Law and Public Safety

Law and Public Safety

Radiation: Frequently Asked Questions

How does precipitation affect radiation levels in the environment?

It is very common for natural background radiation levels to rise during precipitation events such as rain, sleet or snow. Radon and radon decay products, which occur naturally and normally dissipate in the air, are held closer to the ground by cloud cover, pressure changes, and may also be captured in the precipitation itself and brought to the ground. This causes the gamma radiation levels to rise in some measurements.


How do I protect myself in the event of a radiation emergency?

TIME: The amount of radiation exposure increases and decreases with the time people spend near the source of radiation. In general, we think of the exposure time as how long a person is near radioactive material. It's easy to understand how to minimize the time for external (direct) exposure. Gamma and x-rays are the primary concern for external exposure. However, if radioactive material gets inside your body, you can't move away from it. You have to wait until it decays or until your body can eliminate it. When this happens, the biological half-life of the radionuclide controls the time of exposure. Biological half-life is the amount of time it takes the body to eliminate one half of the radionuclide initially present. Alpha and beta particles are the main concern for internal exposure.

DISTANCE: The farther away people are from a radiation source, the less their exposure. How close to a source of radiation can you be without getting a high exposure? It depends on the energy of the radiation and the size (or activity) of the source. Distance is a prime concern when dealing with gamma rays, because they can travel long distances. Alpha and beta particles don't have enough energy to travel very far. As a rule, if you double the distance, you reduce the exposure by a factor of four. Halving the distance, increases the exposure by a factor of four.
SHIELDING: The greater the shielding around a radiation source, the smaller the exposure. Shielding simply means having something that will absorb radiation between you and the source of the radiation (but using another person to absorb the radiation doesn't count as shielding). The amount of shielding required to protect against different kinds of radiation depends on how much energy they have.

  • (Alpha) A thin piece of light material, such as paper, or even the dead cells in the outer layer of human skin provides adequate shielding because alpha particles can't penetrate it. However, living tissue inside body, offers no protection against inhaled or ingested alpha emitters.


  • (Beta) Additional covering, for example heavy clothing, is necessary to protect against beta-emitters. Some beta particles can penetrate and burn the skin.


  • (Gamma) Thick, dense shielding, such as lead, is necessary to protect against gamma rays. The higher the energy of the gamma ray, the thicker the lead must be. X-rays pose a similar challenge, so x-ray technicians often give patients receiving medical or dental X-rays a lead apron to cover other parts of their body.

What is being done to track radiation in our environment from Fukushima?

EPA’s RadNet system continuously monitors radiation levels in the air throughout the U.S., 24 hours a day, 7 days a week. RadNet sample analyses and monitoring results of precipitation, drinking water, and milk provide baseline data on background levels of radiation in the environment and can detect increased radiation from radiological incidents. RadNet has not found any radioactive elements associated with the damaged Japanese reactors since late 2011, and even then, the levels found were very low—always well below any level of public health concern. You can see near real-time air monitoring results and download sample analysis results at: www.epa.gov/radnet. The EPA, along with the National Oceanic and Atmospheric Administration (NOAA), the Food and Drug Administration (FDA), and the Nuclear Regulatory Commission (NRC) are working together to carefully follow the situation at Fukushima.


How does radiation cause health effects?

Living tissue in the human body can be damaged by ionizing radiation. The body attempts to repair the damage, but sometimes the damage is too severe or widespread, or mistakes are made in the natural repair process.


What does radiation dose mean?

Radiation dose identifies the quantity of radiation or energy absorbed. It is measured in Roentgens. Dose may be looked at or calculated in various ways: Absorbed Dose: The absorbed dose is the amount of energy deposited per unit of mass (e.g., ergs per gram). It has traditionally been measured in rads (Roentgen Absorbed Dose), which is being replaced by grays (100 rad). Equivalent Dose: This is the absorbed dose adjusted for the relative biological effect of the type of radiation being measured (a certain amount of one kind of radiation can cause more harm to living tissue than the same amount of a different kind of radiation). Equivalent dose is measured in rem (Roentgen Equivalent Man). Committed Dose: This is a dose that accounts for continuing exposures over long periods of time (such as 30, 50, or 70 years). For more information on radiation terminology, click here to go to the EPA's Radiation Glossary.


What limits does EPA set on exposure to radiation?

Health physicists generally agree on limiting a person's exposure beyond background radiation to about 100 mrem per year from all sources. Exceptions are occupational, medical or accidental exposures. (Medical X-rays generally deliver less than 10 mrem). EPA and other regulatory agencies generally limit exposures from specific source to the public to levels well under 100 mrem. This is far below the exposure levels that cause acute health effects.


For more information, go to the EPA's radiation protection site.


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