In the UK, there are action levels (200 Bq/m3) and target levels (100 Bq/m3) set for the average radon concentration in homes (over a 3 month period for detectors in a bed room and a living room). I don't know the details of the derivation of these levels, but they relate to dose to residents.

As for whether these levels are "safe", that rather depends on what you mean by "safe". There is a probability of radon exposure causing health effects that increases with increased radon concentration (with various adjustments for lifestyle, age etc). There is no scientific basis for a safe/unsafe threshold. There is a threshold between acceptable and unacceptable probability of health effects, which is derived as much from political considerations as it is from hard science.

In Italy we have specific regulations only for workers. The regulations provides an action limit of 500 Bq/m3 averaged over one year, for workers, and (I do not why) an action level express in term of effective dose of 0,3 mSv/year for public who attend in worker rooms. (use a conversion coefficient of 3,0 *10-9 Sv/Bq*hr*m-3)

In my opinion a limit concentration of Radon, equivalent to the limit of 0,3 mSv/year, is a good value even in homes.

Backing the information given by Alan Cresswell, take a look at IAEA safety report No. 33, esp. section 3.5, to see how action levels can be derived.

This publication also emphasises the importance of the assumed (or measured) equilibrium factor between radon and its progeny.

Furthermore the “soil gas” alone might only give you a part of the picture as water supply in a building might add to the radon burden, while particular building technologies might strongly reduce radon levels in a specific room.

Further reading: ICRP Publication 65 and 82

http://www-pub.iaea.org/MTCD/publications/PDF/Pub1168_web.pdf

Generally speaking, safety limits are not applied to soil gas. This is because soil gas radon concentration is only one of quite a number of factors determining radon concentration in a building, making it impossible in practice to predict the radon level in a building before the building is built. After the building is built, it is clearly the radon -in-air concentration which is measured. In addition, radon-in-soil-gas measurements can vary dramatically over relatively small distances, even less than the dimensions of a typical home.

The radon concentration in Soil Gaz is not the concentration in air as you said below. Its concentration varies between 0 and 100 kBq m-3, depending on the type of rocks and the depth at which it is measured, being variable in the first 25 cm of the soil due to erratic soil moisture content, the greater the depth of 0.5m on the surface, relatively stable 0.8-1m layer which are considered as reference values ​​greater than the depth of 1-2 m which can reach 100 kBq m-3

As long the Radon is in the soil it will not bother you even at extremely high concentration. It is an alpha-emitter and alpha radiation will not penetrate your dead skin. Therefore it has no effect on your health whatsoever.

However, if incorporated, alpha-emitters exhert a high level of radiotoxicology. The good part of Rn being a noble gas is, that it will not easily accumulate in the body. So inhalation is by far not as toxic as with, say, Uranium. The contamination threshold is 3kBq/m3 (U-238: 1 Bq/m3). That means: If you are exposed 8h/day and 5d/week to this concentration, you accumulate a dose of 20 mSv/year.

There is not acceptance level of radon in soil. Emanation rate of radon gas in the soil depend on the soil porosity and the grate of uranium . Thus, rate of soil radon gas (exhalation rate) is variable form country to country , and from region to another. I don't agree with those selected an acceptance level for soil radon .

There are no limits for radon in soil-gas. There are limits for radon release rates to the atmosphere. The limits are for specific potential sources of large radon releases, such as uranium mill tailings and phosphogypsum. The limits are on the order of 1 Bq/s over a square meter. Limits cannot be imposed on soil-gas because radon emanation is affected by soil properties such as radium content, mineral composition, moisture content, density, and soil porosity. Radon concentrations in soil may also be affected by meteorological conditions on the surface.

I agree with Dr.Joseph there are no limits and safe level of soil radon concentration , it's depending on grade of uranium and radium contend

I think that the question of "how much radon is in the soil gas" is not an important one for radiation protection, the important questions I think are

1. What is the gamma dose rate from the soil

2. How much radon is in the air outside

3. How much radon is in the air indoors

4. How much radon is in well water

5. How much radium is in the well water

6. How easy is it to transfer radioactivity to plants grown in the soil.

7. How mobile is the radioactivity in the soil (what is Kd for the elements of interest)

There no single safer/upper limit for the radon in the soil given, however the important thing is that how much it contribute to the water used and the air inhale

Nelida Florea and many others just explained clearly and correctly that there is no relation between the measured Radon activity concentrations in soil gas and the radioprotection issues of the human exposure to indoor Radon. The exhalated Radon from soil can represent only a potential Danger for human health that can be translated in a Risk when in a High Radon Exhalation area buildings are constructed there and Radon can enter the confinded spaces.

i agree with Michele that there is no limit, the important thing is how contribution to air or even to water

The air in the soil is heavily loaded with radon, with concentrations between 2000 and 106 Bq m–3, usually measured 0.5–1 m beneath the soil surface.

The latest ICRP publication related to radon exposure is Publication 126 "Radiological Protection against Radon Exposure" ( http://www.icrp.org/publication.asp?id=ICRP%20Publication%20126 ) issued in 2014.

Then, an update was given in April 2015 as below ( http://www.icrp.org/admin/Summary%20of%20April%202015%20Main%20Commission%20Meeting%20Sydney.pdf ).

Progress on the calculation of dose coefficients was reviewed, in particular for exposure to 222Rn and progeny. There is a remarkable consistency between radon dose coefficients obtained by dosimetric calculations and conversion coefficients based on epidemiological comparisons. In an upcoming publication, ICRP intends to recommend a single coefficient for use in most circumstances, with a value of 12 mSv/Working Level Month (3.4 mSv per mJ h m–3). Additional data will be provided for circumstances significantly divergent from typical conditions where sufficient and reliable information is available to support an adjustment.