Key facts
- UNSCEAR 2000 gives dose-rate coefficients of 0.0417 nGy/h per Bq/kg for K-40, 0.462 for the U-238 series and 0.604 for the Th-232 series.
- Absorbed dose in air is converted to effective dose with about 0.7 Sv/Gy for adults.
- UNSCEAR assumes people spend about 80% of their time indoors and 20% outdoors.
- Radium equivalent activity, with a commonly used screening level of 370 Bq/kg, combines the three activities into one index.
External gamma exposure from the ground and from walls
Rocks, soil and construction materials such as concrete, brick, stone, and by-products like fly ash or phosphogypsum contain potassium-40 and the long-lived uranium and thorium series. The gamma rays from these nuclides cause a dose to anyone nearby even without any release. Most of the gamma dose from the uranium series comes from lead-214 and bismuth-214, which follow radon in the chain (see radioactive properties); for the thorium series, from actinium-228, thallium-208 and others.
This page covers only external gamma dose. Radon accumulation indoors is a separate pathway and often the larger contributor; intake of radionuclides through food and water is another. All are included in the total natural background discussed on the dose and risk page.
From activity concentration to absorbed dose rate in air
If the activity concentrations in the material are known in becquerels per kilogram, the absorbed dose rate in air about one metre above a large, uniform surface can be estimated with the UNSCEAR 2000 (Annex B) coefficients:
D (nGy/h) = 0.0417 × AK-40 + 0.462 × AU-238 series + 0.604 × ATh-232 series
The calculator above uses this formula. It assumes the U-238 and Th-232 series are in equilibrium, so one value represents the whole series (usually via Ra-226 or Pb-214/Bi-214 for uranium and Ra-228 or Ac-228 for thorium). With typical world soil values of about 400 Bq/kg for K-40, 35 Bq/kg for U-238 and 30 Bq/kg for Th-232, the formula gives about 51 nGy/h, close to the population-weighted outdoor average reported by UNSCEAR of roughly 59 nGy/h from direct measurements.
Radium equivalent activity as a screening index
To compare materials with different mixtures of nuclides, researchers often use radium equivalent activity: Raeq = ARa + 1.43 ATh + 0.077 AK, in Bq/kg. It rests on the assumption that 370 Bq/kg of Ra-226, 259 Bq/kg of Th-232 or 4810 Bq/kg of K-40 give the same gamma dose rate. A value at or below 370 Bq/kg is the usual screening threshold for building materials. It is a screening index only; it does not replace a dose assessment, and national regulations use their own indices.
From dose rate and time to annual dose
Annual effective dose is the dose rate multiplied by the conversion coefficient and the time spent in each place. UNSCEAR uses a conversion of 0.7 Sv/Gy for adults and an occupancy of 0.8 indoors and 0.2 outdoors:
E = D × 0.7 Sv/Gy × (hours indoors + hours outdoors), using the corresponding indoor and outdoor dose rates.
The calculator also supports a simpler mode. If a handheld survey meter shows a dose rate in microsieverts per hour, multiply by the hours spent there. For example, 0.10 µSv/h for 8760 hours (a full year) is 876 µSv, or about 0.88 mSv. If indoor and outdoor rates differ, calculate the two places separately and add the results.
A worked example
Take UNSCEAR's rounded averages of 59 nGy/h outdoors and 84 nGy/h indoors for absorbed dose rate in air. Outdoors, for 20% of 8760 hours: 59 × 8760 × 0.2 × 0.7 ≈ 72 µSv. Indoors, for 80% of the year: 84 × 8760 × 0.8 × 0.7 ≈ 412 µSv. The sum is about 0.48 mSv per year, which agrees with the value UNSCEAR gives for terrestrial external exposure. Local conditions can move this by a factor of several, for instance on granitic soils or when building materials contain unusually high levels of radium or thorium.
Limitations of the method
- The coefficients assume an infinite, flat, uniform source. Small plots, thin layers, rooms with different wall materials and steep terrain give different results.
- Moisture, snow cover, and sealed or paved surfaces reduce outdoor dose rates.
- A dose-rate instrument must be calibrated for the energy range of natural gamma rays, and readings include cosmic radiation, which is not from the ground.
- Different population groups have different conversion factors: UNSCEAR gives 0.8 Sv/Gy for children and 0.9 for infants.
- The calculator uses defaults for occupancy; real behaviour differs and can be entered instead.
- Radon, thoron and intake are not included. Contaminated sites, such as tailings, need site-specific assessment by qualified professionals.
Where the indoor environment changes the picture
Indoor gamma dose rates are typically higher than outdoor ones, because walls, floors and ceilings surround the occupant, which is why UNSCEAR reports an indoor average of about 84 nGy/h against about 59 nGy/h outdoors. Materials differ widely. Natural stone such as some granites, concrete made with certain aggregates, bricks from particular clays, and industrial by-products such as fly ash, phosphogypsum or slag can carry higher activity than average soil.
Many countries and the European Union set reference levels or indices for building materials, and these use the same three activities that appear in the formula above. When a house is built on, or with, uranium-mining or milling residues, the radon exposure may dominate and should be measured separately; see the page on mill tailings.
If the question is whether a measurement is unusual, compare it with the local background range rather than the world average, since dose rates vary severalfold between geological regions.
Frequently asked questions
What data do I need for the dose estimate?
Activity concentrations of K-40, the U-238 series and the Th-232 series in Bq/kg, or alternatively a measured gamma dose rate and the hours spent in the place.
Why is 0.7 Sv/Gy used?
UNSCEAR uses 0.7 Sv/Gy to convert absorbed dose in air to effective dose in adults for environmental gamma radiation; values for children and infants are slightly higher.
What does a radium equivalent of 370 Bq/kg mean?
It is a commonly used screening level for building materials. Materials above it are not automatically unsafe, but they merit a closer assessment under national rules.
Does this include radon?
No. The method covers only external gamma exposure. Radon and radon progeny are assessed separately and are often the larger indoor contribution.
Can I use this for a contaminated site?
Only as a rough indication. Contaminated sites need measurements and dose assessment by qualified professionals under the applicable regulation.
Sources and further reading
- United Nations Scientific Committee on the Effects of Atomic Radiation — UNSCEAR 2000 Report, Volume I: Sources and Effects of Ionizing Radiation (Annex B) (2000)
- United Nations Scientific Committee on the Effects of Atomic Radiation — UNSCEAR 2008 Report, Volume I: Sources of Ionizing Radiation (2010)
- arXiv preprint — Gamma radiation measurements and dose rates in commercially-used natural tiling rocks (granites) (2002)
- arXiv preprint — Indoor and outdoor in situ high-resolution gamma radiation measurements in urban areas of Cyprus (2006)
- International Commission on Radiological Protection — ICRP Publication 103: The 2007 Recommendations of the ICRP (2007)
- International Commission on Radiological Protection — Dose limits (ICRPaedia) (2024)
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