Post-Mitigation Radon Verification: Who Should Test After the System Goes In

A radon mitigation system is not finished when the fan is switched on. It is finished when someone has measured the building and shown that it worked — and Health Canada is specific about who that someone should be.

TL;DR: Start a short-term test at least 24 hours after the fan is activated, ideally in the same locations originally measured. Follow it with a long-term test of at least three months during the next heating season to confirm the annual average. Retest at least every five years thereafter. And, in Health Canada's words, "to avoid conflict of interest, the test should not be performed by the company that installed the radon mitigation system." Note that this guidance is residential — Canada has no workplace-specific post-mitigation verification standard.

First, a note on where this guidance comes from

This matters, and most sources do not say it.

Health Canada's post-mitigation guidance — the timing sequence and the recommendation about who should perform the test — is set out in the Radon — Reduction Guide for Canadians, a document addressed to homeowners. The Guide for Radon Measurements in Public Buildings, which governs workplace measurement, covers measurement duration, devices, measurements in public buildings and interpretation of results, with annexes on follow-up measurements to determine the method for mitigation. It does not set out a post-mitigation verification protocol.

So there is a genuine gap in Canadian guidance: no workplace-specific post-mitigation verification standard exists. What fills it in practice is the residential sequence, applied by extension — which is defensible for two reasons. The public-buildings guide expressly treats such buildings as "dwellings" for the purposes of radon testing, and the underlying logic is physical rather than building-type specific: a fan either reduces soil gas entry or it does not.

Read what follows on that basis. It is the recognised practice, not a workplace standard, and where a result is likely to be examined, saying so in your own record is better than implying a certainty that does not exist.

Health Canada's position on who tests

This is the part most building owners have never been told. Health Canada's Radon — Reduction Guide for Canadians — the residential guidance discussed above — recommends that a certified C-NRPP professional carry out a short-term test after a system is activated to demonstrate that it is working, and then states plainly:

"To avoid conflict of interest, the test should not be performed by the company that installed the radon mitigation system."

It is a short sentence with real consequences. Mitigation involves three roles — measuring the building, designing and installing the remedy, and verifying the remedy worked — and they are routinely bundled into one contract with one firm. Health Canada's guidance is that the third role should sit apart from the second.

The reasoning is the same as in any other trade: a building inspection is not usually performed by the builder. The party that designed and installed a system has an interest in the number that says whether it worked. That does not imply anyone acts improperly; it means the result is more robust when the interest is removed.

The verification sequence

Why the short-term test alone is not enough

Health Canada is explicit that short-term measurements "should never be used to determine if the radon concentration in a home exceeds the Canadian Guideline or to assess the need for remedial actions," and that the result of any short-term measurement should be confirmed with a follow-up long-term measurement at the same location.

Applied to mitigation: a short-term test tells you the fan is moving air and the number has dropped. It does not tell you the building's annual average is below 200 Bq/m³, because radon varies by hour, day and season. A system that performs well in April may perform differently in January.

The long-term test during the following heating season is the one that closes the file. A mitigation project that stops at the short-term test is a project without a conclusion.

What this means for workplaces

For federally regulated employers the stakes are higher than for a homeowner, because the record is retained and the duty is continuing. Since no workplace-specific verification protocol exists, an employer applying the residential sequence should document that choice and its basis rather than leave it implicit — the qualified person's report under s. 10.5 is the natural place, since it must set out recommendations respecting sampling and testing methods.

• The verification result is part of the compliance record. The date, time and location of the test, the method, the result and the name and occupation of the person who performed it all belong in the file — see what a compliant radon record contains.
• Test the same locations. Verification in different rooms from the original measurement is not a comparison. Where the original identified specific elevated rooms, those are the rooms that matter.
• The five-year cycle is a scheduled obligation, not a reminder. Put it in a calendar that survives staff turnover, and attach it to the building rather than to a person.
• Independence is easier to arrange before the contract is signed than after a disputed result.

Where mitigation timelines fit

Health Canada recommends remediation within two years for results between 200 and 600 Bq/m³, and within one year above 600 Bq/m³. Note also a point that surprises people: if a long-term test came back above the guideline, Health Canada does not recommend retesting to confirm it — the recommendation is to act. Retesting a high result delays the remedy by a season without changing the conclusion.

What else triggers a retest

Beyond the post-mitigation cycle, Health Canada identifies several situations that warrant retesting the following heating season:

• Any renovation affecting the structure or ventilation of the building
• Energy retrofits — new windows, insulation, air sealing. C-NRPP notes that work making a building more airtight is likely to increase indoor radon
• Significant excavation or earthwork adjacent to the building — drainage systems, exterior waterproofing, service replacement
• New noticeable structural damage to parts of the building in contact with the ground, such as slabs and foundation walls
• A level that was not previously tested becoming the lowest occupied level
• After any mitigation system is installed

The excavation trigger is easy to overlook and mechanically obvious once stated: work that disturbs the ground beside a foundation changes the soil-gas path into it.

Practical questions to settle before mitigation starts

1. Who performs the verification? Settle it before signing, not after. Health Canada's recommendation is that it not be the installer.
2. Which locations? Agree that verification will repeat the original measurement locations.
3. What happens if the long-term test still exceeds the guideline? A short-term pass followed by a long-term failure is exactly the scenario the sequence exists to catch. Know who is responsible for further work.
4. Who holds the record? The building owner needs it for the retest cycle, and for whoever asks in fifteen years.
5. When is the five-year retest scheduled? Put a date in the system on the day the file closes.

Common questions

Can the company that installed our system also do the verification test?

Health Canada recommends against it: "to avoid conflict of interest, the test should not be performed by the company that installed the radon mitigation system." That recommendation appears in Health Canada's residential guidance; there is no equivalent statement in the public-buildings guide, which does not address post-mitigation verification. The reasoning applies equally to a workplace, but an employer relying on it should record that it is applying residential guidance by extension. Many firms are certified for both measurement and mitigation, so this is a question to raise at contracting.

How soon after the fan starts can we test?

The short-term test should begin at least 24 hours after the fan is turned on. A measurement of about two to seven days is acceptable for confirming immediate effectiveness.

Is the short-term test enough?

No. It confirms the system is working now. A long-term test of at least three months during the following heating season is what confirms the annual average is below the guideline.

How often after that?

At minimum once every five years, and after any renovation, ventilation change, energy retrofit, adjacent excavation, or structural damage to ground-contact elements.

Our result was above the guideline — should we retest to be sure?

Health Canada's guidance is that a retest is not necessary where a long-term test measured above the guideline; the recommendation is to take action to reduce levels.

Read next

What a compliant radon record contains · How commercial radon testing works · Our result came back at 180 — are we finished? · Radon, HVAC and ventilation

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 guidance and is not legal advice.

Sources

Note on scope: the post-mitigation sequence and the conflict-of-interest recommendation are set out in Health Canada's residential guidance. The Guide for Radon Measurements in Public Buildings does not contain a post-mitigation verification protocol. Sources: Health Canada, Radon — Reduction Guide for Canadians (post-mitigation testing and conflict of interest — residential); Health Canada, Guide for Radon Measurements in Residential Dwellings (retest triggers); Health Canada, Guide for Radon Measurements in Public Buildings; C-NRPP, Steps to Reduce Radon; C-NRPP Technical Bulletin, Radon and the Impact of Energy Efficiency; CAN/CGSB-149.12-2024, Radon mitigation options for existing low-rise residential buildings.