Q. Your answer about the radiation risk of mammography didn't address the concern I have about the number of x-rays I've received. So far, I've had two barium enema exams, three stomach x-rays and a gallbladder x-ray, as well as routine chest and dental x-rays. In addition, I travel abroad at least twice a year, which exposes me to considerable cosmic radiation. Recently, my doctor recommended a baseline mammogram. What would be the impact of all this radiation on my future health?

A. As I mentioned in my discussion of mammography, the risk of low doses of radiation, if it exists at all, is too small to measure. In your case, however, the cumulative dose of radiation is beginning to add up.

Even so, experts are not able to predict with certainty whether such amounts of radiation are harmful. The best advice I can give is to work with your doctor to limit the radiation you receive to only what's necessary. Radiation is both natural and man-made; each contributes about half of the average person's radiation exposure. Medical and dental procedures account for about 90 percent of exposure to man-made radiation.

On average, Americans receive 90 millirems -- a measure of radiation -- each year from medical and dental sources. For comparison, people receive about 125 millirems of radiation from natural sources, including about 50 millirems of cosmic radiation from the sun and space.

As you go higher in the atmosphere, you're exposed to more cosmic radiation, although not a great amount. A transcontinental flight will expose you to an additional 20 to 40 millirems, roughly the equivalent of a standard chest x-ray. Although many experts consider this amount of radiation negligible, others have compared its maximum risk to the risk involved in smoking two cigarettes or driving 78 miles in a car.

Not everyone realizes that different x-ray exams produce different exposures. You might like to have some idea of the amount of radiation involved in some common procedures. The numbers in parentheses refer to the relative number of millirads, another measure of x-ray dose:

High-dose x-rays include barium enema exams (875), stomach or upper GI x-rays (535), lower spine x-rays (450) and kidney IVP x-rays (420). Mid-dose procedures include abdomen (147), ribs (143) or pelvis x-rays (93). Low-dose exams are thigh (21), chest (10) and dental x-rays (10).

To put these amounts of radiation from medical sources into perspective, people receive about 7,000 to 10,000 millirads of radiation from natural sources during their lifetimes. Although I want to emphasize the relative benefits of x-ray procedures over their risks, I wouldn't want you to think that radiation is totally harmless. For example, some experts have calculated that about 1 percent of all cases of leukemia and just under 1 percent of all cases of breast cancer may be due to medical x-ray procedures.

To limit your exposure to medical x-rays: Discuss with your doctor the reasons for each x-ray exam and what is to be gained from the procedure. See if there are alternative ways to obtain the same information -- for example, having a sonogram, which uses no radiation, instead of an x-ray. Ask for lead-lined shields for parts of the body not relevant to the exam, especially for the genital area.

Jay Siwek, a family physician from Georgetown University, practices at the Fort Lincoln Family Medicine Center and Providence Hospital in Northeast Washington. Send questions to Consultation, Health Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071. Questions cannot be answered individually.