Radon is an invisible, odourless, tasteless, radioactive gas. It is formed by the disintegration of radium, which is a decay product of uranium. Radon emits alpha particles and produces several solid radioactive products called radon daughters.
Some amounts of radon gas and radon daughters are present everywhere in the soil, water, and air. Particularly high radon levels occur in regions where the soil or rock is rich in uranium. Radon is emitted by radium in the ground, groundwater and building materials. It can enter the indoor air where it and its decay products accumulate in poorly ventilated areas. Harmful levels of radon and radon daughters can accumulate in confined air spaces, such as basements and crawl spaces.
Radon daughters are inhaled with air and deposit in the lungs. The lung absorbs alpha particles emitted by the radon daughters. The resulting radiation dose increases the risk of lung cancer.
Inhaling radon daughters increases the risk of lung cancer. The link between the concentration of radon daughters in the air and the risk of lung cancer was first based on data from a study of lung cancer mortality among uranium miners and other workers exposed to very high levels of radon daughters.
Smoking increases the risk of lung cancer. Smokers exposed to radon daughters are at greater risk of developing lung cancer. According to Health Canada, the risks are as follows: "if you are a lifelong smoker but are not exposed to radon, your risk of getting lung cancer is one in ten. If you add exposure to a high level of radon, your risk becomes one in three. On the other hand, if you are a non-smoker, your lifetime lung cancer risk at the same high radon level is only one in twenty."
Radon daughters are solid particles. Most of the radon daughters become attached to tiny dust particles (aerosols) in the indoor air. A variable proportion remains unattached. When these particles are inhaled, a fraction of both attached and unattached radon daughters is deposited in the lungs. Inside the lung, radon daughters emit alpha particles that are absorbed in the nearby lung tissues. Since alpha particles cannot penetrate more than a fraction of a millimeter into the tissue, the damage is confined to the lung tissue in the immediate area.
The following figure illustrates the radioactive decay chain that produces radon and radon daughters.
Figure--Production of radon and radon daughters from uranium
Each radioactive isotope decays at a unique rate described as the half-life of that isotope. This is the time required for half the atoms of a radioactive substance to disintegrate. Radon's half-life is 3.8 days. This means that, in the absence of its parent radium, the intensity of alpha particles from a given sample of radon will decrease by one-half in 3.8 days; to half the remainder (i.e., one-quarter of the original) in another 3.8 days; to an eighth in another 3.8 days; and so on. However, this does not happen indoors because as old radon decays new radon continuously comes out from the decaying radium present in the ground and walls.
Radon daughters have very short half-lives ranging from a fraction of a second to 27 minutes. As a result, radon daughters are present in significant quantities only as long as radon is present. If all the radon gas is removed, the radioactivity of radon daughters will fade away quickly.
The concentration of radon in the air is measured in units of picocuries per litre (pCi/L) or becquerels per cubic meter (Bq/m3). One Bq corresponds to one disintegration per second. One pCi/L is equivalent to 37 Bq/m3.
The concentration of radon daughters is measured in units of working level (WL) which is a measure of the potential alpha particles energy per litre of air. One WL of radon daughters corresponds to approximately 200 pCi/L of radon in a typical indoor environment. However, the relative concentration of radon and radon daughters may vary from one building to another. In the extreme case 1 WL corresponds to 100 pCi/L of radon. This situation is called full equilibrium and is extremely unlikely to occur. Occupational exposure to radon daughters is expressed in working level months (WLM) and a working level month is equivalent to the exposure at an average concentration of 1 WL for 170 working hours. Measurement data are reported in either of the above units. For making comparisons between the data from different sources, the following conversion chart may be useful:
1 pCi/L = 37 Bq/m3
1 m3 = 1000 L
0.01 WL = 74 Bq/m3 = 2 pCi/L
0.02 WL = 148 Bq/m3 = 4 pCi/L
0.1 WL = 800 Bq/m3 = 20 pCi/L
The document Quantities and Units of Ionizing Radiation provides more details on the units of ionizing radiation.
Radium in the soil directly under a building is normally the major source of indoor radon. Less important sources of radium are in ground water and building materials.
The presence of uranium in soil and rock is an important indicator of places where radium and radon can be present. Because radon is a gas, a fraction of the radon produced in the soil can find its way into a building. The rest is trapped in the soil. In the air, radon decays to radon daughters that are solids, and are present in the building air as fine particles.
The concentration of radon and radon daughters in the indoor air depends on:
Because radon is a gas, changes in the atmospheric pressure also affect its emission from the ground and its accumulation in the building air.
The concrete floor and walls in the basement slow down the movement of radon from the soil into the building. However, cracks in the floor, wall slab joints, and the drainage system allow radon to enter a building.
Indoor radon concentrations are almost always higher than outdoor concentrations. Once inside a building, the radon cannot easily escape. The sealing of buildings to conserve energy reduces the intake of outside air and worsens the situation. Radon levels are generally highest in cellars and basements because these areas are nearest to the source and are usually poorly ventilated.
In Canada, the Canadian Nuclear Safety Commission (CNSC) sets radiation exposure limits. It gives two types of exposure limits--one for occupationally exposed persons and another for the general public. The annual occupational exposure limit is an effective dose of 4 mSv (milli-Sievert). The annual exposure limit for the general public is an effective dose of 1 mSv. These values are found in the Radiation Protection Regulations (SOR/2000-203).
Acceptable levels of radon in "dwellings" which includes homes or public buildings (schools, hospitals, long term care facilities and correctional facilities) is 200 Becquerels per cubic metre (200 B/m3) based on the Government of Canada Radon Guideline.
The threshold limit value (TLV®), or occupational exposure limit, established by the American Conference of Governmental Industrial Hygienists (ACGIH®) is 4 working level months (WLM/year) (2012).
A 2012 Cross-Canada survey, conducted by Health Canada’s National Radon Program, found that no areas of Canada are "radon free." One of the main purposes of the study was to estimate the number of Canadians living in homes with radon gas levels above the guideline of 200 Bq/m3, which identified 6.9% of the population living in this situation. This level is similar to the results of a 1970’s Cross Canada survey, which showed that 5% of Canadians were living in homes that were above the 200 Bq/m3 guideline.The results of the survey are not meant to be used to determine radon risk potential. The only way to know for sure if your home or workplace has levels of radon higher than the guidelines is to conduct testing in each home or workplace of concern.
Indoor radon level is measured by air sampling and by alpha dosimetry using radon track etch dosimeters. A number of companies manufacture and sell measuring instruments.
Since radon levels vary greatly from day to day, Health Canada recommends long-term sampling (3 to 12 months) to get a more accurate reading (e.g., at least 3 months and ideally during the winter to get a representative sample).
Health Canada also indicates that testing can be done by professionals or by using testing kits that can be purchased over the internet or from some home improvement stores. Costs for the kits range from $50 to $100. Be sure to follow the testing kit’s instructions carefully to get accurate values.
Health Canada recommends that the higher the radon concentration, the sooner you should take steps to remediate the issue.
Health Canada continues "If your home tests above the guideline you should hire a certified radon professional to determine the best and most cost effective way to reduce the radon level in your home. The most common radon reduction method is called sub-slab depressurization. With this solution a pipe is installed through the basement sub-flooring to an outside wall or up through to the roof line with a small fan attached which draws the radon from below the house to the outside before it can enter your home. This type of system can reduce the radon level in a home by over 90%. Increasing ventilation and sealing major entry routes can also help reduce radon levels but their effectiveness will be limited depending on how high the radon level is and the unique characteristics of each home."
Health Canada recognizes the certification programs offered by the National Environmental Health Association.
Document last updated on February 5, 2013