Radon is a chemically inert, naturally occurring radioactive gas without odour, colour or taste. It is produced from radium in the decay chain of uranium, an element found in varying amounts in all rocks and soil all over the world. Radon gas escapes easily from the ground into the air and disintegrates through short-lived decay products called radon daughters or radon progeny. The short-lived progeny, which decay emitting heavily ionizing radiation called alpha particles, can be electrically charged and attach to aerosols, dust and other particles in the air we breathe. As a result, radon progeny may be deposited on the cells lining the airways where the alpha particles can damage the DNA and potentially cause lung cancer.
When radon gas itself is inhaled, most is exhaled before it decays. A small part of the inhaled radon and its progeny may be transferred from the lungs to the blood and finally to other organs, but the corresponding doses and associated cancer risk are negligible compared to the lung cancer risk.
Due to dilution in the air, outdoor radon levels are usually very low. Radon can also be found in drinking water, the concentration depending on the water source, and this can sometimes present a hazard. Radon levels are higher indoors, and much higher radon concentrations can be found in places such as mines, caves and water treatment facilities. Health effects have been found in, for example, miners. However, the lower concentrations - found, for example, in normal buildings and to which large populations are exposed – also confer health risks. For most people, by far the greatest exposure to radon comes in the home.
The concentration of radon in a home depends on the amount of radon-producing uranium in the underlying rocks and soils, the routes available for its passage into the home and the rate of exchange between indoor and outdoor air. Radon gas enters houses through openings such as cracks at concrete floor-wall junctions, gaps in the floor, small pores in hollow-block walls, and through sumps and drains. Consequently, radon levels are usually higher in basements, cellars or other structural areas in contact with soil.
Exchange of indoor air with the outside depends on the construction of the house, ventilation habits of the inhabitants, and sealing of windows. The radon concentration in houses directly adjacent to each other can be very different. Radon concentrations within a home can vary with the time of the year, from day to day and from hour to hour. Because of these fluctuations, estimation of the annual mean concentration of radon in indoor air requires reliable measurements of mean radon concentrations for at least three months and preferably longer. Short term radon measurements give only limited information.
Radon radioactivity is measured in becquerels (Bq). One becquerel corresponds to the transformation (disintegration) of one atomic nucleus per second. Radon concentration in air is measured as the number of transformations per second in a cubic metre of air (Bq/m3). The average radon level outdoors varies between 5 and 15 Bq/m3, but both higher and lower values have been observed.
Based on a series of surveys, the global mean indoor radon concentration is estimated to be 39 Bq/m3, with marked variation between countries reported by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). Very high radon concentrations (>1000 Bq/m3) have been found in countries where houses are built on soils with a high uranium content and/or high permeability of the ground. In specific geological formations found, for example, in many European countries, radon released from underground waters easily permeates through the rock to the surface and into buildings. Overall, many countries around the world may have tens of thousands of houses with indoor radon concentrations above levels considered acceptable.
The main health hazard from high radon exposure is an increased risk of lung cancer. This has been substantiated in many studies of uranium miners. Based on these studies, the International Agency for Research on Cancer (IARC), a WHO agency specializing in cancer, and the US National Toxicology Programme have classified radon as a human carcinogen. Scientists have also been investigating whether levels of radon found in homes and other places are a significant hazard to health. These studies are now complete and pooled analyses of key studies in Europe, North America and China have confirmed that radon in homes contributes substantially to the occurrence of lung cancers world-wide. Recent estimates of the proportion of lung cancers attributable to radon range from 6 to 15%. The pooling studies all agree on the magnitude of the risk estimates.
The recent pooled analysis of key European studies estimated that the risk of lung cancer increases by 16% per 100 Bq/m3 increase in radon concentration. The dose-response relation seems to be linear without evidence of a threshold, meaning that the lung cancer risk increases proportionally with increasing radon exposure.
From the results of the same study, when a non-smoker is exposed to radon concentrations of 0, 100 and 400 Bq/m3, the risk of lung cancer by age 75 years will be about 4, 5 and 7 in a 1000, respectively. However, for those who smoke, the risk of lung cancer is about 25 times greater, namely 100, 120 and 160 in a 1000, respectively. Most of the radon-induced lung cancer cases occur among smokers.
In many countries, some homes obtain drinking water from groundwater sources (springs, wells and boreholes). Underground water often moves through rock containing natural uranium and radium that produce radon. This is why water from deep drilled wells normally has much higher concentrations of radon than surface water from rivers, lakes, and streams. Radon concentrations of 20 Bq/l and more, in some instances well above 100 Bq/l, have been measured in individual water supplies in many countries. Calculations indicate that some small risk exists due to radon in drinking water but the few epidemiological studies conducted to date have not found an association between radon in drinking water and cancer of the digestive and other systems. However, more data are needed to better quantify the risk from radon in drinking water.
Most countries have adopted a radon concentration of 200–400 Bq/m3 for indoor air as an Action or Reference Level above which mitigation measures should be taken to reduce the level in homes. Other countries have chosen higher or lower Action Levels. The choice of Action Levels generally has been based on the concept of acceptable risk, i.e. these levels are thought to represent population health risks similar to other everyday risks.
Concerning drinking water, the 2004 WHO Guidelines for Drinking Water Quality and the European Commission recommend that controls - for example repeat measurements - should be implemented if radon in public drinking-water supplies exceeds 100 Bq/l. The United States has proposed a Maximum Contaminant Level for radon of 150 Bq/l for private water supplies. For public or commercial water supplies, the European Commission recommends that remedial action be taken if the radon level exceeds 1000 Bq/l. A tap water radon concentration of 1000 Bq/l contributes 100 to 200 Bq/m3 to indoor air and thus corresponds to the indoor air radon Action Levels discussed above.
Radon levels in indoor air can be lowered in a number of ways, from sealing cracks in floors and walls to increasing the ventilation rate of the building. The five principal ways of reducing the amount of radon accumulating in a house are:
Radon safety should be considered when new houses are built, particularly in high radon areas. In Europe and the United States, the inclusion of protective measures in new buildings has become routine for some builders and - in some countries - has become a mandatory procedure. Passive systems of mitigation have been shown to be capable of reducing indoor radon levels by up to 50%. When radon ventilation fans are added (active system) radon levels can be reduced further.
Recent studies of people exposed to radon have confirmed that radon in homes is a serious health hazard that can be easily mitigated. As a result WHO has established the International Radon Project in which over 20 countries have formed a network of partners to identify and promote programmes that reduce the health impact of radon. The first meeting of the Project was held in Geneva in January 2005 to develop a strategy for dealing with this important health issue. The key objectives of the Project are to:
The International Radon Project will be issuing detailed recommendations on radon risk reduction that will target:
The International Radon Project will provide sound, science-based information on radon control measures and investigate the cost-effectiveness of alternative approaches. The Project will also provide a world-wide estimate of the number of lung cancers attributable to radon exposure that can be used to highlight the global scope of the problem. This estimate will also allow assessment of progress made through future prevention and mitigation programmes.