Radon gas exposure is the second leading cause of lung cancer after smoking. Epidemiological studies…
“Radon the silent killer is blamed for 1 800 cancer deaths a year” (Daily Mail 1998).
This dramatic statement in the Daily Mail was typical of the reports carried on Tuesday, 19 May 1998, in the majority of national newspapers. The Guardian commented:
“Radioactive gas link to cancer” (The Guardian 1998).
The Times was equally alarming:
“Radon blamed for 1 in 20 lung cancer deaths” (The Times 1998).
Such headlines were commenting upon a report by the Imperial Cancer Research Funds epidemiology unit at Oxford that demonstrated for the first time, in the UK, that there was significant evidence to confirm that radon was a human carcinogen at the levels found in domestic dwellings (Darby et al 1998). Up until this report, the impact of radon on health had largely been evaluated from studies on miners, working in much elevated levels of the gas.
These were not isolated stories. Newspapers, national and local, have been carrying quite regular, high-grade, informative statements on radon for the past decade as knowledge of the growing geographical extent of the problem increased. Between 1992 and 1997 alone, The Times had 48 stories on radon and the Guardian and Observer together had 56. These are not large numbers when compared to other environmental issues, such as stratospheric ozone, for which there are many times more reports, but this is more than offset by their quality as many of them are scientifically accurate and well written. More importantly, they inform concerned members of the public whom to contact if they want further advice about how to take preventive action. Television and radio, national and local, have also made significant contributions to the public awareness, in the UK, of the link between high radon levels in buildings and lung cancer.
The chemical element
Radon is a colourless, odourless gas found in Group 0 of the Periodic Table. The only isotope abundant enough to be a health problem is 222Rn , formed by the decay of 226Ra, which is part of the 238U series, this contributes around 50% of the dose to the average person in the UK. The other possibly significant isotope is 220Rn, formed via the thorium decay series, but its half-life is so short (55 seconds) that it is, on the whole, unlikely to contribute more than 4% of the dose received by the average UK person. Radon gas levels are normally quoted in Bq m-3 (1 Bq is one decay per second).
Average indoor levels in the UK are about 20 Bq m-3. Local geology that has high levels of uranium would be expected to produce high levels of radon in soil gas, but for it to escape to the surface the soil should also be highly porous and allow gas to migrate. On the whole, only radon produced in the top two metres of soil will pose an environmental threat. Even as late as the early 1970s, it was considered that unusual soil conditions were required to produce high gas levels (Scott 1994). Simple models were developed that predicted house radon concentration and these were based mostly upon the soil radium concentration and to some extent its permeability. As more data were collected, it became apparent that such models were far from perfect, as soils that were previously considered to be of little risk were in fact radon-prone, e.g. clay soils. More realistic models had to be developed to take account of soil fracture patterns, as they dry out, as well as permeability. What is surprising, in retrospect, is that geological data indicating that high radon levels could be found in a wide variety of environments was available from a range of research areas, e.g. to predict earthquakes (Editorial 1983), study monsoon circulations (Rangarajan et al 1984) as well as initial work for `The National Uranium Resource Evaluation Programme`. What was lacking was a programme to bring together and analyse all the diverse data in the primary literature but as there was no apparent conceptual framework with which to handle it, data was not processed. There was little recognition of the danger to the public, no risk assessment was carried out so no effective management plan was developed.
222Rn decays with a half-life of 3.82 days into 218Po, which in turn decays into 214Pb, 214Bi and 214Po. These radionuclides are commonly known as radon daughters or progeny and they become attached to particles in the air and get breathed into lungs. The vast majority of the radiation damage caused to living tissue is from the progeny, not from the radon itself. 222Rn, 218Po and 214Po are all alpha particle emitters; these result in very localised energy deposition in the respiratory system. Alpha particles are high linear energy transfer (high LET) radiation and, as such, deposit most of their energy over a short distance, causing massive chemical and biological damage.
The amount of energy that ionising radiation imparts to tissue is called the absorbed dose. When the degree of damage of different types of radiation is considered then the equivalent dose can be calculated and finally when the sensitivity of different tissues is taken into account it is possible to calculate the effective dose, the unit of which is the Sievert (Sv), values are normally reported in milliSieverts (mSv). The annual dose to the average person in Cornwall is 7.8 mSv, with radon making up 81% of that, while to the average UK person the value is much less at 2.5 mSv. The common assumption in radon studies is that health risk is proportional to radiation dose and so a linear, no-threshold model is commonly employed.
The Action Level, above which action should be taken to reduce radon, in UK homes is 200 Bq m-3 and for the workplace it is 400 Bq m-3. These values vary between countries, the USA having a domestic level of 148 Bq m-3, the Netherlands has set a target as low as 20 Bq m-3 for average levels in homes, while Canada has a level as high as 800 Bq m-3. The Basic Safety Standards for Radiation (International Atomic Energy Agency) is 200-600 Bq m-3.
Radon in buildings
Why does radon enter buildings and reach such relatively high concentrations compared to outside air? In most buildings the vast majority of radon comes from the subjacent soil rather than the external air or building materials. The air in most buildings is at a lower pressure than the external air. This small pressure difference (only a few pascals) is enough to draw radon from the surrounding soil into a building via cracks in floors and walls. Modern buildings that minimise draughts and reduce ventilation are prone to higher levels, on the same geology, than those that are poorly insulated and leave windows and doors open for long periods. It is interesting to speculate what levels were like previous to the 1960s, after which a large proportion of the population began to insulate their homes and install double glazing
4. RADON IN THE WORKPLACE
Surveys of radon in mines, in the UK, have taken place since the mid-1960s when it was found that around 40% of miners in noncoal mines were exposed to levels considered dangerous to health. When these occurred there were no statutory regulations controlling radiation exposure for radon. The Ionising Radiation Regulations introduced statutory control of radon in workplaces for the first time in 1985 (Health and Safety Executive 1985).
When elevated levels of radon were first found in domestic dwellings in Cornwall it was realised that above ground workplaces were likely to be affected in a similar proportion. A planned survey was carried out for local authorities in the southwest of England, especially in schools and offices, which confirmed the original suspicions. Those above the Action Level for the workplace (400 Bq m-3) have either had to reduce levels to below 400 Bq m-3 or restrict staff doses by applying the Ionising Radiations Regulations with the designation of a supervised area. Above 1 000 Bq m-3 there is the requirement for the designation of a Controlled Area.
What has comprehensive testing of workplace radon demonstrated? In 1996, it was reported that results were available for around 6 000 workplaces in the UK in areas with high radon (Dixon et al 1996). Cornwall was the worst affected with 21% of workplaces above the Action Level, Northamptonshire having some 14% and Somerset, the lowest, with 5%.
The application of the Regulations is the responsibility of the Health and Safety Executive for certain types and size bands of business (mostly large and the total is 450 000 for the whole UK) while for others it is the responsibility of local authority Environmental Health Officers. How do these agencies check if employers are following legislation? Both are under resource constraint so visits to check on employers are unlikely. Discussions, when visits are made for other reasons, are one way but the preferred method has been the targeted mailshot according to postcode, to hit those with the highest probable level of radon. Around 700 registered premises in Northamptonshire and around 300 in Derbyshire were so selected for a recent campaign. Employers need to be educated about the need to include radon in their risk assessments.
Radon in the workplace must be considered a priority area for the future as there is legislation that can be used to control levels. One of the big stumbling blocks is the cost of such radon programmes to employers. In 1992, one of us commenced a programme of testing in National Health Service premises in Northamptonshire and found elevated levels of radon (Denman 1994). Further investigation found that there were certain workers who were receiving very high doses of radiation (Denman et al 1996). To deal with the problem, a large radon mitigation programme was carried out and some 1 038 locations were tested with the highest level being 3 750 Bq m-3 (Denman et al 1997). The total cost of the programme was in the region of £100 000 and it did enable the NHS to achieve a large dose reduction to staff at a value of £184 000 per Man-Sievert. This is around half the amount the NRPB calculate is required to achieve similar dose reductions to patients from dental X-rays and which they considered justified when compared to the costs of the effects of radiation. Can most companies afford extensive surveys? Is the expertise available to carry them out? These are serious questions for the coming years. When will the first litigation occur when a worker, who has developed lung cancer, suggests it was due to a high level of radon and that the employer had not taken the steps to test for and reduce it?
Information taken from: http://oldweb.northampton.ac.uk/aps/env/wastes/radon_hotline/radonstory.htm