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Radiation dose limits and health effects: background, NRC limits, LNT

Dose limits only make sense against two reference points: what everyone receives from background and medicine, and the doses at which harm has actually been observed.

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Study text drafted from public-domain US government sources. Practice questions are reviewed before publication.

Everyday doses

The NRC puts the average US dose at about 620 millirem (6.2 mSv) per year. Roughly half is natural — radon in indoor air is the largest part, plus cosmic rays, terrestrial radiation and the body's own potassium-40 — and roughly half comes from medical imaging and treatment. Commercial nuclear power contributes a very small fraction.

NRC dose limits

For adult radiation workers, 10 CFR 20.1201 limits TEDE to 5 rem (50 mSv) per year, the lens of the eye to 15 rem, and skin or any extremity to 50 rem. Minors are limited to 10 percent of those values (§20.1207), and the embryo/fetus of a declared pregnant worker to 0.5 rem over the whole pregnancy (§20.1208). Members of the public are limited to 0.1 rem (1 mSv) per year from licensed operation, and no more than 2 mrem in any one hour in unrestricted areas (§20.1301). Limits are ceilings; ALARA programs keep actual doses far lower.

How radiation causes harm

Deterministic effects, or tissue reactions, such as skin burns, cataracts and acute radiation syndrome, have thresholds and get more severe with dose. CDC describes acute radiation syndrome as requiring a large, penetrating whole-body dose above about 0.7 Gy received over a short time. Stochastic effects, mainly cancer, are assumed to have no threshold: dose raises the probability, not the severity.

For protection purposes regulators use the linear no-threshold (LNT) model, assuming risk is proportional to dose even at low doses. Rapidly dividing cells, such as those in bone marrow and the gut lining, are the most radiosensitive, which shapes the early symptoms of high exposures.

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Frequently asked questions

What is the annual radiation dose limit for workers?
Under NRC rules, 5 rem (50 mSv) total effective dose equivalent per year for adults, with separate limits for the lens of the eye (15 rem) and skin or extremities (50 rem).
What is the dose limit for the public?
0.1 rem (1 mSv) per year from a licensed operation, excluding background and medical exposure.
How much radiation is a CT scan compared to background?
It varies widely by scan type; medical imaging overall makes up roughly half the US average annual dose of about 620 mrem. Ask the imaging provider for the dose of a specific exam.
Is any dose of radiation safe?
Regulators assume, via the LNT model, that any dose carries some small risk, and they keep doses ALARA. At everyday background levels that assumed risk is very small and has not been directly observed.

Sources

  1. 10 CFR Part 20, Standards for Protection Against Radiation (U.S. NRC) www.ecfr.gov
  2. U.S. NRC, Doses in Our Daily Lives
  3. U.S. NRC Backgrounder, Biological Effects of Radiation
  4. U.S. EPA, Radiation Sources and Doses www.epa.gov
  5. U.S. NRC Regulatory Guide 8.29, Instruction Concerning Risks from Occupational Radiation Exposure
  6. U.S. CDC, Acute Radiation Syndrome: A Fact Sheet for Clinicians

Explanations on this page are original. Cited US government works are in the public domain; they do not endorse this site.