The detectors came back. The building averaged 180 Bq/m³. The limit is 200. Are you finished?
Legally, for now, yes — a result below 200 Bq/m³ means no action is required. Practically, a result in the 150–200 range is the one worth thinking about hardest, and there are four reasons why.
TL;DR: 200 Bq/m³ is an action level, not a safety threshold. Radon moves year to year with weather and building operation. Every measurement carries uncertainty, so a reported 180 sits in a range that may cross 200. And the federal regulations direct the qualified person to consider whether levels are likely to exceed 50 per cent of the applicable limit — 100 Bq/m³ — as one of the criteria in a hazard investigation. Common practice is to treat 150–200 as a watch band: retest and monitor rather than close the file.
1. Two hundred is an action level, not a safety line
Health Canada's guideline of 200 Bq/m³ is the concentration at which remedial action is recommended. It is not a threshold below which radon is harmless.
There is no radon concentration considered risk-free. Risk is a function of concentration multiplied by duration of exposure, and it declines continuously as concentration falls — it does not switch off at any particular number. Health Canada's own position is that building owners should consider mitigation where results approach the guideline, even without exceeding it.
The World Health Organization's reference level is 100 Bq/m³, and the United States Environmental Protection Agency's action level is 148 Bq/m³. A Canadian building at 180 sits above both.
2. Radon moves year to year
A long-term measurement establishes an average over the test period. It does not fix the building's concentration permanently.
Indoor radon varies with weather, ground conditions, barometric pressure, how the heating and ventilation systems are run, and how occupants use the building. A building that averages 180 in one heating season can sit above 200 in the next with nothing about the structure having changed.
That is why Health Canada notes levels vary year to year, and why a borderline result is better treated as a data point than a verdict.
3. Every measurement carries uncertainty
A reported result is an estimate with a range around it, not a single exact value. Laboratories report a lower limit of detection and detectors carry a coefficient of variation; the C-NRPP quality control framework exists precisely because measurement systems drift and need monitoring.
The practical consequence is straightforward: a result near the guideline sits in a range, and part of that range may be on the other side of it. At 180, "below 200" is the best estimate, not a certainty. This is also why quality control duplicates matter — they are what tell you how much confidence the number deserves.
4. The regulations themselves point at half the limit
This is the part almost nobody discusses, and it is in the text.
Section 10.4(2) of the Canada Occupational Health and Safety Regulations lists the criteria a qualified person must take into consideration when carrying out a hazard investigation. Paragraph (h) is: whether "the concentration of an airborne chemical agent or the level of ionizing or non-ionizing radiation is likely to exceed 50 per cent of the values referred to in subsection 10.19(1) or the levels referred to in subsections 10.26(3) and (4)."
Radon sits in subsection 10.26(4). Once the workplace limit becomes 200 Bq/m³ on 30 January 2027, 50 per cent of it is 100 Bq/m³.
Be precise about what this does and does not mean. It is not a second exposure limit, it does not create an obligation to act at 100 Bq/m³, and nothing is contravened by a result between 100 and 200. What it does mean is that the regulation directs a qualified person's attention well below the number everyone is planning around — which makes "we were under 200, so we stopped thinking about it" a harder position to defend than it first appears.
The watch band: 150 to 200
Common industry practice is to treat results between roughly 150 and 200 Bq/m³ as a watch band — retest and monitor, rather than closing the file. That is prudent practice rather than a regulatory requirement, and it is easy to document.
| Result | Legal position | Sensible practice |
|---|---|---|
| Below ~100 Bq/m³ | No action required | File the record. Retest on change. |
| ~100–150 Bq/m³ | No action required | File the record. Note it as a building to retest sooner rather than later. |
| 150–200 Bq/m³ | No action required | Watch band. Retest, ideally a following heating season. Record the decision and the reasoning. |
| 200–600 Bq/m³ | Above the limit from 30 January 2027 | Health Canada recommends remediation within 2 years, then verification testing. |
| Above 600 Bq/m³ | Above the limit | Remediation within 1 year, then verification testing. |
What to actually do with a borderline result
- Record it properly. Date, time and location of the test, the method used, the result, and the name and occupation of the person who performed it — retained per the applicable period. See what a compliant radon record contains.
- Look at the spread, not just the average. A building average of 180 made up of rooms at 120 and 240 is a different situation from one where every room is near 180. The high room is where people are actually working.
- Check your quality control. If duplicates were out of tolerance, the number deserves less confidence and a repeat is warranted.
- Consider the occupancy. A borderline result in a room occupied continuously matters more than the same number in a room used four hours a day.
- Decide on a retest, and write down why. A documented decision to monitor is a defensible position; a result filed and forgotten is not.
- Note what would change it. Renovation, HVAC modification, altered airflow, or a change in how ground-contact rooms are used all reset the question.
If mitigation is on the table anyway
Some building owners choose to mitigate below the guideline, and it is not an unreasonable decision. Where a building sits at 180 with continuously present occupants, where a mitigation system is being installed nearby anyway, or where the building is undergoing work that makes installation cheap, acting early can cost less than acting later. Health Canada's guidance that mitigation should be considered where results approach the guideline supports that choice.
The point is that it is a choice, made on the facts, and worth recording as one.
Common questions
Is 180 Bq/m³ risk-free?
No radon concentration is risk-free; risk declines with concentration but does not disappear. 180 Bq/m³ is below Health Canada's action level of 200, above the WHO reference level of 100, and above the US EPA action level of 148. Health Canada is the health authority on radon in Canada and its guidance should be your reference.
Do we have to do anything if we are under 200?
No action is required. Whether you should do something is a separate question, and the answer depends on how far under, how the result is distributed across rooms, who occupies the building and for how long, and how much confidence the measurement deserves.
Is 100 Bq/m³ a legal limit in Canada?
No. The federal workplace limit is 200 Bq/m³ as an annual average from 30 January 2027. The 50 per cent figure in s. 10.4(2)(h) is a criterion a qualified person must consider during a hazard investigation, not a limit in itself.
How soon should we retest a borderline building?
There is no prescribed interval for a building that tested below the guideline — the regulatory trigger is change rather than the calendar. For a watch-band result, a retest in a following heating season is a reasonable, documentable choice.
Could the real level be above 200?
A reported result is an estimate with uncertainty around it, so a value close to the guideline sits in a range that may cross it. That is one of the reasons a borderline result warrants a second measurement rather than a conclusion.
Two-thirds of Canada's radon-exposed workers sit in exactly this band: what the research says about who is actually exposed.
Read next
What a compliant radon record contains · How commercial radon testing works · Workplace radon FAQ · Canada radon map
RadonTest.ca provides testing logistics and laboratory submission. We do not perform radon mitigation and we do not interpret health risk — Health Canada is the health authority on radon in Canada. This page describes publicly available regulatory information and is not legal advice, nor health advice.
Sources
Canada Occupational Health and Safety Regulations (SOR/86-304), ss. 10.4(2), 10.19, 10.26, consolidated on the Justice Laws website; Canada Gazette, Part II, Vol. 160, No. 3 (SOR/2026-10); Health Canada, Canadian radon guideline; Health Canada, Guide for Radon Measurements in Public Buildings; World Health Organization, WHO Handbook on Indoor Radon; United States Environmental Protection Agency action level; C-NRPP Quality Control and Quality Assurance Manual.