Handbook of Radon.
6. A note on units used in radon work.
Two special scientific terms need to be understood in radon work. These are Bq/m3 (becquerels per cubic metre) and mSv (millisieverts).
Bq/m3 is a unit of activity concentration, a measure of radon per unit volume. This can be thought of as the concentration of radon in the air, whether in a house, a school or outdoors.
Values for activity concentration in Bq/m3 can range from about 3 (outdoors in the UK), through 50 (reported outdoors in some States of America), to 150,000 or more (found in the worst affected American and East German houses). In soil gas, levels of 20,000 or 30,000 are quite normal.
Over 1,200,000 Bq/m3 has been recorded by the author in Cornwall, equalling reported concentrations in mines in Czechoslovakia that were at the centre of lung cancer studies decades ago.
The average concentration of radon in UK housing is around 20 Bq/m3. The average for houses in Cornwall is around 170 and in Devon about 70 Bq/m3. This compares with an average of between 50 to 60 Bq/m3 in the USA.
In general, radon levels in UK housing are low: many countries average more than 20 Bq/m3.
However, the radon concentration in a building does not represent the associated risk - because the amount of time spent in the building is of equal importance. To assess potential harm from any type of ionising radiation, a concept called 'dose' is used.
Millisieverts (mSv) are a modern unit for dose, and are used here in the context of effective dose equivalent, a concept introduced by the ICRP in 1977.
Effective dose equivalent takes into account that some organs of the body are more sensitive than others to an assault by unit radiation dose. Formally, the effective dose equivalent is that dose equivalent which if delivered uniformly to the body would result in the same total potential harm as results from the actual dose. Effective dose equivalent can be used directly to compare the chance of harm (usually cancer) resulting from various doses to different organs and by different types of radiation.
There is a conversion between Bq/m3 of radon and mSv/year for any type of building, determined by the average time that it is occupied. For houses, the conversion factor is 20, so 20 Bq/m3 in a house will give an annual effective dose equivalent of 1 mSv, assuming that the occupants spend most of their time at home indoors. If the average radon concentration is 800 Bq/m3, the annual dose will be 40 mSv, (800/20) and so on. It is assumed that house occupancy is around 80 or 90%.
However, the situation in schools and offices is different. School buildings are occupied by classes at work, or during breaks, for around 1000 hours per year, a fraction of only 11 or 12%. For office workers, typical occupancy factors are around 20%. Thus, typically, the radon concentration in a school could be 6 or 7 times as high as in a house before it gave rise to the same concern for an individual, (see Sections 27 and 41) and in an office building 3 or 4 times as high.
KEY FACTS:
High radon levels in houses are of most concern simply because people spend so long at home compared with time in other buildings.
Radon concentrations in air are measured in Bq/m3, but dose (and thereby potential harm) is expressed in mSv of effective dose equivalent, and usually in mSv per year.
Conversion factors for different types of buildings depend upon the time people spend in them. Radon in schools has proved to be an emotive issue, especially in the USA, but dose to occupants is generally low.