LUNG CANCER RISKS DUE TO THE RADON 209
210 Po
238 U decays to 222 Rn( a gas) and
then to 210 Pb, which settles on tobacco leaves and later decays into 210 Po
222 Rn
Fertilizer made of uraniumrich phosphate rock
210 Pb
238 U
210 Pb from the soil is absorbed through the roots
Fig. 1. Sticky hair-like structures on both sides of tobacco leaves [ 4 ].
from use of these tobacco samples ranged from 1.67 to 3.52 mSv year – 1. The result refers to the dual( chemical and radioactive) effect of smoking as a risk factor for lung cancer. Laith [ 8 ] showed that the 222 Rn concentrations in cigarette tobacco samples ranged from 228 to 778 Bq m – 3 with an average of 432 Bq m – 3, with the radon-induced lung cancer risks varying from 103 to 353 cases per million people with an average value of 196. Excellent correlation was observed between the radon concentration and lung cancer cases per year per million people for different brands of tobacco.
The aims of the present work were to evaluate the activity concentrations of radon and radium in tobacco and to calculate the risk parameter, namely, the number of smokers( per million smokers per year) who can be expected to get lung cancer as a result of smoking a particular type of cigarettes.
THEORY Activity Concentrations of Radon Gas and Radium
The activity concentration of radon( C Rn, Bq m – 3) in cigarette tobacco at secular equilibrium is given by the equation [ 9 ]
C Rn = ρ /( KT),( 1)
where ρ is the track density( mm – 2); K, experimental calibration factor equal to 0.0022( mm – 2)/( Bq day m – 3); and T, exposure time( days).
To calculate the activity concentration of 226 Ra( C Ra, Bq kg – 1) in cigarette tobacco samples, we used the following equation [ 10 ]:
C Ra = ρhA /( mKT e),( 2) where ρ is the track density( track mm – 2); h, distance between the detector and the top of sample( 0.07 m); A, surface area of the sample in the plastic cylinder; K, calibration coefficient for CR-39; T e, effective exposure time( h); and m, sample weight [ 11 ]:
T e = T +( 1 / λ)( e – λT – 1),( 3)
where λ is decay constant of radon gas( 0.1814 day – 1), and T is the exposure time( days).
Radon Dose Estimation
The annual effective dose from indoor radon concentration, E Rn( mSv year – 1), was calculated using the UNSCEAR model [ 12 ]:
E Rn = C Rn FHTD,( 4)
where F is an equilibrium factor( 0.4), H is the occupancy factor( assumed equal to 0.8 for this work), T is the number of hours in a year( 8760 h year – 1), and D is the dose conversion factor [ 9.0 × 10 – 6 mSv /( Bq m – 3 h)].
The annual effective dose for smoker lung from radon gas inhalation, E L( mSv year – 1), was calculated using the following equation [ 12 ]:
E L = E Rn W R W T,( 5)
where W R is radiation weighting factor equal to 20 for α-particles, and W T is the tissue weighting factor equal to 0.12 for lungs.
The equivalent dose for bronchial areas of human lungs was calculated using the conversion factor of 1.0 × 10 – 5 mSv /( Bq h m – 3) [ 13 ]. It should be taken into
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