Radiation Dose and Risk: Sievert, Background and Limits

Reference article · Updated 2026-11-06

Key facts

Becquerel measures decays per second; gray measures energy absorbed; sievert measures biologically weighted dose.
UNSCEAR's 2008 estimate for average natural background was 2.4 mSv per year; its newer estimate is about 3.0 mSv per year.
ICRP recommends 1 mSv per year for the public and 20 mSv per year averaged over 5 years for workers, with no single year above 50 mSv.
Dose limits do not apply to patients receiving medical exposure, which is justified individually.
Short whole-body doses of about 1 Sv (1000 mSv) and above can cause acute radiation effects.

Approximate doses on a logarithmic scale; see the sources listed below for the underlying values. Original schematic.

Activity, absorbed dose and effective dose

Three ideas are often confused. Activity counts how many nuclei decay each second; one decay per second is one becquerel (Bq). It says how much radioactive material is present, not what it does to a person. Absorbed dose is the energy deposited per kilogram of tissue; one joule per kilogram is one gray (Gy). Effective dose is meant for radiation protection.

It multiplies absorbed dose by a radiation weighting factor (for example 1 for gamma rays and beta particles, 20 for alpha particles) and by tissue weighting factors that reflect how sensitive each organ is. Its unit is also J/kg, but is called the sievert (Sv). Because a sievert is large, doses are normally given in millisieverts (mSv, one thousandth of a sievert) or microsieverts. For conversion factors, see the unit converter.

Natural background radiation

Everyone is exposed to radiation from cosmic rays, from radionuclides in rocks and soil, from radon gas indoors, and from natural radionuclides in food and water (including potassium-40 and the uranium and thorium series; see radioactive properties of uranium). UNSCEAR estimated the worldwide average annual effective dose from natural sources at 2.4 mSv in its 2008 report.

A newer UNSCEAR evaluation, announced in 2026, puts the average at about 3.0 mSv, a change that UNSCEAR's announcement attributes mainly to better data and methods rather than to a real rise in radiation levels. Individual doses vary widely: typical values are one to several mSv per year, and some regions with high-background rocks or soils or high indoor radon have much higher values.

This is why a figure of 2.4 mSv in older materials and 3.0 mSv in newer ones are not a contradiction; always check which UNSCEAR report a number comes from.

Typical doses from medical imaging and other sources

The figures below are approximate and depend on the machine, protocol and patient size, so ranges are more honest than single values.

Source	Typical effective dose
Chest X-ray (posterior-anterior)	roughly 0.02 to 0.1 mSv
Head CT scan	about 1 to 2 mSv
Natural background, one year	about 2.4 to 3 mSv worldwide average; wide spread between places
Public dose limit from planned sources, one year (ICRP)	1 mSv
Occupational dose limit (ICRP)	20 mSv per year, averaged over 5 years

Medical doses are not subject to dose limits; they are justified for each patient and optimised to give the needed diagnostic information with the lowest reasonable dose.

Dose limits and what they mean

The International Commission on Radiological Protection (ICRP) recommends limits for planned exposure situations: an effective dose of 1 mSv in a year for members of the public, with a higher value permitted in a single year if the five-year average does not exceed 1 mSv per year, and for workers 20 mSv per year averaged over defined five-year periods, with no more than 50 mSv in any single year.

There are separate limits for the lens of the eye and for skin and extremities. These limits are not a boundary between safe and dangerous. They are regulatory ceilings set on the basis of risk judgements, applied together with two other principles: justification (the activity must do more good than harm) and optimisation (doses are kept as low as reasonably achievable below the limit). Natural background and medical exposure of patients are outside the limits.

Estimating risk: the linear no-threshold model

At high doses, effects such as radiation sickness (acute radiation syndrome) occur reliably above a threshold. Whole-body doses of around 1 Sv in a short time are in the range where symptoms such as nausea and blood changes begin to appear, and larger doses cause progressively more severe illness.

At low doses the effects are a statistical increase in cancer risk, which is too small to observe directly in most studies. ICRP therefore uses the linear no-threshold (LNT) model for protection purposes, assuming risk rises in proportion to dose with no safe level. ICRP's nominal risk coefficient for cancer is around 5 to 6 per cent per sievert.

The LNT model is a prudent regulatory assumption, not a proven description of biology at very low doses. Critics argue that repair mechanisms could make real risks lower at low dose rates, and others argue that risks might be higher for some groups. UNSCEAR cautions against multiplying very small doses by very large numbers of people to produce collective cancer counts. This page does not take a side; it only notes that the model is the basis for most regulations.

Using the dose scale sensibly

Comparing doses on one scale helps give proportion, but there are limits to this. A dose delivered over a lifetime is not equivalent to the same dose in an hour; a dose to one organ is not equal to a whole-body dose; and internal emitters such as radon progeny or inhaled uranium dust irradiate specific tissues. When assessing a local situation, such as soil or building material, use measured data and a consistent method, as explained on the page on dose from soil and building materials. For health effects of uranium itself, see uranium toxicity.

Putting uranium work in context

Uranium mining and milling workers are classified as workers exposed to natural sources and are mostly assessed for radon progeny, long-lived alpha emitters in dust and gamma dose. UNSCEAR's occupational evaluations report average annual doses for workers exposed to natural sources around 2 mSv, with higher averages for underground mining in some data sets; check the current report for specific figures. For nearby residents, dose assessments for mine and mill sites usually consider radon, dust, water and food pathways, and compare the added dose against the 1 mSv per year public benchmark and against natural background at the site.

The dose scale diagram above uses rounded values that are approximate, in order of magnitude only. A single bar cannot show differences in dose rate, tissue or time.

Frequently asked questions

What is the difference between a gray and a sievert?

A gray measures absorbed energy per kilogram of tissue. A sievert applies weighting factors for the type of radiation and the organ, to estimate biological effect for protection purposes.

How much natural radiation does the average person receive?

UNSCEAR's 2008 average was 2.4 mSv per year. Its newer estimate is about 3.0 mSv per year. Individual values vary considerably with geology, altitude and indoor radon.

What are the ICRP dose limits?

For the public, 1 mSv per year. For workers, 20 mSv per year averaged over five years, with no year above 50 mSv. They do not apply to medical exposure of patients.

Does the linear no-threshold model prove that every dose is harmful?

No. It is a precautionary assumption used for radiation protection. Evidence at very low doses is uncertain and the model is debated.

How does a chest X-ray compare with background radiation?

A chest X-ray gives roughly 0.02 to 0.1 mSv, which is comparable to a few days to a few weeks of natural background, depending on the technique and location.