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Irish Medical Times: Health Threats from Radon Radiation

Irish Medical Times: Health Threats from Radon Radiation

Stephanie Long and Dr Éamann Breatnach examine the problems caused by exposure to high levels of radon radiation and ways to address the issue.

The Irish population is constantly exposed to ionising radiation of both natural and man-made origins. Natural radiation comes from long-lived radionuclides present in the earth’s crust since the formation of the planet and from outer space. For most people, by far the greatest source of exposure is from naturally-occurring radiation.

The largest source of natural radiation is radon gas, which accounts for 56 per cent of the radiation dose received by the Irish population. Radon gas is also the exposure pathway where the greatest reduction is possible. Most other pathways either make a much smaller contribution to the dose or are not amenable to control.

This article compares the radiation dose received by the Irish population from radon with that received from other sources and explains how exposure to radon can be reduced.

Radon is released from rocks and soil during the decay of the radioactive element uranium. It can then move through the soil and is constantly released into outdoor air. Because of the natural pressure differential that exists between indoor and outdoor air, radon from soil is preferentially ‘sucked’ into buildings through any gaps that may exist in the foundations. In this way, it can build up to high levels in some buildings.

In order to quantify the extent of the radon problem in Ireland and to identify those areas most at risk from high indoor-radon levels, the Radiological Protection Institute of Ireland (RPII) carried out a national radon survey. The results of this were used to generate a radon predictive map, which indicates ‘high-radon areas’.

A high radon area is defined as an area in which it is predicted that 10 per cent or more of homes have radon levels in excess of 200 Bq/m3 (the national reference level). An interactive version of this map is available on the RPII’s website, www.rpii.ie.

Radon has been classified by the International Agency for Research on Cancer as a Group I carcinogen. International epidemiological studies have established the link between long-term exposure to high radon levels and an increased risk of developing lung cancer. The risk of developing lung cancer is dominated by one’s exposure to tobacco smoke, however, the synergistic effect between radon and tobacco smoke means that the risk to active smokers is approximately 25 times greater than that to a lifelong non-smoker (the health risks from radon were described in detail in a 2010 IMT article, see http://bit.ly/oRJvIh). In Ireland, exposure to radon is linked to up to 200 lung cancer deaths each year.

Sources of radiation exposure
The biological impact of radiation is measured in units called sieverts (Sv). In practice, radiation doses are measured in terms of fractions of a sievert, for example millisievert (mSv) or microsievert (μSv). The average individual radiation dose per year in Ireland has been calculated by the RPII to be 3,950 μSv.

The estimated annual dose to each individual from radon in the home and workplace are 2,050 μSv and 180 μSv, respectively, giving a total of 2,230 μSv. Radon in the home is the largest component, as people generally spend more time at home than at work. Radon is also the most variable source of radiation, with levels in Irish homes ranging from 10 Bq/m3 to 49,000 Bq/m3.

A level of 10 Bq/m3 results in an annual radiation dose of 250 μSv; while a level of 200 Bq/m3 (the national reference level) results in an annual dose of approximately 5 mSv. This dose increases linearly with increasing radon levels.

To put these doses in context, a typical dental x-ray results in a radiation dose of about 10 μSv, a chest x-ray results in a dose of about 20 μSv, while a CT scan results in a dose of about 5.4 mSv. Clearly, exposure to very high radon levels can result in significant radiation doses.

For example, the occupants of a recently-identified home in north Kerry with 37,000 Bq/m3 of radon received an annual radiation dose of the order of 925 mSv and a daily dose of about 2.5 mSv. This is an extreme example; however, the RPII estimates that there are about 91,000 homes in Ireland that have radon levels exceeding the reference level of 200 Bq/m3. Consequently, there are many members of the public that are unknowingly exposed to an unnecessary radiation dose.

A summary of the contribution of each source of radiation exposure to the population is as follows:

  • Cosmic radiation, which accounts for an annual average dose of 345 μSv;
  • Gamma radiation in soils accounts for an annual dose of 310 μSv;
  • Thoron in homes, which accounts for an annual dose of 280 μSv;
  • Radioactivity in the diet, which accounts for an annual dose of 240 μSv (5 μSv of this being due to man-made radionuclides); and
  • Exposure to radioactivity at work, which accounts for an annual average dose of 5 μSv in addition to the 180 μSv due to radon in above-ground workplaces.

The main source of exposure to artificial radioactivity is the medical exposure of patients for diagnostic purposes. The highest individual doses to patients are associated with interventional radiology procedures such as percutaneous transluminal coronary angioplasty, angiocardiography and certain CT scans. These account for 43 per cent of the procedures carried out but 74 per cent of the total dose.

Conventional radiology accounts for 57 per cent of all procedures, but only 26 per cent of the total dose. The use of all x-ray procedures in hospitals results in an estimated dose per person of 500 μSv. Nuclear medicine procedures are estimated to result in a dose per person of 40 μSv, giving a total annual dose of 540 μSv. Doses due to radiotherapy are not included in these figures. This is because these patients are not representative of the general population.

Reducing the radiation dose

Radon exposure can be reduced by either identifying buildings with high levels of radon and remediating them, or by incorporating radon prevention into new buildings. The most commonly used methods to achieve these are described below.

Reduction of radon levels in existing homes.

A number of methods are used to reduce radon levels in homes. For moderately high levels (200 Bq/m3 to 400 Bq/m3), increasing indoor ventilation (by installing or unblocking air vents or installing trickle vents in windows) can reduce radon levels by up to 50 per cent.

For homes with higher radon levels, the most effective and most commonly used method is the installation of an active sump.

This is carried out by creating a cavity about the size of a bucket in the ground immediately under the floor slab that is open to the surrounding under-floor hardcore and linked by pipework to the outside. It operates by reversing the pressure differential between the space under the floor and the room above. This pressure differential, which exists as a result of the temperature difference between inside and outside the home, is responsible for radon-laden air from the ground being drawn into the home.

This radon-laden air is drawn out from under the floor slab by an electric fan in the pipeline and so is prevented from entering the occupied indoor space.

Work to install a sump is carried out outside the home with very little disruption. Normally, work is complete within one day and costs on average €1,100.

A standby pump

‘Building Regulation 1997, Technical Guidance Document C — Site Preparation and Resistance to Moisture’, published by the Department of Environment, Community and Local Government, requires that all new homes built since 1 July 1998 are installed with a standby sump. A standby sump comprises the cavity under the floor slab, vented to outside and capped. Sumps are activated by extending the pipework and installing a fan. Thus, installation of an active sump in any home built since this date is less expensive and simpler than for homes without a standby sump.

An active sump will reduce radon levels on average by about 90 per cent and so is a very effective way of significantly reducing the radiation dose. Sumps are effective for all levels of radon, even extremely high levels. In fact, it has been shown that the higher the initial radon level, the greater the percentage reduction when an active sump is installed.

Prevention of radon ingress into new homes.

‘Technical Guidance Document C’ also specifies that all homes built since 1 July 1998 in high-radon areas are installed with a radon barrier during construction. A radon barrier is a fully-sealed membrane of low permeability that is placed over the entire footprint of the building and should be impermeable to radon.

The impact of the requirement to install a radon barrier has been assessed using data from measurements of homes by local authorities in Co Cork. The radon levels in homes measured before 1998 (without barriers installed) was compared to levels in homes built from 1999 onwards (with barriers installed). These data show that, on average, the installation of a barrier has resulted in a reduction in radon level of approximately 50 per cent and, in some cases, up to 70 per cent.

These results also agree well with a study of the impact of the barrier requirement on privately-owned homes in the high-radon areas of Ennis, Kilkenny and Tralee carried out between 2001 and 2003. This study showed an average reduction of between 40 per cent and 55 per cent in the radon level in homes fitted with barriers.

Dose reduction due to remedial and preventive work.

Since radon is the main source of human exposure to radiation in Ireland, there is considerable scope for significant radiation dose reduction to the occupants of homes with elevated levels of radon by using the simple reduction techniques described above.

For example, the home with measured radon levels of 37,000 Bq/m3 was remediated by installing two active sumps, with a reduction in radon levels to 90 Bq/m3. This resulted in a reduction in the radiation dose from 2.5 mSv per day to 6 μSv per day. Whilst this was a particularly unusual measurement, homes with very high radon measurements are measured regularly.

For example, during 2011, some 15 homes with radon levels above 2,000 Bq/m3 have been identified. These homes are unusual; however, there are many homes with moderately high radon levels in which the individual radiation dose to the occupants of the house may be easily reduced. For example, the average radon level in 130 north Cork homes in high-radon areas was 660 Bq/m3; following remediation by the local authority this was reduced to 41 Bq/m3. This work will have resulted in a significant reduction in the radiation dose to the tenants of these homes.

As discussed, the installation of radon barriers has been shown to decrease the radon level by about 50 per cent in new homes. This will result in a corresponding 50 per cent decrease in the dose to occupants of new homes installed with radon barriers.

Conclusions

In Ireland, exposure to radon gas is the most significant source of radiation exposure and is linked to up to 200 lung cancer deaths each year. However, this dose can be easily and significantly reduced by using straightforward preventive and remediation measures.

Further information regarding radon measurement and reduction is available from the RPII at www.rpii.ie.

View this article: http://www.imt.ie/features-opinion/2011/11/health-threats-from-radon-radiation.html.