Researchers have studied the prevalence of radon-induced lung cancer in mining and residential populations. In miners, statistically significant increases in lung cancer have been observed, exceeding 465 WLM (Roscoe et al. 1989) and in residential populations exceeding an average 14.65 pCi/L.
As charged particles, the unattached radon progeny can adhere to lung fluid or the respiratory epithelium.
But the attached fraction is what clings more effectively to the respiratory epithelium. Through mucociliary action, those progeny floating unattached in lung fluid are rapidly cleared from the respiratory tract. And because of the alpha particles’ short track length, only the fluid is exposed to any released radiation, with no adverse health effects.
- When progeny transform within the lungs and their energy deposits in tissue (and not fluid), the genetic material of cells lining the airways can be damaged. If a cell lives but repair is incomplete, lung cancer can develop (NRCC 1999).
- Attached progeny preferentially deposit in the bronchi, the site of most lung cancers.
- The total amount of energy deposited in successive transformations of the progeny is several times
that produced in the initial radon decay.
An exact systematic description of how cancers form as a result of exposure to radiation is only partially understood. Cancer is a monoclonal disease that starts as a single cell with heritable damage to the deoxyribonucleic acid (DNA); this damage confers a proliferative advantage relative to normal cells (Iannaccone 1987). Most of the lung cancers associated with radon are bronchogenic, with all histologic types represented.
Smaller lungs and faster respiration rates in children generally results in higher estimated radiation doses to children’s lungs relative to adults.
Cigarette smoking and radon decay products synergistically influence lung cancer risk in a supra-additive manner. Miner studies found that if smoking started before occupational radon exposure, the effect was submultiplicative, or, if these occurred in the opposite order, more-than-multiplicative (ATSDR 2009).
The analysis of results from thirteen European residential case-control studies showed an increase in lung cancer risk proportionate to the unit increase in radon concentration, similar in lifelong nonsmokers and cigarette smokers (ATSDR 2009).
The lung cancer risk for cigarette smokers may be up to 25 times greater than that of nonsmokers exposed to high residential radon levels (up to 10.8 pCi/L) (Darby et al. 2005, 2006).
The lung cancer risk due to radon exposure is second only to that of smoking (Alberg 2007; Copes 2007; EPA 2009a).
- Although the synergistic mechanism(s) of cigarette smoking and radon exposure are unknown, the combination’s
adverse health effects are well known.
Among both smoking and nonsmoking populations of underground miners, small-cell carcinoma occurs at a higher frequency in the initial years following exposure compared with the pattern of similar histologic types in the general population. This is believed due to the high levels of radon exposure underground, but could be due in part to high-level silica dust exposure.
Other types of lung cancers seen in radon exposed miners include
- Adenocarcinoma,
- Large cell carcinoma, and
- Squamous cell carcinoma.
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