Radon in Ontario Office Buildings: The OHSA Testing Duty Most Commercial Landlords Don’t Know They Have
Say the word radon and people picture a detached-home basement. But the place radon quietly becomes a legal problem is the office building — and it can legally be your problem.
Does Ontario law actually require radon testing before you buy, sell, or operate a commercial office building?
Here’s the part that trips up most landlords: no Ontario statute specifically orders a commercial office building to be tested for radon. The duty is indirect. It flows from the general duty clause of the Occupational Health and Safety Act — s.25(2)(h) — which requires an employer to take “every precaution reasonable in the circumstances for the protection of a worker.” Radon is a recognized occupational carcinogen, so once you have reason to believe a workspace (especially below grade) could hold radon, testing becomes the practical way to show you met that duty — not an optional extra. The line that matters is Health Canada’s action guideline of 200 Bq/m³: not a legal penalty threshold, but the level at which you’re expected to act.
Sources: Ontario Occupational Health and Safety Act (OHSA) s.25(2)(h); Ontario.ca “Radon in the workplace”; Health Canada Radon Guideline (200 Bq/m³). Verified 2026-08-04.
I’m Arthur Zhao. After years of handling commercial deals, I’ve noticed one due-diligence blind spot almost nobody raises at the table: radon. Say the word and people picture a detached-home basement — a residential problem. But the place radon quietly becomes a legal problem is the office building.
Here’s the fact that reframes it: the most radon-exposed workers in Canada aren’t miners. They’re the administrative and reception staff sitting in lower-floor offices (the numbers are below). If you own or are buying that building, its radon can legally be your problem. This piece takes the commercial and workplace side of the story — where the OHSA duty lives, which floors actually matter, and who pays when a test comes back high. The residential radon primer is a separate article; I won’t repeat it here.
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The counterintuitive part: Canada’s most radon-exposed workers sit in offices
When people hear radon plus workplace, they think uranium mines. The data says otherwise. According to CAREX Canada, roughly 190,000 Canadians are exposed to radon at work, and the occupations with the most exposed workers are administrative assistants (about 9,200), general office support staff (about 8,700), receptionists (about 5,400) and elementary school teachers (about 5,100) — people who spend long hours indoors, often on lower floors.
The national stakes are real too: Health Canada estimates radon is the second leading cause of lung cancer after smoking, responsible for roughly 16% of lung cancer deaths — more than 3,200 Canadians a year. The most typical radon-exposed workplace, in other words, looks a lot like an ordinary office building.
What the law actually says — and what it doesn’t
It’s worth being precise, because this is where owners get it wrong in both directions. Ontario does have a regulation governing workplace exposure limits for chemical and biological agents — O. Reg. 833 (Control of Exposure to Biological or Chemical Agents) — but its schedule of controlled substances does not list radon. The only radon-specific Ontario regulation is Reg. 854 (Mines and Mining Plants), sections 289–293, which covers underground mines only.
For an office, the duty is the general duty clause, s.25(2)(h). Ontario applies Health Canada’s NORM dose tiers to gauge severity: at or below 200 Bq/m³ is unrestricted (≤1 mSv/year); 200–800 Bq/m³ calls for NORM management (1–5 mSv); above 800 Bq/m³ requires radiation-protection management (>5 mSv); and the ceiling for occupationally exposed workers is an annual average of 3,000 Bq/m³, equal to a 20 mSv effective dose.
Why basements and mechanical floors carry the risk
Radon is heavier than air. It seeps up through soil and foundation gaps and pools in the lowest, earth-contact rooms of a building. Health Canada’s guide for public buildings flags rooms on the lowest floor and any room in direct contact with the earth as the highest-risk locations — which in an office means below-grade or partially below-grade suites, floors sitting above underground parking, mechanical rooms, and slab-on-grade server rooms.
A test placed in a sunny top-floor boardroom will read low and tell you nothing. The point of the exercise is the space where staff actually spend time on the lowest floors.
How a defensible commercial radon test is done
A screening kit that gives you a number in a few days is not the same thing as a test that holds up to a reasonable-precaution standard. Do it in this order.
Use a long-term detector — minimum three months
alpha track detector, which accumulates a reading over the placement period and is then sent to a lab. A few-day short-term test is a screen only — it can’t confirm you’re above or below the guideline.Place C-NRPP-certified detectors in the lowest-floor rooms — then read against 200 Bq/m³
C-NRPP, which is what makes the reading defensible. Then compare the annual average to Health Canada’s 200 Bq/m³ guideline: below it is the acceptable range; at or above it, you’re into the mitigation decision below.Above 200: how fast, and how much
Health Canada sets clear deadlines. Between 200 and 600 Bq/m³, reduce the level within two years. Above 600 Bq/m³, act within one year. In both cases the goal is to get below 200 Bq/m³, as low as reasonably achievable, as soon as possible.
The most common and reliable fix is active sub-slab depressurization — suction points under the slab, piping that vents soil gas outdoors, and a continuously running fan. On cost, Health Canada’s consumer figure for a home system is roughly CAD 2,000–4,000; a commercial building runs materially higher because of footprint, complex foundations, and integration with existing HVAC, with no standard price — it’s quoted per building. That’s exactly why it belongs in due diligence, not in a post-closing surprise.
ℹ️There’s no standard commercial mitigation price: footprint, foundation type, and HVAC integration vary enormously. Have a C-NRPP mitigation contractor quote the specific building — don’t estimate an office from a residential range.
Who pays: in a purchase vs in a lease
⚠️Legal duty is not the same as a testing mandate — don’t state it backwards. The “duty” here comes from the OHSA general duty clause. Ontario has no statute requiring commercial owners to test for radon on a schedule. Treating a test as the sound way to meet a reasonable-precaution obligation is correct; claiming in marketing or a contract that “the law requires office buildings to test for radon” is not accurate.
💡 My honest take: radon is under-managed in commercial real estate not because the risk is small, but because “nobody specifically requires it” gives everyone permission to do nothing. The general duty clause works the opposite way — the more reason you had to suspect a problem and still didn’t test, the harder it is to defend later. For a buyer, a three-month, few-hundred-dollar test buys knowledge of a potential structural liability in the building. For an owner, testing, documenting and mitigating turns a legal soft spot into proof of diligence.
- Ontario.ca — Radon in the workplace (OHSA s.25(2)(h), NORM dose tiers, Reg. 854 mines provisions)
- Health Canada — Government of Canada Radon Guideline (200 Bq/m³, mitigation deadlines)
- Health Canada — Guide for radon measurements in public buildings (3-month long-term, lowest-floor rooms)
- CAREX Canada — Radon (Occupational Exposures): ~190,000 workers exposed
Elevators and Escalators in Commercial Buildings: TSSA Inspection Cycles, Entrapment Alarms, and Owner Liability →The Boiler Room Page Most Commercial Buyers Skip — and the Insurance Link That Trips Owners Up →Radon Testing Home Ontario →Ontario Home Buying Guide →
Frequently Asked Questions
Is radon testing legally required in a commercial office building in Ontario?
Not by any direct statute. No Ontario regulation names offices for radon testing; O. Reg. 833’s schedule doesn’t include radon, and the only radon-specific regulation, Reg. 854, covers mines. The obligation for an office flows from the OHSA general duty clause, s.25(2)(h) — an employer must take every reasonable precaution to protect workers. Because radon is a recognized carcinogen, testing is treated as the practical way to meet that duty: best practice and reasonable diligence, not an express mandate.
We’re only on the 3rd floor — do we still need to worry about radon?
The duty attaches to the building’s occupied spaces, and radon concentrates on the lowest, earth-contact floors — basements, below-grade suites, floors over parking, mechanical rooms. If your tenancy is entirely on upper floors with no earth-contact space, your own exposure is low, but the building owner’s duty toward workers on the lower floors doesn’t go away. For a whole-building owner or buyer, the lowest floor is what sets the risk.
How long does a proper radon test take in an office building?
For public buildings including offices, Health Canada asks for a long-term test — the detector stays in place at least three months. Radon swings with season and ventilation, so only a multi-month average is representative. A few-day short-term kit is a screen; it can’t confirm you’re above or below the 200 Bq/m³ guideline. Use a C-NRPP-certified long-term alpha track detector for a defensible result.
A test came back above 200 Bq/m³ — how fast do we have to act?
Health Canada’s timeline: between 200 and 600 Bq/m³, reduce within two years; above 600 Bq/m³, act within one year; in both cases aim to get below 200 as soon as practical. The usual fix is a sub-slab depressurization system. A home system runs roughly CAD 2,000–4,000; a commercial building is materially higher and quoted per building by a mitigation contractor.
Should a radon test be a condition in a commercial purchase agreement?
It’s worth considering when the property has significant below-grade or slab-on-grade office space. You can make a long-term radon test a condition in the agreement of purchase and sale, and set out who bears mitigation cost — or how price adjusts — if it comes back high. Mitigation is a capital expense; discovering it after closing leaves you with no leverage, so it belongs in due diligence.
Arthur Zhao
Real Estate Broker · FRI · ABR · SRS · PSA · MCNE · E-PRO · CLHMS & GUILD Elite · REAIS
VP & Branch Manager, Bay Street Group Inc.
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