at all," says Mark Barnett, assistant director for education at the Food and Drug Administration's Center for Devices and Radiological Health. "Even after severe burns, people tried to explain these burns away and not relate them to X-rays."

It wasn't until the 1920s, when radiation-caused diseases such as lung cancer began to show up in uranium miners and bone cancers appeared in women who painted luminescent faces on clocks with radium paint, that attitudes began to change. Physicians began to acknowledge that X-rays were harmful, but only in large doses, says Barnett. "When people talked about medical X-ray protection in the 1930s, they were talking about protecting the doctor and the nurse, not the patient."

Then came Hiroshima and Nagasaki and the advent of above-ground atomic bomb testing by the United States and the Soviet Union. The effects of large amounts of radiation were apparent, and people began to worry about the subtle effects of low-level radiation.

"What we know now is that radiation in large amounts can cause delayed effects, including cancer and genetic effects," says Barnett. "The prudent assumption that we make at this point is that the smaller amounts from a diagnostic X-ray can increase the risk of cancer and genetic mutations, but the degree of risk is so small that it cannot be measured in epidemiologic studies. But there is no point in taking any risk that isn't needed."

That means don't have an X-ray study performed unless it is clinically needed, says Barnett. Studies may be done unnecessarily for a number of reasons: because the patient demands one, out of habit (chest X-rays on admission to the hospital, for example), or to protect the physician in the event of a malpractice suit.

While no one knows whether the total dose of X-rays to the entire population has gone up or down or stayed the same overall, advances in technology have lowered the X-ray dose delivered by certain tests.

For example, the doses from "mammography exposure really have come down," Barnett says. In the mid-1960s, when mammography was first used to search for early indications of breast cancer, the machines used industrial X-ray film, which required large amounts of radiation to get a picture. Newer, faster films used with intensification screens lowered the dosage. In one FDA study, the dose from mammography dropped from 0.90 roentgens in 1977 to an average of 0.56 roentgens seven years later. (A roentgen is a measure of the radiation used in all types of imaging studies, including CAT scans, fluoroscopy, nuclear medicine scans and even traditional chest X-ray, which delivers 0.02 roentgens.)

Screening for scoliosis, a curvature of the spine which typically forms during growth spurts in adolescence, is a new concern for radiation exposure. Two-thirds of the children sent for follow-up exams are girls, and most of them get at least one X-ray, sometimes many more.

But X-raying the spine also exposes the developing beasts at a time when they are most susceptible to X-ray damage. An FDA study of some 200 centers that perform scoliosis screening found that few were taking simple precautions to lower the X-ray dose to the breasts of these young girls.

The average dose was about 0.16 roentgens (about the same dose as eight chest X-rays), says FDA's Charles Showalter. With proper shielding, they "can reduce the exposure to less than 0.005 roentgens."

Even though consumers and professionals take a more enlightened approach to the risks of X-rays, it's still up to the consumer to ask the hard questions, says Barnett.

Before having an X-ray exam, the FDA recommends:

*Ask how the X-ray will determine what is wrong, and how that information will change the treatment.

*Don't refuse an X-ray if the doctor explains why it is medically needed. The risk of not having a needed X-ray is greater than the tiny risk from the radiation.

*Don't insist on an X-ray.

*Tell the doctor if you are, or think you may be, pregnant.

*Ask for a gonad shield for you or your children if X-rays are being taken of the back, abdomen, or near the sex organs. Sometimes such shields cannot be used.

More Information:

Write to Division of Consumer Affairs, Center for Devices and Radiological Health, Food and Drug Administration, HFZ-210, 5600 Fishers La., Rockville, Md., 20857, or call 443-4190.