Why Health Canada lowered the radon guideline from 800 to 200 Bq/m³

What the guideline was before 2007

From 1988, Canada’s guideline for radon in homes was 800 Bq/m³. It was one of the highest action levels among developed countries. Most homeowners who tested in the 1990s and early 2000s compared their results to that number, and most came in well below it.

What changed the science

Early estimates of radon risk relied largely on studies of underground miners, who were exposed to very high concentrations. In the early 2000s, researchers pooled data from residential case-control studies: thousands of people, comparing those who developed lung cancer with those who didn’t, and measuring radon in the homes they had lived in for years.

The European pooling found lung cancer risk rose by about 16% for every 100 Bq/m³ increase in long-term indoor radon, with no evidence of a safe threshold. North American pooled studies pointed the same way. Health Canada’s 2006 proposal for a new guideline noted both points: the old 800 Bq/m³ level was out of step with most other countries, and pooled studies now showed an association between radon and lung cancer down to about 100 Bq/m³.

The new guideline: 200 Bq/m³

After a public consultation, the federal, provincial and territorial governments adopted the new guideline in June 2007. Its main points:

• Remedial action should be taken when the average annual radon concentration exceeds 200 Bq/m³ in the normal occupancy area of a home.
• The higher the concentration, the sooner action should be taken: within 2 years for 200–600 Bq/m³, and within 1 year above 600 Bq/m³.
• Levels should be reduced as low as reasonably achievable after mitigation.
• New buildings should be built to minimize radon entry and to make future mitigation easy. Ontario wrote this into its 2024 Building Code.

What it means if your home was tested before 2007

A home that measured 500 or 700 Bq/m³ in 2005 “passed” at the time. Under today’s guideline it calls for action within one or two years. Even a result in the 200s or 300s, once reassuring, is now above the line. If your home was tested before 2007, or if you bought it with only a short test during the sale, test again with a long-term detector.

Same house, same air, same lungs. Only the yardstick changed.

Why many people still haven’t heard

There was no recall notice and no mandatory testing. The change was announced, guidance documents were updated, and awareness campaigns followed, but research from the University of Calgary has found that even among households that learn their radon is high, only about a third mitigate within a year or two. Awareness and follow-through remain the biggest gaps, not technology.

Is 200 Bq/m³ “safe”?

It’s an action level, not a safety line. Risk rises steadily with exposure, and a study of Ontario estimated that most radon-related lung cancers actually occur in homes below 200, simply because there are so many of them. The World Health Organization recommends a reference level of 100 Bq/m³. Health Canada advises keeping levels as low as reasonably achievable, and many families with results between 100 and 200 choose to mitigate.

Common questions