An X-ray for knee pain. A CT scan for a head injury. Mammograms every other year, starting at age 50. Over a typical lifetime of radiation exposure from medical tests, a person can start to wonder: How much is too much?

There is no formula for answering that, experts say, in part because the health effects of radiation do not add up linearly. And while massive doses of radiation are known to be harmful, the small doses used in routine tests are usually safe, especially compared with other health-care choices people make without thinking twice.

Effect of radiation exposure on health

“Radiation does have some risk,” says Russ Ritenour, a medical physicist at the Medical University of South Carolina in Charleston. “But it is important for medicine. And in most cases, the risk is quite small compared to the risk of taking too much Advil over your life and other things like that.”

Ionizing radiation the type that can damage cells is a daily fact of life even for people who never go to the doctor. Rocks and soil contain radioactive materials, which also appear in our food, our bones and the air we breathe. Cosmic rays barrage us with radiation from space, with higher doses at altitude and on airplanes.

Overall, a person in the United States gets an annual average of about 3 millisieverts (mSv) of background radiation. (Millisieverts are units that measure radiation absorbed by our bodies.)

Added exposure, totaling another 3 mSV each year for the average American, comes from such man-made sources as power plants that run on coal and nuclear fuel, and consumer products including TVs and computer screens. But most of the extra radiation we get comes from X-rays and CT scans, Ritenour says.

Most routine diagnostic tests emit extremely small amounts of radiation. A patient will get about 0.001 mSv from an arm X-ray, 0.01 mSv from a from a panoramic dental X-ray, 0.1 mSv from a chest X-ray and 0.4 mSv from a mammogram, according to Harvard Medical School. (Those estimates vary somewhat, depending on the source and on the specific device used, the size of the patient and other factors.)

CT scans, which take multiple X-rays to create cross-sectional images, deliver higher doses: 7 mSv for a chest CT, and 12 mSV for a full-body scan, according to the National Cancer Institute. Studies have found doses of 25 mSv or more from a PET/CT, an imaging test that requires ingesting a radioactive substance.

With the increasing availability and affordability of imaging technologies, people are getting more tests than they used to. Today, Americans receive more than 85 million CT scans each year, compared with 3 million per year in the 1980s.