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It is harder to measure the much smaller increase in cancer risk that might come from much lower levels of radiation exposure. Most studies have not been able to detect an increased risk of cancer among people exposed to low levels of x-rays or gamma rays. For example, people living at high altitudes, who are exposed to more natural background radiation from cosmic rays than people living at sea level, do not have noticeably higher cancer rates.

Still, most scientists and regulatory agencies agree that even small doses of gamma and x-radiation can increase cancer risk, although most likely by a very small amount. In general, the lower the exposure dose is, the smaller the increase in risk. But there is no threshold below which this kind of radiation is thought to be totally safe.

What do studies show?

Atomic bomb survivors

Much of what we know about cancer risks from radiation is based on studies of the survivors of the atomic bombs in Nagasaki and Hiroshima. These people had higher risks of some, but not all cancers. Studies have found an increased risk of the following cancers (from higher to lower risk):

• Most types of leukemia (although not chronic lymphocytic leukemia)
• Multiple myeloma
• Thyroid cancer
• Bladder cancer
• Breast cancer
• Lung cancer
• Ovarian cancer
• Colon cancer (but not rectal cancer)
• Esophageal cancer
• Stomach cancer
• Liver cancer
• Lymphoma
• Skin cancer (besides melanoma)

For most of these cancers, the risk was found to be highest for those exposed as children, and was lower as the age at exposure increased. Those exposed while still in the womb (in utero) had lower risks than those exposed during childhood.

Exposure to higher doses of radiation exposure was linked to higher risk of cancer, but even low amounts of radiation were linked to an increased risk of getting and dying from cancer. There was no clear cut-off for a safe level of radiation exposure.

These cancers took years to develop, but some cancers appeared sooner than others. Deaths from leukemia started to go up about 2 to 3 years after exposure, with the number of cases peaking after about 10 years and going down after that. Solid tumors took longer to develop. For example, excess deaths from lung cancer began to be seen about 20 years after exposure.

Chernobyl accident

Children and teens living near the Chernobyl plant at the time of the accident had an increased risk of thyroid cancer linked to exposure to radioactive iodine. This increased risk was not seen in adults living in the area.

Workers in cleanup operations from 1986 to 1990 had an increased risk of leukemia (all types). These people had higher and more prolonged radiation exposures than the people living near the plant.

Nuclear weapons testing

Studies suggest that some people who were children when above-ground nuclear testing was being done in the US may develop thyroid cancer as a result of exposure to radioactive iodine in milk.

Radiation therapy

To treat benign conditions

Radiation therapy is now used mainly to treat cancer. But in the past, before the risks of radiation were clearer, it was also used to treat some benign (non-cancerous) diseases. Studies of people treated for these conditions have helped us learn about how radiation affects cancer risk.

For example, some studies have suggested links between:

• The use of radiation to treat peptic ulcer disease and a higher risk of cancers of the stomach and pancreas.
• The use of radiation to treat ringworm of the scalp (a fungal infection, also called tinea capitis) and an increased risk of basal cell skin cancers.
• The injection of radium to treat ankylosing spondylitis (an autoimmune disease) and an increased risk of leukemia, bone sarcoma, and possibly some other cancers.
• The use of radiation to treat some benign head and neck conditions and a higher risk of cancers of the salivary gland, brain and spinal cord tumors, and thyroid cancer.

To treat cancer

Studies have linked radiation therapy to treat cancer with an increased risk of leukemia, thyroid cancer, early-onset breast cancer, and some other cancers later in life. The increase in risk depends on a number of factors, including:

• The dose of radiation
• The part of the body being treated
• The person's age (younger people are generally at greater risk)
• The use of other treatments such as chemotherapy at or around the same time

Other factors might also play a role in how likely it is that a person exposed to radiation will develop cancer. For example, some genetic conditions can make a person’s cells more vulnerable to radiation damage, which might in turn raise their risk more than in someone without these gene changes.

Cancers after radiation therapy don't happen right away

If cancer does develop after radiation therapy, it doesn't happen right away. Most leukemias develop within about 5 to 9 years after exposure. In contrast, most other cancers are not seen for at least 10 years after radiation therapy, and some are diagnosed more than 15 years later.

When considering getting radiation therapy to treat cancer, the benefits generally outweigh the risks. Overall, radiation therapy alone does not appear to be a very strong cause of second cancers. This is probably because doctors focus the radiation on the cancer cells as much as possible, while limiting the exposure of nearby normal cells. Doctors do their best to ensure the treatment destroys the cancer while limiting the risk that a second cancer will develop later on.

For more information, see Second Cancers Related to Treatment.

Imaging tests

Some studies have estimated the risk of radiation exposure from imaging tests based on the risks from similar amounts of radiation exposure in the studies of the atomic bomb survivors. Based on these studies, the US Food and Drug Administration (FDA) estimates that exposure to 10 millisieverts (mSv) from an imaging test would be expected to increase the risk of death from cancer by about 1 chance in 2,000.

It can be hard to study cancer risks from imaging tests that use radiation. In order to find a small increase in risk (such as 1 in 2,000), a study would have to look at tens of thousands, or even hundreds of thousands of people. For each person, information about other exposures that could affect cancer risk would need to be collected, to help ensure any increase in cancer risk came from the radiation exposure and not something else. And because cancers caused by radiation take many years to develop, the study would need to follow people for decades.

Often, scientists use other types of studies that can be done more quickly and require fewer resources, but the conclusions from these types of studies often are not as strong.

For example, researchers might use questionnaire studies to look for possible causes of cancer. These studies compare exposures among people who have a certain cancer to those who don’t. Or they may compare people who had a certain exposure (like radiation) to those who didn’t. However, this is hard to do for radiation exposure from imaging tests, because people often can't accurately recall information about things that happened many years before (such as in childhood), and information about all the imaging tests they've had is often not available. There is also a concern that people with cancer are more likely to report exposures that they think might have affected their cancer risk than people who do not have cancer, which can affect study results.

Studies that have found an increased risk of cancer after imaging tests that use radiation often involve people who have had many imaging tests or high-dose procedures. For example:

• Studies of women who had been imaged many times with fluoroscopy as a teen or young woman during treatment for tuberculosis have found an increased risk of breast cancer years later.
• Teens and young women who had many x-rays of the spine to monitor scoliosis have been found to have an increased risk of breast cancer later on.
• People with meningioma (a brain tumor that is most often benign) have been found to be more likely to have had certain types of dental x-rays every year.
• Some studies have suggested a link between higher doses of radiation from CT scans in children to increased risks of leukemia and brain tumors, although the overall risk was still low.

What do expert agencies say?

Several national and international agencies study different exposures in the environment to determine if they can cause cancer. (An exposure that causes cancer or helps cancer grow is called a carcinogen.) The American Cancer Society looks to these organizations to evaluate the risks, based on the available evidence.

Based on studies done on people and studies done in the lab, several expert agencies have evaluated the cancer-causing nature of x-rays and gamma rays.

The International Agency for Research on Cancer (IARC) is part of the World Health Organization (WHO). One of its major goals is to identify causes of cancer. Based on the data available, IARC classifies x- and gamma radiation as a “known human carcinogen.”

The National Toxicology Program (NTP) is an interagency program of different US government agencies, including the National Institutes of Health (NIH), the Centers for Disease Control and Prevention (CDC), and the Food and Drug Administration (FDA). The NTP has classified x- and gamma radiation as “known to be a human carcinogen.”

The US Environmental Protection Agency (EPA) sets limits for exposure to x-rays and gamma rays in part because it recognizes that this form of radiation can cause cancer.

For more information on the classification systems used by these agencies, see Determining if Something Is a Carcinogen and Known and Probable Human Carcinogens.