We've made some changes to EPA.gov. 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

The mission of EPA’s Radiation Protection Program is to protect human health and the environment from unnecessary exposure to radiation. This page provides basic information about the health effects of radiation. EPA uses current scientific understanding of the health effects of radiation exposure to create protective standards and guidance.

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 cause chemical changes in cells and damage them. Some cells may die or become abnormal, either temporarily or permanently. By damaging the genetic material (DNA) contained in the body’s cells, radiation can cause cancer. Fortunately, our bodies are extremely efficient at repairing cell damage. The extent of the damage to the cells depends upon the amount and duration of the exposure, as well as the organs exposed.

A very large amount of radiation exposure (acute exposureHelpacute exposureExposure to an amount of radiation all at once or from multiple exposures in a short period of time. In most cases, a large acute exposure to radiation causes both immediate (see radiation sickness) and delayed effects (cancer or death).), can cause sickness or even death within hours or days. Such acute exposures are extremely rare.

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:

Chronic Exposure

In general, the amount and duration of radiation exposure affects the severity or type of health effect. There are two broad categories of health effects: chronic (long-term) and acute (short-term).

Chronic exposureHelpChronic exposureContinuous or intermittent exposure to radiation over a long period of time. With chronic exposure, there is a delay between the start of the exposure and the observed health effect, such as cancer, benign tumors, cataracts, and potentially harmful genetic changes. is continuous or intermittent exposure to radiation over a long period of time. With chronic exposure, there is a delay between the exposure and the observed health effect. These effects can include cancer and other health outcomes such as benign tumors, cataracts, and potentially harmful genetic changes.

Low Levels of Radiation Exposure

Current science suggests there is some cancer riskHelpriskThe probability of injury, disease or death from exposure to a hazard. Radiation risk may refer to all excess cancers caused by radiation exposure (incidence risk ) or only excess fatal cancers (mortality risk). Risk may be expressed as a percent, a fraction, or a decimal value. For example, a 1% excess risk of cancer incidence is the same as a 1 in a hundred (1/100) risk or a risk of 0.01. from any exposure to radiation. However, it is very hard to tell whether a particular cancer was caused by very low doses of radiation or by something else. While experts disagree over the exact definition and effects of “low dose,” U.S. radiation protection standards are based on the premise that any radiation dose carries some risk, and that risk increases directly with dose. This method of estimating risk is called the "linear no-threshold model (LNTHelpLNTThe assumption that the risk of cancer increases linearly as radiation dose increases. This means, for example, that doubling the dose doubles the risk and that even a small dose could result in a correspondingly small risk. Using current science, it is impossible to know what the actual risks are at very small doses.)." The risk of cancer from radiation also depends on age, sex, and factors such as tobacco use.

Top of Page

Acute Effects

Acute health effects occur when large parts of the body are exposed to a large amount of radiation. The large exposure can occur all at once or from multiple exposures in a short period of time. Instances of acute effects from environmental sources are very rare. Examples include accidentally handling a strong industrial radiation source or extreme events like nuclear explosions. Learn about protecting yourself from radiation.

It takes a large radiation exposure — more than 75 rad HelpradThe 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.— in a short amount of time to cause acute health effects like radiation sicknessHelpradiation sicknessA serious illness that can happen when a person is exposed to very high levels of radiation, usually over a short period of time.. (This level of radiation would be equivalent to an x-ray dose of 75,000 milliremHelpmilliremThe millirem is the U.S. unit used to measure effective dose. One millirem equals 0.001 rem. The international unit is milliSievert (mSv).. For comparison, the average dose from an adult chest x-ray is about 4 millirem. See Radiation Sources and Doses for more information about radiation sources.)

Exposures between 5 and 10 rad usually result in no acute health effects. However, exposures in this range slightly increase the risk of getting cancer in the future.

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

Top of Page

Exposure Pathways

Understanding the type of radiation received (alphaHelpalpha particleA form of particulate ionizing radiation made up of two neutrons and two protons. Alpha particles pose no direct or external radiation threat; however, they can pose a serious health threat if ingested or inhaled., betaHelpbeta particleA form of particulate ionizing radiation made up of small, fast-moving particles. Some beta particles are capable of penetrating the skin and causing damage such as skin burns. Beta-emitters are most hazardous when they are inhaled or swallowed., gammaHelpgamma raysA form of ionizing radiation that is made up of weightless packets of energy called photons. Gamma rays can pass completely through the human body; as they pass through, they can cause damage to tissue and DNA., x-rayHelpx-raysA form of ionizing radiation made up of photons. X-rays are capable of passing completely through the human body. Medical x-rays are the single largest source of man-made radiation exposure.), 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 radioactive element depends on:

The risk that exposure to a radioactive element will cause a particular health effect also depends on whether exposure is internal or external.

  • Internal exposure is when radioactive material gets inside the body by eating, drinking, breathing or injection (from certain medical procedures). Alpha and beta particles pose a serious health threat if significant quantities are inhaled or ingested. Outside the body, alpha particles are too large to pass through the skin or a thin layer of clothes.
  • External exposure (also known as direct 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.

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

Top of Page

Sensitive Populations

Pregnant women and children 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 accounts for these differences in sensitivity due to age and sex when revising radiation protection standards.

Top of Page