Radiation Terms and Units | US EPA (2024)

Scientists measure radiation in different ways. Sometimes, they measure the dose that a person receives from a radioactive source, and sometimes they measure the amount of radioactivity in water, or in soil, or in the air. These measurements are taken to determine if safety actions are needed.

There are different but interrelated units for measuring radioactivity and estimating health effects.

On this page:

  • Radioactivity
  • Absorbed Dose
  • Effective Dose


Radioactivityis the release of ionizing radiation that occurs when the nucleus of a radioactive atom decays. Activity refers to the frequency of radioactive decay (disintegrations per unit time) produced by a given amount of radioactive material. The ionizing radiation released can be in the form of alpha particles, beta particles, or gamma rays. The different forms of ionizing radiation have different potentials to damage human tissue.Learn more about different types of ionizing radiation here.

A material's radioactivity is measured in becquerels (Bq, international unit) and curies (Ci, U.S. unit). Because a curie is a large unit, radioactivity results are usually shown in picocuries (pCi). A picocurie is one trillionth of a curie. The higher the number, the more radiation released by the material.

Radiation Terms and Units | US EPA (1)

Radiation Terms and Units | US EPA (2)


  • The natural radium-226 level of surface water generally ranges from 0.0037 to 0.0185 becquerels per liter (Bq/L), or 0.1 to 0.5 picocuries per liter (pCi/L).1
  • The radium limit in drinking water for daily consumption is 0.185 becquerels per liter (Bq/L), or 5.0 picocuries per liter (pCi/L).2

Unit Conversions and Calculations


Becquerel (Bq) | International or SI unit

Curie (Ci)|U.S. unit

How the unit is derivedRadioactivity represents the rate of radioactive decay. One becquerel (Bq) is equal to one radioactive decay per second.One curie (Ci) is the approximate number of radioactive decays in one gram of radium per second – approximately 3.7 x 1010decays per second.
Conversions1 becquerel (Bq)= 2.703×10−11curie (Ci)1 curie (Ci)= 3.7 x 1010becquerel (Bq)
Common Metric Prefixes

1gigabecquerel(GBq) = 1,000,000,000 Bq

1megabecquerel (MBq) =1,000,000Bq

1 kilobecquerel (kBq) =1,000Bq

1 millicurie (mCi) = 0.001 Ci

1 microcurie (µci) = 0.000 001 Ci

1picocurie (pCi)= 0.000 000 000 001 Ci

Absorbed Dose

Absorbed dosedescribes the amount of energydeposited per unit mass in an object or person. The units for absorbed dose are gray(Gy, international unit) and rad (rad, U.S. unit).

Radiation Terms and Units | US EPA (3)

Radiation Terms and Units | US EPA (4)


  • A dose to the lens of the eyes from a brain CT scan is about 60 milligray (mGy) or 6 rad.
  • A dose to the thyroid from a chest CT scan is about 10 milligray (mGy) or 1 rad.3

Unit Conversions and Calculations


Gray (Gy) | International or SI unit

Rad (rad) | U.S. unit

How the unit is derived

Absorbed dose is energy per unit mass

1 gray (Gy) = 1 joule(J)/kilogram(kg)

1 rad = 0.01 joule(J)/kilogram(kg)
Conversions1gray (Gy)= 100 rad1 rad = 0.01 gray (Gy)
Common Metric Prefixes

1 centigray (cGy) = 0.01 Gy

1 milligray(mGy) = 0.001 Gy

1 millirad (mrad) = 0.001rad

1 kilorad (krad) = 1,000 rad

Effective Dose

Effective dose takesthe absorbed dose(see above) and adjusts it forradiationtypeand relative organ sensitivity. The result is an indicator for the potential for long-term health effects (i.e., cancer and hereditary effects) from an exposure. It is used to set regulatory limits that protect against long-term health effects in a population. It also allows experts to compare anticipated health effects from different exposure situations. Because this value is a calculated approximation, not a physical quantity, it cannot be used to predict individual health effects. The units for effective dose are sievert (Sv, international unit) and rem (rem, U.S. unit).

Radiation Terms and Units | US EPA (5)

Radiation Terms and Units | US EPA (6)


  • The annual radiation dose limit for workers is 0.05 sieverts (Sv) or 5 rem.4
  • During an emergency, the guidance for when to evacuate or shelter in place is when the total projected dose exceeds 10-50 millisieverts (mSv) or 1-5 rem over the course of four days.5

Unit Conversions and Calculations

TopicSievert (Sv) | International or SI unitRem (rem) | U.S. unit
Conversions1sievert (Sv)= 100 rem1 rem = 0.01sievert (Sv)
Common Metric Prefixes

1 millisievert (mSv) = 0.001 Sv

1 microsievert (µSv) = 0.000 001 Sv

1 millirem (mrem) = 0.001 rem

1 microrem (µrem) = 0.000 001 rem

Use the Radiation Dose Calculator to estimate your yearlydosefrom sources ofionizing radiation.

View a PDF version of the Radiation Terms and Units infographic here.

1Agency for Toxic Substances and Disease Registry, 1990. "Toxicological Profile for Radium" (p. 53). Retrieved fromhttps://www.atsdr.cdc.gov/toxprofiles/tp144.pdf
2Radionuclides Rule. Retrieved fromhttps://www.epa.gov/dwreginfo/radionuclides-rule
3 Gao, Yiming et al., 2020. Patient-Specific Organ and Effective Dose Estimates in Adult Oncologic CT. American Journal of Roentgenology, 214:4, 738-746. Retrieved from https://www.ajronline.org/doi/full/10.2214/AJR.19.21197
4 United States Nuclear Regulatory Commission, 1991. "Regulations (10 CFR) Subpart C Occupational Dose Limits."Retrieved fromhttps://www.nrc.gov/reading-rm/doc-collections/cfr/part020/part020-1201.html
5 United States Environmental Protection Agency, 2017. "PAG Manual: Protective Action Guides and Planning Guidance for Radiological Incidents" (p. 50). Retrieved fromhttps://www.epa.gov/sites/default/files/2017-01/documents/epa_pag_manual_final_revisions_01-11-2017_cover_disclaimer_8.pdf

Radiation Terms and Units | US EPA (2024)
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