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Radon is a naturally occurring inert gas produced as a part of the decay chain of uranium, which is found in many types of soils. Previous studies have shown that significant exposure to radon can be a cause of lung cancer (Masters, 1998). In the United States it is widely recognized that radon gas released from soils can build up in residential homes, in some cases resulting in a significant health hazard. As a result of this, numerous studies have been undertaken in the US, focusing on radon levels in homes and ways to mitigate them. In other countries, including Australia, public awareness of this problem is extremely limited and as such little research has been undertaken to determine whether or not elevated radon concentrations are present in homes.
Introduction
Radon gas is an inert gas that forms naturally as part of the decay chain of uranium, see Figure 1.1. It is formed as the uranium found naturally in most soils, decays over time. Radon gas formed between pore spaces in soils can work its way to the surface, where it can enter building through cracks in the floor (Gates, 1992). While radon itself is chemically inert, its daughter products, polonium, bismuth and lead are chemically active. These daughter products can easily become attached to small particulate matter that is inhaled deep into the lungs, where they can cause significant damage as carcinogens (Masters, 1998).
Figure 1 - Decay Chain of Uranium 238 and Half-lives, (Gates, 1992)
While radon is also produced from other sources, the biggest single source of radon in houses is from soil (Mueller Associates, 1988). As such it is suspected that the underlying geology of a particular area has a significant effect on the prevalence of radon problems in that area. In particular, areas where igneous rocks of granitic composition are common, have shown themselves to be particularly prone to elevated levels of radon gas in buildings (Gates, 1992).
Radon
The term radon applies to any one of the isotopes of the element with the atomic number 86. These isotopes are colourless, odourless, chemically inert gases. The principle isotope of radon is 222Rn. For the remainder of this article , the term radon will be used to refer to 222Rn. The other isotopes of radon are not considered to be significant health risks due to their short half-lives (Gates, 1992), with the exception of thoron ( 220Rn), which does occur in sufficiently high levels to be a concern in some areas (Gates, 1992). Radon is a naturally occurring product of the decay of 238U (see Fig 1), and has a half-life of 3.82 days. Radon in turn leads to the progeny 218Po, 214Pb and 214Bi. 214Bi in turn produces 214Po. Radon and its three progeny are known carcinogens (Masters, 1998). Figure 2 shows radon and its decay products.
Figure 2 - Radon decay chain. (U.S. Geological Survey, 2002)
Health Effects From Radon Exposure
While the health effects of exposure to elevated radon levels in residential homes is the subject of much debate, it has been well documented that exposure to high levels of radon progeny in underground mines can lead to an elevated risk of lung cancer (National Radiation Laboratory, NZ, 2000) (Gates, 1992). Radon produces two short-lived progeny, 218Po, 214Po, which are solids and can attach themselves to aerosol particles. Polonium, when inhaled can lodge deep in the lungs and cause damage to lung tissue by alpha radiation as it decays. The primary sources of data relating to the carcinogenic effects of radon and its progeny come from studies of underground uranium miners the United States, the former Czechoslovakia and Canada; iron and zinc miners in Sweden and Great Britain and fluorspar miners in Newfoundland. It was shown that 3-8% of miners studied developed lung cancer that was directly attributable to radon exposure (Cross, 1987).
Using data from miners to estimate risks due to residential exposure introduces large uncertainties. Miners could be expected to be exposed to much higher concentrations of particulate matter, and have much higher breathing rates than a person resting in their home. A number of studies have been conducted to try and find a link between cancer rates and radon levels in homes. The largest such study, conducted in Sweden showed that cancer rates increased by 30% amongst those who lived in homes with radon levels of 140 Bq/m3 and above, when compared to houses with a level below 58 Bq/m3 (Pershagen, 1994).
In the United States, indoor radon is seen as a significant health risk, with over 16 000 lung cancer deaths per year attributed to radon. Radon is ranked as the sixth most important health risk in that country, behind tobacco smoking, motor vehicle accidents and homicide, but ahead of alcohol related deaths and skin cancer (Masters, 1998).
Sources of Radon
There are numerous sources of radon, both natural and human catalysed, that can give rise to elevated levels on indoor radon. By far the dominant source of radon gas in houses is the soil on which they are built. Other sources however include groundwater, earth derived building materials and natural gas. Each of these sources will be examined in detail.
Soil
By far the dominant source of radon in buildings is the surrounding soil. Radon is produced continually within soil as a result of the decay of radium-226, which is in turn a result of the decay of uranium-238 (Mueller, 1998). When a radium atom decays to radon, energy is released which results in what is known as alpha recoil. This energy is strong enough to drive the newly formed radon atom up to 40 nanometres (nm) (Gates, 1992). It is largely as a result of this recoil action that radon is forced into the air filled pores between soil grains and is able to then percolate to the surface (Gates, 1992, Mueller Associates, 1988).
Figure 3 - The decay of radium into radon showing the emission of an alpha particle. (U.S. Geological Survey, 2002)
The rate of radon production in soils is heavily influenced by the distribution of uranium in the soil. As such the uranium content of soils is a commonly used method of accessing the radon potential in an area. In the United States, geological data along with other measurements, have been used extensively to map areas that have high radon potential and good correlation has been shown between high uranium content geology and high levels of indoor radon (Gates, 1992). Table 1 and Figure 4 show the correlation between soil uranium and radon concentrations for soils from Virginia in the United States. The rock types listed identify the underlying geological unit.
Table 1 - Comparison of Uranium Concentrations to Radon Concentration for Soils. (Gates, 1992)
Figure 4 - Comparison of Uranium Concentrations to Radon Concentration for Soils. (Gates, 1992)
Water
Studies have shown that water can be the source of up to 35% of indoor radon in homes (Mueller Associates, 1988). This result is significantly dependent on the source of the water in question. Surface water can be expected to have relatively low concentration of radon due to atmospheric exchange. Groundwater however can show high radon concentrations if it is sourced from aquifers with high radium and uranium concentrations. This radon can be released into the indoor atmosphere when water is used for domestic purposes within the dwelling (i.e. showering, laundering, dishwashing). Studies conducted in the United States have shown that indoor radon levels are increased by approximately 10 Bq/L for every 37 000 Bq/L concentration in water. Whilst most domestic water supplies have radon levels of less then 74 000 Bq/L some groundwaters have been shown to have concentrations of up to 3 700 000 Bq/L. (Mueller Associates, 1988).
Building Materials
Building materials derived from soil and rock can be important contributors to indoor radon concentrations. These materials diffuse radon into indoor living spaces and can significantly elevate indoor levels. The emanation of radon from building materials is affected by two factors, namely the type of material and the environmental conditions. The most significant material in this regard is concrete whilst the least significant is wood. The amount of radon emitted by concrete is also significantly effected by the types of aggregate used, leading to significant variations in the level of radon emitted by different types of concrete. A further complication arises from the fact that for a given type of building material the rate of radon emission varies significantly from one sample to another (Mueller Associates, 1988).
Different environmental conditions have also been found to affect the rate of radon emission. Studies have been undertaken which show that pressure differentials effect the amount of radon emission, (Jonassen, 1977), with the rate of emission increasing with a decrease in ambient air pressure. The effect of temperature is less clear cut. Stranden et al, 1984 found that the rate of emission of radon increased with concrete temperature however Auxier et al, 1973, has shown that for the range of temperatures expected to be found in a residential dwelling, temperature is not a significant factor. Moisture on the other hand has been shown to have significant influence on radon emission with a doubling of moisture content in concrete resulting in an increase of the order of 10-20% in the rate of radon emission (Auxier et al. 1973).
Home Heating Fuels
The use of heating fuels can lead to elevation of indoor radon levels, though the increase from this source is usually small. Radon is present in small amounts in natural gas and can find its way into the home when this fuel is used for home heating purposes. The effect of this source can be largely mitigated if the combustion products are vented to the external environment. The low occurrence of unvented heaters results in the contribution from this source usually being negligible. The fact that nearly all natural gas undergoes processing prior to distribution also results in a significant reduction in the amount of radon present in natural gas. This has the spin off effect however of increasing radon concentrations in liquefied petroleum gas (LPG). Processing of natural gas involves the separation of hydrocarbons and the removal of impurities. Some of these hydrocarbons are bottled for sale as LPG. Radon tends to be removed from natural gas with the hydrocarbons, resulting in a significantly higher level of radon in LPG, in some cases an increase of ten fold or more, (Bernhardt, 1973). Once again, the effects of radon from LPG can be largely mitigated by the use of vented heaters and in most cases does not figure as a significant source of radon in homes.
Having discussed what Radon is, its potential health impacts and how it can come to be present in your home, in a subsequent post, I will outline factors that effect the concentration in a given home.
References
Bernhardt, D.E., Memorandum to D.W. Hendricks, Radon in Natural Gas, May 24, 1973.
Canadian Centre for Occupational Health and Safety, 2002
http://www.ccohs.ca/oshanswers/phys_agents/radon.html#_1_5
Imperial Cancer Research Fund, Lung Cancer and Radon in the Home:
First Direct Evidence of an Effect in the UK Population, Press release, London 1988.
http://www.uicc.org/publ/pr/archives/98051901.shtml#fs
Jonassen, N. and J.P. Mc Laughlin, Radon in Indoor Air, II, Lingby: Technical University of Denmark, Laboratory of Applied Physics 1, (Research Report 7), 1977.
Langaroo, M.K., Wise, K.N., Duggelby, J.C. & Kotler, L.H., A Nation-Wide Survey of Radon and Gamma Radiation Levels in Australian Homes, Australian Radiation Laboratory, Yallambie, 1990.
National Council on Radiation Protection, Exposures from the Uranium Series with Emphasis on Radon and Its Daughters, Report No 77, Washington, D.C., 1984.
National Radiation Protection Institute, Czech Republic, 2002
http://www.suro.cz/en/faq/radonvdome.html
Natural Resources Canada, 2002
http://gamma.gsc.nrcan.gc.ca/radon_e.html
Stranden, E., and A.K. Kolstad, and B. Lind, Radon Exhalation: Moisture and Temperature Dependence, Health Physics, Vol. 47, No 3, September 1984.