1In this Safety Guide, as in GSR Part 3, the term ‘radon’ is used to mean any combination of the two main isotopes of the element radon (²²²Rn and ²²⁰Rn).

2The reference level for dwellings also applies to other buildings with high occupancy factors for the public.

3Criteria for controlling exposure due to radon in drinking water can be found in Section 9 of the current WHO Guidelines for Drinking-water Quality [10].

4Reference [17] sets out the calculation of the total inhalation dose from ²²²Rn. The calculation assumes equilibrium factors for ²²²Rn of 0.4 indoors and 0.6 outdoors, an average activity concentration for ²²²Rn indoors of 40 Bq/m³ and an average activity concentration for ²²²Rn outdoors of 10 Bq/m³ , an occupancy factor per year of 7000 hours indoors and 1760 hours outdoors, and a dose conversion factor of 9 nSv/(Bq·h·m^–3) for equivalent equilibrium concentration

5On the assumption of an equilibrium factor for ²²²Rn of 0.4 and an annual occupancy rate of 7000 hours, the value of activity concentration of 300 Bq/m³ corresponds to an annual effective dose of the order of 10 mSv.

6In some States, such areas are referred to as ‘high radon areas’ or ‘radon affected areas’.

7Studies [18] have shown that when doses due to gamma radiation are limited to levels below 1 mSv/a, the activity concentrations of ²²⁶Ra in building materials have to be limited, in practice, to levels that are unlikely to cause activity concentrations of ²²²Rn indoors to exceed 200 Bq/m³.

8Equilibrium equivalent concentration is the activity concentration of ²²²Rn and ²²⁰Rn in radioactive equilibrium with their short lived progeny that would have the same potential alpha energy concentration as the actual (non-equilibrium) mixture.

9Of the ²²⁰Rn progeny, only ²¹²Pb and ²¹²Bi make a significant contribution to the potential alpha energy concentration [32]. The contribution per becquerel of the parent nuclide ²²⁰Rn is nearly three orders of magnitude lower than that of ²¹²Pb. Since the majority of the potential alpha energy concentration is provided by ²¹²Pb, measurement of the activity concentration of ²¹²Pb in air usually allows a good estimate of the potential alpha energy concentration.

10The derivation of the activity concentration index, and the derivation of the upper values for the activity concentration index (see para. 4.21), can be found in annex I of Ref. [18].