Measurement Of Radon Levels

Measurements may be made of radon or radon progeny concentrations and may be instantaneous or last for days or months. The NRPB required that the quantity to be measured should be as closely related as possible to risk and measurements should be cheap and convenient since large surveys need to be undertaken by the organisation. As indicated in 2.7 above, essentially all the dose to lung tissues arises from inhaled short-lived radon progeny. However, the dose depends strongly on the `unattached’ fraction of these progeny (i.e. those in very small particles, up to 3nm) and on the proportion of the attached progeny on small particles below lOnm.

The proportion of activity in these categories is dependent on the submicron aerosol concentrations. Determination of the size distribution of radon progeny in air requires sophisticated equipment with skilled operators, precludes its use for large surveys. However, other things being equal, as the ventilation rate decreases the fraction of the total activity that is on submicron aerosols decrease while the total activity increases. As a result, the average radon gas concentration is a surrogate for the activity on submicron aerosol particles and thus a better indicator of dose to lung than is a simple measurement of the total energy potentially released by the decay of all the radon progeny (Vanmarcke, Berkvens and Poffijn 1989).

Radon gas concentrations can be determined by taking samples of air and measuring the activity with appropriate electronic apparatus. However, such short-term measurements can be misleading in assessing the exposure of people to radon because of the considerable variations in levels from night to day as shown in Figure 2.5 and from season to season as shown in Figure 2.6.







Figure 2.5 Variation of radon levels in a house over a 24-hour period – Source (NRPB, 1994)






Figure 2.6 Annual variation in radon concentrations in ground-floor living rooms and first-floor bedrooms – Source (NRPB, 1994)

To determine annual average values it is far better to make measurements lasting a few months so that the effects of short-term variations are averaged. This can be done by using passive radon detectors which are left in place in dwellings during the measurement period. The detector consists of a small chamber containing a sensitive plastic material, PADC. Radon diffuses into the chamber and decays through its chain of decay products. Some of the alpha particles emitted damage the plastic detector, and this damage is revealed later by etching the plastic in a solution of sodium hydroxide. The damaged tracks are counted with an automatic image analyser, and their number is proportional to the exposure of the detector to radon. The detectors are small enough to go through the post and are relatively inexpensive and are suitable for large surveys of radon in dwellings.

Two detectors are placed in each dwelling, one in the living room and one in an occupied bedroom, and left in place for 3 months. The seasonal variation in average radon concentrations are derived using correction factors for measurements lasting 3 or 6 months.