RADON AND CANCER
In most parts of the world, the largest source of ionising radiation is radon, a product of radium. Radon-222 is an inert gas that has a half-life of four days and arises from radioactive decay of uranium-238, which is present throughout the Earth’s crust. The average radon concentration in British homes is 21 Bq/m3 and the estimate is that 1,110 lung cancer deaths per year (about 3.3% of lung cancer deaths) are caused by this background source of alpha particles (Fig. 1).
1. Lung tumours
In the European Union, the figure is 20,000, in the USA 21,000 and worldwide one million deaths are caused by radon, with smokers being more vulnerable. Significant radon concentrations normally accumulate only inside buildings but this can readily be prevented by installing a sealed membrane at ground level. Although there is a statutory level at which this preventative measure must be applied (200 Bq/m3), the Gray report (Gray et al., 2009) estimates that 85% of deaths caused by radon arise from lower levels for which preventative measures are not legally required. Much higher concentrations of radon can, however, be encountered in some thermal springs, notably those in Boulder, Colorado and Lurisia in Northern Italy.
The energy carried by the UV region of the electromagnetic spectrum is ~4 electron volts (eV), which means that it is capable of breaking covalent bonds and thus damaging biological molecules either directly or indirectly (by a generation of free radicals). Most of the radiation reaching the Earth from the Sun is absorbed by the ozone layer, and nearly all the UV radiation that reaches us is UVA (98.7%). Nevertheless, UVB is important because it is responsible for producing vitamin D in the skin, a deficiency of which causes a high proportion of premature deaths and has been linked to many diseases including cancers.
Thus UV radiation is necessary for good health but it is also a carcinogen, so its effects become a matter of balance. As a contribution to this equilibrium, we make melanin pigments in our skin that absorb UV. The cells that do this are melanocytes (they’re also present in the eye and the bowel). They make both black eumelanin and the second type of pigment, phaeomelanin, that is red. The relative amounts of these two pigments determine our hair and skin colour. When we get suntanned it’s because our skin makes eumelanin as a protection against UV light. Dark-skinned people make more eumelanin and are thus better protected against than their brethren in northern climes whose lighter skin may have evolved as a response to lower levels of sunshine, thereby boosting their vitamin D production.
Melanocytes have come to prominence in the cancer field in recent years because their uncontrolled growth gives rise to a malignant tumour called a melanoma (Fig. 2).
2. Moles and melanomas
Melanoma is a relatively rare form of skin cancer – the non-melanoma skin cancers basal cell carcinoma and squamous cell carcinoma are the most common – but it is the most serious. It causes 75% of skin cancer deaths of which worldwide there are 48,000 per year with over 160,000 new cases. In the USA there are over 68,000 new malignant melanoma cases and 8,700 deaths every year, the corresponding UK figures being 8,000 and 1,800, respectively. In Britain and the USA, it is the second most common cancer in young people (aged 15 to 34) and the incidence is rising by 1% per year in both countries. White Americans are almost 30 times more likely to develop melanoma than African Americans. For non-melanoma skin cancers, UV exposure appears to be a major cause, and have had this condition increases the risk of malignant melanoma. Because malignant melanomas develop from moles on our skin they are the easiest cancers to detect at an early stage and if you have any such mark on your skin that changes colour, size or shape you should seek medical advice. The encouragement to do so is that, if identified early, melanomas can be treated by surgery alone and the prognosis is then excellent (five-year survival >95%).