Radon gas and its byproducts occur naturally everywhere - in soil, water and air. Rarely does it occur in concentrations that you need to worry about. However, radon gas can accumulate in confined spaces such as basements and crawl spaces in homes. If the levels are high enough, it can be a health hazard.

As radon decays, it produces decay products called "radon daughters," They also decay rapidly and emit alpha particles. While your skin is enough to protect you from these particles, when they become attached to dust and you breathe them in, you could be at risk. The risk arises from the alpha radiation that is emitted by radon and certain of its daughters. When alpha radiation strikes the cells of the soft tissue of the lung it can bring about changes in these cells. These changes may lead to lung cancer. Studies have confirmed that long exposure to elevated radon levels in our homes is the leading cause of lung cancer next to smoking.

It depends what country you live in. The levels that had been considered safe in Canada were substantially higher than in the rest of the world — as much as five times higher. They were equivalent to the exposure considered safe for someone who spent their working hours in a uranium mine.

But Health Canada reviewed those standards and recommended the level considered safe be reduced to at least match what's considered safe in the United States.

Radon gas levels are measured in a unit called becquerels (Bq) per cubic metre. One becquerel is described as one event of radioactive emission per second. It is an extremely small unit. The old Canadian standard considered 800 Bq per cubic metre to be an acceptable level.

On June 8, 2007 — 15 months after the Federal-Provincial Territorial Radiation Protection Committee received a report recommending tightening the guidelines — the federal government announced a new standard of 200 Bq per cubic metre. That's a level considered safe by most industrialized countries, including Russia, the Czech Republic and China. The United States recommends a level of 150 Bq per cubic metre.

The Canadian report that recommended tighter guidelines noted that as people make their homes tighter and more energy efficient, they may also be inadvertently raising radon levels. The report suggested the government should explore a system of grants and subsidies to help homeowners with the costs of testing and cleaning up radon gas.

It also recommended that private homes undergo mandatory tests for radon levels as a condition of sale, as is the case in several American states. The new guidelines did not address that recommendation.

The report released in 2009 by the UN Scientific Committee on the Effects of Atomic Radiation, or UNSCEAR, used 20 studies in homes where concentrations of radon were very low, rather than relying on estimates of radon risk that were extrapolated from studies of uranium miners who were exposed to high levels of the gas. This report estimates the risk of radon exposure to members of the public and it recommends a level that is consistent with that recommended by health Canada in 2007.

The World Health Organization also published its findings about the risk of radon in 2009. It recommends the testing of all homes and remedial action where the level exceeds 200 Bq per cubic metre.