An official website of the United States government.

We've made some changes to If the information you are looking for is not here, you may be able to find it on the EPA Web Archive or the January 19, 2017 Web Snapshot.

Radiation Protection

Radiation Health Effects

Ionizing radiationHelpIonizing radiationRadiation with so much energy it can knock electrons out of atoms. Ionizing radiation can affect the atoms in living things, so it poses a health risk by damaging tissue and DNA in genes. has sufficient energy to affect the atoms in living cells and thereby damage their genetic material (DNA). Fortunately, the cells in our bodies are extremely efficient at repairing this damage. However, if the damage is not repaired correctly, a cell may die or eventually become cancerous. Related information in Spanish (Información relacionada en español).

Exposure to very high levels of radiation, such as being close to an atomic blast, can cause acute health effects such as skin burns and acute radiation syndrome (“radiation sickness"). It can also result in long-term health effects such as cancer and cardiovascular disease. Exposure to low levels of radiation encountered in the environment does not cause immediate health effects, but is a minor contributor to our overall cancer risk.

Visit the U.S. Centers for Disease Control and Prevention (CDC) for more information about possible health effects of radiation exposure and contamination.

On this page:

Acute Radiation Syndrome from Large Exposures

A very high level of radiation exposure delivered over a short period of time can cause symptoms such as nausea and vomiting within hours and can sometimes result in death over the following days or weeks. This is known as acute radiation syndrome, commonly known as “radiation sickness.”

It takes a very high radiation exposure to cause acute radiation syndrome—more than 75 radHelpradThe U.S. unit used to measure absorbed radiation dose (the amount of radiation absorbed by an object or person). The international equivalent is the Gray (Gy). One hundred rads are equal to 1 Gray. (0.75 grayHelpgrayA gray is the international unit used to measure absorbed dose (the amount of radiation absorbed by an object or person). The U.S. unit for absorbed dose is the rad. One gray is equal to 100 rads.) in a short time span (minutes to hours). This level of radiation would be like getting the radiation from 18,000 chest x-rays distributed over your entire body in this short period. Acute radiation syndrome is rare, and comes from extreme events like a nuclear explosion or accidental handling or rupture of a highly radioactive source.

View CDC Fact Sheet:  Acute Radiation Syndrome (ARS).

Learn about protecting yourself from radiation.

Learn about radiation sources and doses.

Top of Page

Radiation Exposure and Cancer Risk

Exposure to low-levels of radiation does not cause immediate health effects, but can increase the risk of cancer over a lifetime. There have been studies that kept track of large numbers of people who were exposed to radiation, including atomic bomb survivors and radiation industry workers. These studies show that radiation exposure increases the chance of getting cancer, and the risk increases as the dose increases:  the higher the dose, the greater the risk. Conversely, cancer risk declines as the dose falls: the lower the dose, the lower the risk. According to radiation safety experts, radiation exposures of 5–10 remHelpremThe U.S. unit to measure effective dose. The international unit is sieverts (Sv). (5,000–10,000 millirem or 50–100 millisieverts) usually result in no harmful health effects, because radiation below these levels is a minor contributor to our overall cancer risk.

The mission of EPA’s Radiation Protection Program is to protect human health and the environment from unnecessary exposure to radiation. EPA radiation protection standards are designed to be low enough to limit the population’s risk of cancer from radiation over a lifetime. Learn about EPA's role in radiation protection.

Calculate your radiation dose.

Learn about sources and doses.

Limiting Cancer Risk from Radiation in the Environment

EPA bases its regulatory limits and nonregulatory guidelines for public exposure to low level ionizing radiation on the linear no-threshold (LNT) model. The LNT model assumes that the risk of cancer due to a low-dose exposure is proportional to dose, with no threshold. In other words, cutting the dose in half cuts the risk in half.

The use of the LNT model for radiation protection purposes has been repeatedly recommended by authoritative scientific advisory bodies, including the National Academy of Sciences and the National Council on Radiation Protection and Measurements Exit. There is evidence to support LNT from laboratory data and from studies of cancer in people exposed to radiation.

Top of Page

Exposure Pathways

Understanding the type of radiation received, the way a person is exposed (external vs. internal), and for how long a person is exposed are all important in estimating health effects.

The risk from exposure to a particular radionuclide depends on:

  • The energy of the radiation it emits.
  • The type of radiation (alpha, beta, gamma, x-rays ).
  • Its activity (how often it emits radiation).
  • The rate at which the body metabolizes and eliminates the radionuclide following ingestion or inhalation.
  • Where the radionuclide concentrates in the body and how long it stays there.
  • Whether exposure is external or internal:
    • External exposure is when the radioactive source is outside of your body. X-rays and gamma rays can pass through your body, depositing energy as they go.
    • Internal exposure is when radioactive material gets inside the body by eating, drinking, breathing or injection (from certain medical procedures). Radionuclides may pose a serious health threat if significant quantities are inhaled or ingested.

Learn more about alpha particles, beta particles, gamma rays and x-rays.

Top of Page

Sensitive Populations

Children and fetuses are especially sensitive to radiation exposure. The cells in children and fetuses divide rapidly, providing more opportunity for radiation to disrupt the process and cause cell damage. EPA considers these differences in sensitivity due to age and sex when revising radiation protection standards.

Top of Page