Radon / Thoron Survey in Canadian Metropolitan Areas

Radon and thoron are radioactive gases that occur naturally when uranium and thorium in soil and rock break down through radioactive decay. They are invisible, odourless and tasteless. When released from the ground into the outdoor air, they are diluted and are not a concern. However, in enclosed spaces like homes, they can sometimes accumulate to high levels, which can be a risk to the health of you and your family.

Radon is a member of the uranium decay chain while thoron is a member of the thorium decay chain. Because radon and thoron are members of different decay chains, their concentrations indoors are independent and depend on the uranium and thorium concentrations respectively in local soils and building materials. Many studies have confirmed that there is no clear correlation between radon and thoron concentrations in homes, and thoron concentrations cannot be predicted from widely available radon information.

Radon and thoron break down or decay to form radioactive elements that can be inhaled into the lungs. In the lungs, decay continues, creating radioactive particles that release small bursts of energy. This energy is absorbed by nearby lung tissue, damaging the lung cells. When cells are damaged, they have the potential to result in cancer when they reproduce.

Exposure to high levels of radon and/or thoron in indoor air results in an increased risk of developing lung cancer. The risk of cancer depends on the levels of radon and thoron and how long a person is exposed to those levels.

Radon concentrations are given in units of Bq/m3 (becquerels per cubic metre). A becquerel is the unit used to measure the amount of radioactivity.  Health Canada recommends that steps should be taken to lower the radon level in a home whenever the average annual radon concentration exceeds 200 Bq/m3 in the normal occupancy area (the lowest lived-in location where you spend more than 4 hours per day).  The results from this long-term (3 month) radon test are considered representative of your average annual radon exposure level.

Currently, a recommendation for thoron does not exist. In recent years, exposure to thoron and its health effects have gained increasing attention internationally by organisations such as the World Health Organization (WHO).  While there is a general consensus among scientists that thoron has less health impact than radon at the same concentration in the air, little data exists in Canada or internationally on levels of thoron indoors.

In 2008 through 2011, simultaneous radon and thoron measurements were performed in a total of 370 Canadian homes in 5 communities. Based on these results, it is estimated that thoron contributes only about 8% of the radiation dose due to indoor radon and thoron exposure. To confirm this estimate and whether there is a need for a Canadian thoron guideline, a larger survey of simultaneous radon and thoron measurements in all Canadian metropolitan areas will be conducted by Health Canada in the fall and winter of 2012.

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