Radiation Exposure and Cancer

What is radiation?

Radiation is the emission (sending out) of energy from any source. X-rays are an example of radiation, but so is the light that comes from the sun and the heat that is constantly coming off our bodies.

When talking about radiation and cancer, many people think of specific kinds of radiation such as x-rays or the radiation made by nuclear reactors. But there are different types of radiation, and many of them are not linked to cancer.

Types of radiation

Radiation exists across a spectrum from very high-energy (high-frequency) radiation to very low-energy (low-frequency) radiation. This is sometimes referred to as the electromagnetic spectrum. From highest to lowest energy, the main forms of radiation are:

• Gamma rays
• X-rays
• Ultraviolet (UV) rays
• Visible light
• Infrared rays
• Microwaves
• Radiofrequency (radio) waves
• Extremely low-frequency (ELF) radiation

An important distinction that affects the health risks from radiation is whether the energy is ionizing or non-ionizing.

Ionizing radiation is high-frequency radiation that has enough energy to remove an electron from (ionize) an atom or molecule. Ionizing radiation has enough energy to damage the DNA in cells, which in turn may lead to cancer. Gamma rays, x-rays, some high-energy UV rays, and some sub-atomic particles such as alpha particles and protons are forms of ionizing radiation.

Non-ionizing radiation is low-frequency radiation that does not have enough energy to remove electrons or directly damage DNA. Low-energy UV rays, visible light, infrared rays, microwaves, and radio waves are all forms of non-ionizing radiation. Aside from UV rays, these types of radiation are not known to increase cancer risk.

It is important to understand the difference between these types of radiation. For example, the non-ionizing radiation given off by a cell phone or a television screen is not the same as the ionizing radiation you might get from x-rays taken in the hospital.

The types of radiation exposures discussed here include:

• Ionizing radiation from sources such as medical and other man-made forms of radiation, and natural background radiation from the earth and sky.
• Ultraviolet (UV) radiation from natural and man-made sources.
• Some sources of non-ionizing radiation, including power lines, cell phones, TV and computer screens, and microwaves.

Ionizing radiation

Ionizing radiation has enough energy to knock electrons off of atoms or molecules. This is called ionization. Ionized molecules are unstable and quickly undergo chemical changes.

If ionizing radiation passes through a cell in the body, it can lead to mutations (changes) in the cell's DNA, the part of the cell that contains its genes (blueprints). This could contribute to cancer, or to the death of the cell. The amount of damage in the cell is related to the dose of radiation it receives. The damage takes place in only a fraction of a second, but other changes such as the beginning of cancer may take years to develop.

Types of ionizing radiation include x-rays, gamma rays, some high-energy UV rays, and particles given off by radioactive materials such as alpha particles and protons. These forms of radiation have different energy levels and can penetrate cells to different extents, but all are capable of causing ionization.

Does ionizing radiation cause cancer?

Ionizing radiation is a proven human carcinogen (cancer causing agent). The evidence for this comes from many different sources, including studies of atomic bomb survivors in Japan, people exposed during the Chernobyl nuclear accident, people treated with high doses of radiation for cancer and other conditions, and people exposed to high levels of radiation at work, such as uranium miners.

Most studies on radiation and cancer risk have looked at people exposed to very high doses of radiation in the settings above. 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 radiation. 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 ionizing radiation increase cancer risk, although by a very small amount. In general, the risk of cancer from radiation exposure increases as the dose of radiation increases. Likewise, the lower the exposure is, the smaller the increase in risk. But there is no threshold below which ionizing radiation is thought to be totally safe.

Although radiation exposure affects the occurrence of various types of cancer, it does not affect their aggressiveness (tendency to grow and spread).

Types of cancer linked to ionizing radiation

Ionizing radiation increases the risk of certain types of cancer more than others.

The thyroid gland and bone marrow are particularly sensitive to radiation. Leukemia, a type of cancer that arises in the bone marrow, is the most common radiation-induced cancer. Leukemias may appear as early as a few years after radiation exposure.

Other types of cancer can also result from radiation exposure, although they may take longer to develop (usually at least 10 to 15 years). Some of the other cancers most strongly linked to radiation exposure in studies include:

• Lung cancer
• Skin cancer
• Thyroid cancer
• Multiple myeloma
• Breast cancer
• Stomach cancer

These are not necessarily the only cancer types that may be linked to radiation, however.

The types of cancer linked to radiation are also affected by the part of the body that is exposed. For example, people who get pelvic radiation therapy would not be expected to have higher rates of cancers in the head and neck because these areas weren't exposed to radiation.

Other factors may also play a role in how likely a person exposed to radiation is to develop cancer. Age is one such factor - children's growing bodies are more sensitive to radiation than adults. A person may also have gene changes that make their cells more vulnerable to radiation damage, which might in turn raise their risk more than in someone without these gene changes.

Sources of ionizing radiation

People may be exposed to ionizing radiation from 3 main sources:

• Natural background radiation comes from cosmic rays from our solar system and radioactive elements normally present in the soil. This is the major contributor to worldwide radiation exposure.
• Medical radiation comes in the form of diagnostic x-rays and other tests, as well as from radiation therapy. Radiation therapy is currently used to treat some types of cancer and involves dosages many thousand times higher than those used in diagnostic x-rays.
• Non-medical, man-made radiation can come from workplace and other sources, and is also a result of above ground nuclear weapons testing that took place before 1962.

Natural background radiation

We are all exposed to some amount of radiation just from being on this planet. This is known as background radiation. For most people, background radiation accounts for most of their exposure to ionizing radiation during the year. It comes from several different 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.

Radon: 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. For more detailed information on radon and its possible health effects, see our document, Radon.

Medical radiation

Ionizing radiation is used in the diagnosis and treatment of some medical conditions. This can be in the form of radiation that penetrates from outside the body, or radioactive particles that are swallowed or inserted into the 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 in order to create internal pictures of the body. (MRI and ultrasound exams do not use ionizing radiation.)

The increased risk of cancer from exposure to any single test is likely to be very small. Still, concerns have been raised in recent years as the average amount of radiation a person is exposed to from medical tests has risen. Children's growing bodies are especially sensitive to radiation.

Because of the very small but real risk, and the fact that radiation exposure from all sources can add up over one's lifetime, imaging tests that use radiation should only be done if there is a good medical reason to do so. The usefulness of the test must always be balanced against the possible risks from exposure to the radiation. In some cases, other imaging tests such as ultrasound or MRI may be an option. But if there is a reason to believe that an x-ray or CT scan is the best way to look for cancer or other diseases, the patient will most likely be helped more than the small dose of radiation can hurt.

For more detailed information on the possible radiation risks from imaging tests, see our document, Imaging (Radiology) Tests.

Radiation therapy: Ionizing radiation is an effective way to treat certain kinds 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 lead to DNA mutations in other cells that survive the radiation, which may eventually lead to the development of a second cancer.

Overall, radiation therapy alone does not appear to be a very strong cause of second cancers. This is probably due to the fact that doctors try to focus the radiation on the cancer cells as much as possible, which means few normal cells are exposed to radiation. Still, some studies have linked radiation therapy with an increased risk of leukemia, thyroid cancer, early-onset breast cancer, and some other cancers. The amount of increased risk depends on a number of factors, include the dose of radiation, the location in the body, and the age of the person getting it (younger people are generally at greater risk later on).

If cancer does develop after radiation therapy, it does not happen right away. For leukemias, most cases develop within 5 to 9 years after exposure. In contrast, other cancers often take much longer to develop. Most of these cancers are not seen for 10 years after radiation therapy, and some are diagnosed even more than 15 years later.

When considering radiation exposure from radiation therapy treatment, the benefits generally outweigh the risks. However, some combinations of radiation therapy and chemotherapy are more risky than others. Doctors do their best to ensure the treatment that is given destroys the cancer while minimizing the risk that a secondary cancer will develop later on.

For more information, see our document, Second Cancers Caused by Cancer Treatment.

Other forms of man-made radiation

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

Nuclear tests and facilities: The United States government conducted above-ground nuclear tests in the South Pacific and in the state of Nevada between 1945 and 1962. 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. More information on this topic is available in our document, Cancer Among Military Personnel Exposed to Nuclear Weapons.

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.

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

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 that smokers are exposed to.

Some building materials used in the home or other structures may contain low levels of naturally occurring radiation. This 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.

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 United States Department of Agriculture (USDA), irradiating food does not cause it to become radioactive and does not change nutritional value or flavor of the food.

Airport security scanners: In recent years, some airports have begun to use whole body scanners as a way to detect objects hidden by clothing. These scanners are different from the metal detectors most people are familiar with.

One type of body scanner, based on backscatter technology, uses very weak x-rays aimed at the surface of the body to capture a whole body image. The Transportation Security Administration (TSA) says the radiation from such a scan is about the same amount a person gets from cosmic rays when flying for 2 minutes in an airplane at 30,000 feet. A person would need to get more than 1,000 backscatter scans in a year to reach the same dose they'd get from a standard chest x-ray, according to the American College of Radiology (ACR).

The other type of body scanner is based on millimeter wave technology. Neither millimeter wave scanners nor metal detectors use ionizing radiation.