X-ray Safety

How safe are X-ray exams?

We are surrounded by radiation. Most of this radiation is in the form of visible light, ultraviolet and infrared light, radio waves and microwaves. Only a small portion is ionizing radiation.

X-rays are an example of ionizing radiation. Unlike most other forms of radiation, X-rays can pass through body tissue. This makes it possible for X-rays to provide images of the body's internal structures that may help your physician make a diagnosis without performing a surgical procedure.

Safety in low doses

Like other forms of ionizing radiation, X-rays can alter genetic material in cells and cause mutations that promote cancer. However, the amount of X-rays used in most diagnostic procedures is so small that the risk is extremely low — and may even be zero. Richard J. Vetter, Ph.D., is a radiation safety expert at Mayo Clinic, Rochester, Minn. "You are taking a very low risk and trading it for information that may be crucial in discovering or managing disease which may pose a serious risk to your health," Vetter says. "However, for protection purposes, we assume that the same kind of effects that occur at high doses of radiation could occur at low doses, so we try to reduce exposure as much as possible."

Improved technology has made it possible to significantly lower the amount of radiation used for X-rays, according to Vetter. He notes that 50 years ago the doses were much higher while the quality of the images was much lower.

Multiple examinations don't appear to increase risk

The benefit, according to Vetter, of diagnostic information gained from multiple medically necessary X-ray exams makes it worth taking the small risk associated with the procedures. "Studies suggest that risks are not increased from multiple exams," Vetter says. For that reason, no limits have been placed on the number of medically necessary X-ray examinations a person may have.

Measuring radiation

The average person is exposed to only about 360 mrads (millirads, 1/1000th of a rad, a basic unit of radiation measurement) of ionizing radiation annually. And most of this is from natural sources such as cosmic radiation and emissions from rocks and soil that contain radioactive elements such as radon and uranium. The intensity of this "background" radiation exposure varies by geographic location, says Vetter. People who live in Florida are exposed to about 70 millirads of naturally occurring background radiation each year while people living in the mountains of Colorado may receive as much as 235 millirads.

Diagnostic X-rays, produced by an artificial source of ionizing radiation, account for 11 percent of your average yearly exposure, while radon, a radioactive gas that enters buildings through cracks in basements, contributes to about 55 percent of total exposure.

Typical doses received from X-ray exams are measured both at the skin and at the gonads (ovaries and testes) and bone marrow. These organs are chosen because of their effect on the reproductive system and general health. To determine your exposure, it's necessary to add together the doses to a specific organ. These levels are much lower than skin doses. For example, the skin dose for a lumber spine X-ray is 280 mrad while the dose to bone marrow for the same X-ray is 10 mrad.

Risk from all sources is low

Does the average exposure to all sources of ionizing radiation put you at significant risk? Probably not, according to Vetter: "The most likely effect from that is near zero. It's theoretically possible that the average dose could cause problems, but you can't measure it, it's such a small number. Many studies have shown that doses below 5,000 mrads are not harmful."

MRI and ultrasound don't use X-rays

Two other types of diagnostic imaging, magnetic resonance imaging (MRI) and ultrasound, don't use X-rays. "MRI uses radio waves, a form of non-ionizing radiation," Vetter says. "Ultrasound uses sound waves. Neither of those breaks molecular bonds the way ionizing radiation does."

X-rays during pregnancy

The risks to a fetus of exposure to diagnostic X-rays is very low. Nevertheless, be sure to tell your doctor if you think you may be pregnant. If it's necessary for you to have an X-ray even though you're pregnant, special precautions can be taken to minimize radiation exposure to the fetus.

Tests worth taking

X-ray examinations provide valuable information about your health and play an important role in helping your doctor make an accurate diagnosis. Great care is taken to produce the best images with the lowest possible radiation and with as few X-rays as possible.

September 1, 2000
© 2000 Mayo Foundation for Medical Education and Research (MFMER).