Take this quick health quiz: Identify an invisible, odorless and tasteless process millions of Americans are subjected to annually. Doctors apply it often as a life-saving tool, though they know it may ultimately kill patients if misused. Poor records are kept of one's exposure to it, so neither your doctor nor you -- especially you -- may ever know whether you're being exposed to harmful dosages.

The answer (did you guess?) is the X-ray, as personal, beneficial and potentially dangerous as technology gets. The last sentence about poor records and your physician's ignorance should trouble you, but this isn't an attack on doctors or radiologists. It's bad news and good news about a life-threatening problem and its inexpensive solution. First the bad news:

The Food and Drug Administration says one-third of the 600 million X-rays taken annually are unnecessary, and they subject patients to an increased risk of developing cancer.

The New England Journal of Medicine says many of these needless exposures are from 27 million retakes, often the result of equipment failure and errors by technicians and physicians.

The FDA, whose X-ray regulation programs have been hurt by cuts in funding since 1980, says the amount of radiation a patient receives may vary more than a hundredfold for the same exam. Only two states -- Illinois and Vermont -- have set maximum exposure levels.

While radiologists are MDs who specialize (after a four-year residency) in X-ray diagnosis and treatment, they rarely operate the machines themselves -- nor do other physicians, chiropractors, dentists or podiatrists. The FDA estimates that about 20 percent of the nation's 165,000 X-ray operators have no formal instruction. Only 22 states require training for operators, often exempting dental offices, so in many states anyone can use an X-ray machine.

Even if an X-ray machine has been properly calibrated and used correctly, neither the primary doctor nor the radiologist (who interprets the X-ray pictures) nor the machine's operator knows exactly how much radiation the patient has received.

Dr. Derace Schaffer learned about the last item the hard way in 1981 when his daughter Whitney was born with a collapsed lung, requiring numerous diagnostic X-rays. A year later Whitney had a hip problem that needed additional X-rays. "There I was," Schaffer says, "a Harvard-trained radiologist, an X-ray expert. But I had no way of defining how much radiation Whitney had received or what the long-term consequences to her might be."

Once the illogic of the situation hit him, Schaffer says, he had to find the answer. Think about it: Dispensing medicine without a label on it is illegal. Not keeping track of drugs prescribed for a patient is negligent. Yet X-rays, known carcinogens, are widely used with no records kept of the amount of exposure, even though the risk of cancer increases with accumulated radiation.

Schaffer, chairman of the Genesee Hospital Department of Radiology in Rochester, N.Y., and a clinical associate professor at the University of Rochester's School of Medicine and Dentistry, began brainstorming with other radiologists in the winter of 1982. By the summer of 1983 they had filed for a U.S. patent on an original invention: CompuRad, a $39 personal-monitoring device that measures X-ray and background radiation.

People who work around radioactive materials have long worn film-type dose badges or used Geiger counters to monitor their exposure, but CompuRad is unique. The size of a credit card, it contains three thermoluminescent dosimeter slides (TLDs) that measure actual -- not estimated or average -- X-ray and radiation exposure. A fourth slide sealed inside the card measures accumulated background radiation from naturally occurring sources, such as radon, as well as from nuclear power plants, buried wastes and industrial nuclear materials.

CompuRad, which comes with clear, detailed instructions, is easy to use. Let's say you're getting a dental X-ray. Slip one of the TLD slides out of the card and place it on your face in the area to be X-rayed. (Each TLD is only 1 1/4 by 3/4 inches and is self-adhesive.) If you're going to have a routine X-ray such as a mammogram, place a TLD on your skin in the center of the X-ray field, using the same slide for all exposures of the same body part. Then write on the TLD the type of X-ray and number of exposures made.

To get a report, mail CompuRad back to Personal Monitoring Technologies Inc. with $8.75 for each exposed TLD slide. (There's no extra charge for the background radiation TLD, which you carry with you and which should be read annually at least.) PMT will send you and your physician (if you request it) a record of the total radiation you've received from X-rays and the environment. PMT also will record this confidential information in its data base, which it says is the world's largest compilation of actual human radiation exposures. PMT will send you an updated report and a replacement card every time you mail in a CompuRad card. (Call 800/4-DETECT for information on purchasing CompuRad.)

CompuRad may benefit enormous numbers of people, including children, whose developing cells are particularly vulnerable to radiation, and those who need frequent X-rays: sufferers of rheumatic, gastrointestinal and pulmonary problems, and the estimated 600,000 people with scoliosis (a spinal curvature).

But it's also likely to have beneficial social effects, changing the way people relate to their physicians and health services. Knowing the actual amount of radiation received for specific exams could help turn patients into consumer watchdogs, able to pinpoint the sources of their over-exposure to X-rays, whether it's from faulty machines or incompetent technicians. Physicians and dentists also can have continually updated radiation histories for their patients, allowing them to more carefully weigh the risks and benefits of diagnostic X-rays.

Schaffer sees this is as the beginning of a new industry -- technology supplying consumers with a variety of personal monitors, relatively cheap portable devices we'd use to assess the quality of our immediate environment. PMT is already at work developing personal monitors for water and air quality.

It's a trend we should welcome. Being concerned about nuclear weapons or the threat of another Chernobyl is important, but it's clear the public also faces daily ongoing radiation risks from X-rays and unknown nuclear hazards at work and in the home. Devices such as CompuRad give consumers a chance to zero in on these threats and take constructive steps to reduce the danger. ::