Skip to main content

ACS & ASCO are Stronger Together: Cancer.Net content is now available on



Cancer Risk and Prevention

How Are People Exposed to X-rays and Gamma Rays?

People can be exposed to x-rays and gamma rays from 3 main sources:

  • Natural background radiation from cosmic rays from outer space and from radioactive elements normally in rocks and soil. This is the major contributor to worldwide radiation exposure.
  • Medical radiation used for imaging tests such as x-rays, CT scans, and PET scans, as well as for radiation therapy. Radiation therapy is used to treat some types of cancer, and it generally uses doses much higher than those used in imaging tests.
  • Non-medical, man-made radiation exposure can happen in certain workplaces, or in communities as a result of above ground nuclear weapons testing or nuclear accidents. Radiation is also used in small amounts in some consumer products, as well as in food irradiation.

Natural background radiation

We are all exposed to some amount of radiation just from being on this planet. This is known as background radiation. In the United States this averages about 3 millisieverts (mSv) per year. For most people, background radiation is most of their exposure to ionizing radiation during the year. It comes from several sources.

Cosmic rays

Cosmic rays are radioactive particles that hit the earth from outer space. They come from the sun and from other stars. The earth’s atmosphere blocks a portion of these rays, but some of them reach the ground.

Because the atmosphere blocks some cosmic rays, exposure is greater at higher altitudes. For example, people who live in Denver, Colorado, which is at a high elevation, are exposed to slightly more cosmic rays than people living at sea level.

People are also exposed to higher levels of cosmic rays during airplane flights. Airline pilots and flight attendants, who spend many hours at high elevations, are exposed to more of these rays, but it is not clear if they have an increased risk of cancer because of it.

Radiation in the earth

People are also exposed to small amounts of radiation from radioactive elements that occur naturally in rocks and soil. Some of these may end up in building materials used in houses and other structures. Tiny amounts of radiation may even be found in drinking water and in some plant-based foods as a result of being in contact with the soil. For people who smoke, tobacco can account for a significant portion of the yearly radiation they receive.


The largest source of natural background radiation for most people is radon. This is an odorless, colorless gas that is formed from the breakdown of radioactive elements in the ground. Radon levels are usually higher inside buildings and homes, especially in levels closer to the ground such as basements. Radon levels can vary a great deal, depending on where you live. Some people, such as people who work in mines, are exposed to higher radon levels at work.

For more on radon and its possible health effects, see Radon and Cancer.

Medical radiation

X-rays, gamma rays, and other forms of ionizing radiation are used to diagnose and treat some medical conditions. The radiation can be directed at your body from a machine, or radioactive particles can be swallowed or put into your body.

Imaging tests

Certain types of imaging tests, such as x-rays, CT scans, and nuclear medicine tests (such as PET scans and bone scans) expose people to low levels of radiation to create pictures of the inside of the body. (Some imaging tests, such as MRI and ultrasound, do not expose people to ionizing radiation.)

In adults: The amount of radiation a person is exposed to varies depending on the test, as well as on a person's size. For example, the exposure for an averaged-sized adult from a 2-view chest x-ray is about 0.1 mSv, while exposure from a CT scan of the chest is about 6 mSv. The exposure from a PET/CT scan (which combines a PET scan of the body with a CT scan) is typically about 23 mSv. Fluoroscopy, which uses x-rays to make real-time moving images, exposes people to different amounts of radiation depending on how long it is used. The amount of radiation used in many imaging tests has gone down over time as technology has improved.

In children: Exposure levels for a child can be higher than they would be for an adult when using the same amount of radiation, so the settings on the scanner need to be adjusted to account for body size.

For children, exposure to radiation from imaging tests is of particular concern, because:

  • Children (especially younger children) are much more sensitive to radiation than adults.
  • Children are expected to live longer than adults, so they have more time to develop problems from radiation exposure.
  • With tests like CT scans, children might get a higher radiation dose than necessary if the CT scanner settings are not adjusted for their smaller body size.

These factors mean that for a young child, the risk of developing a radiation-related cancer could be several times higher than for an adult exposed to the same imaging test.

The risks from these tests aren't known for sure, but to be safe, most doctors recommend that children only get these tests when they're absolutely needed, and that when such tests are done, they use only the minimum amount of radiation needed to get the image.

To learn more, see Understanding Radiation Risk from Imaging Tests.

Radiation therapy

X-rays, gamma rays, and other forms of ionizing radiation are an effective way to treat some types of cancer. During radiation therapy, high doses of ionizing radiation (much higher than those used for imaging tests) are directed at the cancer, resulting in the death of the cancer cells. However, this can sometimes lead to DNA changes (mutations) in other cells that survive the radiation, which may eventually lead to the development of a second cancer. Radiation therapy is also sometimes used to treat serious medical conditions besides cancer.

For more information about cancer risks from radiation therapy for cancer, see Second Cancers Related to Treatment.

Non-medical sources of man-made radiation

People may also be exposed to ionizing radiation from non-medical man-made sources.

Nuclear weapons

The atomic bombs dropped on Nagasaki and Hiroshima, Japan, exposed many people to radiation from x-rays, gamma rays, and neutrons. Some people died fairly quickly as a result of burns and radiation sickness, but many survived. The survivors were exposed to different amounts of radiation, depending largely on how far they were from the explosions. Much of the information that we have about radiation and cancer risk comes from studies of these survivors.

The United States government conducted above-ground nuclear tests in the South Pacific and in the state of Nevada between 1945 and 1962. Other countries have also conducted above-ground tests. Many people in the military at the time were part of training exercises in the area and were exposed to ionizing radiation from these tests. Others were exposed to radiation while working at facilities making the bombs or at other nuclear sites.

Non-military people living near or downwind of nuclear test sites may have also been exposed to radioactive byproducts. Levels of radiation are likely to be higher near these sites, but some radioactive particles from the tests entered the atmosphere and traveled great distances, landing thousands of miles away from the original site. While exposure levels were likely to be higher at the time of testing, some radiation in the soil today is the result of these tests.

Government programs have been set up to give financial support to people who were exposed to nuclear weapons testing and developed cancer. 

Nuclear power plants

Emissions of radiation from nuclear power plants are carefully monitored and controlled. According to the US Environmental Protection Agency (EPA), nuclear power plant operations account for less than 1/100 of 1% of the average American’s total radiation exposure.

Nuclear power plant accidents: Accidents at nuclear power plants are rare, but they could possibly expose people to high levels of radiation.

In 1986, an accident at the nuclear power plant at Chernobyl (in Ukraine) exposed millions of people living in the area to radiation, either directly or from radioactive elements released into the air that ended up deposited on the ground. The emergency clean-up workers were exposed to the highest levels of radiation.

In 2011, an earthquake and tsunami struck the coast of Japan that resulted in damage to the Fukushima Daiichi Nuclear Power Plant in Fukushima Prefecture, Japan. Radiation was released into the air, contaminating soil, food, and water (both fresh and seawater). An area of more than 300 square miles around the plant was found to be contaminated with radiation, although at lower levels than within the plant. Because of high radiation levels, many areas were evacuated. The health effects of this disaster are still being studied.

Workplace exposures

Some people can be exposed to radiation at work. For example:

  • People who work in nuclear power plants may be exposed to higher levels of radiation than the general public, although their exposure levels are monitored carefully.
  • People who work in uranium mines are monitored because of their exposure to radiation in the form of radon.
  • People who work in health or dental care, particularly those who work with x-ray (or other imaging test) equipment or who work with radioactive isotopes, may also be exposed to radiation at work. Radiation exposure may also occur at some research labs.

In the United States, people who are likely to be exposed to radiation in the workplace are monitored carefully. Exposure is limited to an effective dose of 100 mSv over 5 years, with a maximum of 50 mSv in any single year.

Consumer products

Some consumer products contain small amounts of ionizing radiation.

For example, tobacco products contain low levels of radiation, which may come from the soil it’s grown in or the fertilizer used to help it grow. Tobacco may account for a significant portion of the yearly radiation exposure for people who smoke.

Some building materials used in the home or other structures may contain low levels of naturally occurring radiation, which can be given off in the form of radon gas. The amount of radiation can vary depending on what they’re made of, but the levels are unlikely to contribute much to a person’s overall exposure to radiation, according to the EPA. To learn more, see Radon and Cancer.

Many smoke detectors contain a small amount of a very low-level radioactive material that helps detect the smoke. This material is sealed in a container and does not pose a significant risk of radiation exposure.

Food irradiation

Ionizing radiation can be used to kill bacteria and other germs on certain foods, which may make them safer to eat and help them last longer. Some people may be concerned that irradiated food may itself contain radiation.

It’s important to understand that the radiation does not stay in the food. According to the US Food and Drug Administration (FDA), irradiating food does not make it radioactive and does not change its nutritional value, nor does it noticeably change the taste, texture, or appearance of the food.

The American Cancer Society medical and editorial content team

Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as editors and translators with extensive experience in medical writing.

American College of Radiology/Radiological Society of North America. Radiation Dose. 2021. Accessed at on November 4, 2022

Brenner DJ, Hall EJ. Computed tomography--an increasing source of radiation exposure. N Engl J Med. 2007;357:2277-2284.

Cardis E, Howe G, Ron E, et al. Cancer consequences of the Chernobyl accident: 20 years on. J Radiol Prot. 2006;26:127-140.

National Toxicology Program. Ionizing radiation: x-radiation and gamma radiation. Rep Carcinog. 2011;12:237-240.

National Cancer Institute. Radiation Risks and Pediatric Computed Tomography (CT): A Guide for Health Care Providers. 2018. Accessed at on November 3, 2022.

Ozasa K, Shimizu Y, Sakata R, et al. Risk of cancer and non-cancer diseases in the atomic bomb survivors. Radiat Prot Dosimetry. 2011;146:272-275.

US Food and Drug Administration. Food Irradiation: What You Need to Know. 2022. Accessed at on November 3, 2022.

Zablotska LB, Bazyka D, Lubin JH, et al. Radiation and the risk of chronic lymphocytic and other leukemias among Chernobyl cleanup workers. Environ Health Perspect. 2013 Jan;121(1):59-65. Epub 2012 Oct 24.

Last Revised: November 10, 2022

American Cancer Society Emails

Sign up to stay up-to-date with news, valuable information, and ways to get involved with the American Cancer Society.