Handbook of Radon.

7. Conversion factors for different units.

Many units are used in radon science. The most popular in Europe are explained in Section 6. Other units are also used in radiation work, but not usually for radon.

Activity concentration is measured in Bq/m3 but may be expressed in pCi/l. (picocuries per litre). The conversion is approximately

37 Bq/m3 = 1 pCi/l

Absorbed dose: gray, Gy where 1 Gy = 100 rad.

The rad is an old unit. Both rads and grays express energy imparted by radiation to a mass of tissue: 1 Gy = 1 J/kg.

Dose equivalent: rem, where rem stands for roentgen equivalent man.

(The roentgen is an obsolete unit).

1 rem = 0.01 Sv = 10 mSv

Dose: (effective dose equivalent, usually) is expressed in Sv, or mSv.

One rem, equal to 10 mSv, is sometimes considered equivalent to "a few hundred cigarettes". Thus a house containing 400 Bq/m3 of radon and which delivers occupant doses of 20 mSv (2 rem) per year may be producing the same long term risk of premature death as about 2 cigarettes per day, for non smokers.

Older units in common use in the USA are WL and WLM, respectively working level and working level month. These derive from the days when most concern about radon centred on miners. The units represent a certain concentration and integrated concentration of radon daughters in working environments.

Radon itself does not contribute to calculation of the WL.

Some care is needed in using these units, because equilibrium factors have to be taken into account in converting to radon gas concentrations.

Based on decay product (daughter) concentrations,

1 WL = 3740 Bq/m3.

Assuming an equilibrium factor of 0.5 (normal for houses) 1 WL = 7480 Bq/m3 of radon gas.

Also, 1 WL = 200 pCi/l   (F = 0.5) or

1 WL = 100 pCi/l   (F = 1)

The conversion between pCi/l and Bq/m3 when applied to radon gas (which is the usual usage) is independent of F, the equilibrium factor.

The WL is actually defined in terms of Potential Alpha Energy Concentration (PAEC) in air. In SI units, 1 WL = 2.08 x 10-5 J/m+ or 1.3 x 105 MeV of alpha energy.

Quality factors and other units.

For beta and gamma radiation the so-called quality factor is unity, so 1 Sv is produced by absorption of 1 Gy. For alpha radiation however, it is usual to assume a quality factor of 20, so 1 Gy of absorbed dose will represent 20 Sv of dose equivalent. Higher quality factors have been suggested for alpha radiation in bone marrow, whether from radon daughters or from actinides such as plutonium.

Doses from gamma radiation (from rocks, soil and food) are often expressed in nGy per hour. Doses are small compared with those from radon, and range from about 0.1 to 1 mSv annually in the UK. Typical dose rates indoors are between 10 and 250 nGy/h depending upon the location and construction.

A note on Thoron.

Thoron is often produced alongside radon. The two gases are similar, but differ in that the radioactive half-life of thoron (220 Rn) is only 54.5 seconds, compared with 3.82 days for 222 Rn, usually known simply as radon.

This key difference means that whereas radon can be expected often to enter houses from considerable distances underground (typically 1 to 2 metres), thoron sources need to be either close to the ground surface or to be part of the house construction before they are likely to contribute significantly to airborne radioactivity. However, once in the house, thoron decays to a series of long lived daughters with considerable PAEC.

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