Radon and Thoron in the Environment - Aerosols: Measurement, Dosimetry, and Health Effects - page 548

1 Sv (Harley and Robbins, 2009). The relatively high dose estimate to lymphocytes circulating
through the BE, potential precursor cells for ALL, may provide a dose pathway for an association.

21.11 220 RN (THORON) CONTRIBUTION TO 222 RN (RADON) MEASUREMENTS
Historically, thoron bronchial dose was assessed through the measurement of its decay products,
212 Pb and 212 Po, and not thoron gas itself (Schery, 1985, 1990; Schery and Zarcony, 1985). Thoron
gas itself was rarely measured, because of the dificulty in measuring an alpha-particle-emitting gas
with a very short half-life (t 1/2 = 55 s). The measurement of the two gases required real-time instru-
mentation with various types of decay chambers to permit a difference in signal with and without
the 220 Rn (Israel, 1964; NCRP, 1988). Interest in thoron spurred the development of discriminative
detectors to measure both gases.
Measurements of thoron gas or its decay products are now common. Several discriminative detec-
tors that measure both radon and thoron gas or plate out of the decay products onto surfaces have
been developed (Tokonami et al., 2005; Harley et al., 2010; Janik et al., 2010; Mishra et al., 2010).
Risk assessments are undoubtedly hindered by the presence of thoron in the measured radon gas
signal unless measures were applied to exclude its presence. UNSCEAR (2006) provides central
dose factors for radon and thoron EECs:

− 3

Radon EEC) ( = 9 nSv per Bq m h) (

− 3

Thoron EEC ( ) = 40 nSv per Bq m h ( )

The EEC for radon or thoron is = F (Equilibrium ratio ) × ( gas concentration)

eq

The accepted value of F eq for radon (UNSCEAR, 2006) is 0.4 for indoor environments and 0.6 for
outdoor environments, that is, 40% or 60% equilibrium with the decay products. Harley et al. (2010)
have shown from long-term measurements of thoron gas and the thoron decay product 212 Pb that
the average F eq for thoron is 0.04 indoors and 0.004 outdoors, that is, 4% and 0.4%. Thus, thoron
bronchial dose can be estimated from gas measurement similar to radon dose estimates.
Although the dose factor per unit gas concentration for thoron is larger than that for radon, this is
offset by the much smaller thoron equilibrium factor, F eq . Therefore, the dose from thoron decay prod-
ucts is usually less than that for radon decay products. Because the measurement of total gas has been
used to identify radon, the historic dose and risk assessments may need to be revisited in the future.
It is unlikely that the historic measurements in uranium and other underground mines are
affected by thoron, because the ore was primarily 238 U, the parent of 226 Ra and 222 Rn.

21.12 LUNG CANCER RISK PROJECTIONS
More published information exists concerning the lung cancer risk from radon than for any other
internal radioactive emitter. Eleven underground cohorts have been studied extensively to estimate
lung cancer risk and to develop risk models for the prediction of lung cancer risk in other populations
(NAS/NRC, 1999a,b). The risk estimates for nine cohorts are shown in Figure 21.7 (UNSCEAR,
2006). The combined excess relative risk (ERR) from these miner studies is 0.006 WLM −1 . A very
large study of 58,987 German miners published subsequently determines the ERR for the German
cohort as 0.016 (95% CI: 0.0069-0.014) WLM −1 (Walsh et al., 2010).
Prior to 2003, the estimate of lung cancer risk in residences was derived from models developed
from the underground miner studies. The results of the 23 case control studies and six pooled or