Radon concentration in a building is not fixed by geology alone. It is the product of how much soil gas enters and how much air dilutes it — and the second half of that equation is your ventilation system.
For facilities teams preparing for the federal 200 Bq/m³ workplace limit, that has three practical consequences: ventilation affects the result, scheduled ventilation creates a measurement problem with a specific solution, and changing an HVAC system means the measurement has to be done again.
TL;DR: Buildings pull soil gas in through stack effect and mechanical imbalance. Scheduled ventilation means a 24-hour detector may not reflect what occupants actually breathe — Health Canada provides a specific correction method using a seven-day continuous monitor, with explicit limits on when it may be used. HVAC modification, rebalancing or changed airflow is a retest trigger. Ventilation can reduce radon but is not a substitute for mitigation.
How buildings draw radon in
Soil gas enters wherever a building meets the ground — cracks in foundation walls and floor slabs, construction joints, gaps around service penetrations and support posts, floor drains, sumps and wall cavities. Whether it enters, and how fast, depends largely on pressure.
Stack effect. Warm air rises and escapes at the top of a building, drawing replacement air in at the bottom. In the heating season this creates negative pressure at the lowest level, and some of the replacement air comes from the soil. Taller buildings and buildings with vaulted or open volumes generate stronger stack effect, which is one proposed explanation for why some newer, taller construction shows higher indoor radon than older, lower-profile buildings.
Mechanical imbalance. Any system that exhausts more air than it supplies depressurises the building relative to the soil. Kitchen and washroom exhaust, fume hoods, dryer vents, combustion appliances drawing on indoor air, and unbalanced supply-and-return systems all contribute. A building can be well ventilated in the comfort sense and still be depressurised in the way that matters for radon.
Envelope tightness. A tighter envelope reduces uncontrolled air exchange, and that matters for radon. C-NRPP's technical bulletin on radon and energy efficiency puts it directly: radon levels "can be affected by any renovation work that renders a house more airtight. If the amount of fresh air leaking into a home is reduced, the radon levels inside are likely to increase." A well-designed retrofit ensures good airflow and takes steps to confirm radon levels are acceptable — a poorly designed one can raise them.
The practical consequence for facilities teams is that an energy retrofit is a radon event. Air sealing, window replacement, envelope upgrades and insulation work all change the balance between soil-gas entry and dilution, which is why they belong on the retest trigger list below.
The measurement problem scheduled ventilation creates
Most commercial buildings do not run their ventilation continuously. Systems ramp down or shut off outside operating hours, and on weekends and holidays.
A long-term alpha track detector measures continuously for the whole test period, including every hour the building is closed and unventilated. In a building with a strong occupied/unoccupied ventilation difference, the 24-hour average can be meaningfully higher than what people breathe during working hours — and, conversely, cleaning, security and maintenance staff working after hours may be exposed to more than the daytime average suggests.
This is a real problem and Health Canada addresses it directly rather than leaving it to judgement.
Health Canada's occupied-hours correction
Where a long-term result exceeds the guideline and the building has significantly different ventilation between occupied and unoccupied hours, the guide sets out a method:
- Run a continuous radon monitor for seven days during the heating season, avoiding holiday periods when the building's schedule is atypical.
- Establish the ratio between the average concentration during occupied hours and the overall average across the seven days.
- Apply that ratio to the long-term result to estimate occupied-hours exposure.
The guide is explicit about the limits, and they matter:
- The result is an estimate, and approximate.
- It should only be used where the difference between occupied and unoccupied periods is significant.
- It must not be used if the continuous monitor shows concentrations above the guideline during occupied hours.
- Where there is doubt, decisions rest on the long-term measurement.
Practically: this is expert work. It involves calibrated continuous monitors, a defensible definition of "occupied hours," and a judgement about whether the difference is significant enough to apply the method at all. It is one of the clearest cases for engaging a C-NRPP certified professional — see the qualified person question.
Where the correction does not apply
In continuously occupied buildings — hospitals, long-term care, correctional facilities, some 24-hour operations — there are no unoccupied hours to correct for. The 24-hour long-term average is what occupants actually experience, which makes interpretation simpler and the result more directly meaningful. See hospitals, care facilities and correctional centres.
Detector placement and your air distribution
Placement rules exist partly because of ventilation. Health Canada's protocol requires detectors 0.5–2 m from the floor, at least 30 cm from an exterior wall, 10 cm clear of surrounding objects, and away from HVAC air currents, heat sources and direct sunlight.
A detector in a supply-air stream reads diluted air, not room air. A detector above a return grille reads mixed air on its way out. Both understate or distort the room's actual concentration. In buildings with high-velocity distribution or exposed ductwork, the placement walk-through is worth doing with someone who knows where the air actually goes.
HVAC changes are a retest trigger
Health Canada sets no fixed retest interval for a building that tested below the guideline. What triggers a retest is change — and much of that list is HVAC work:
- Ventilation or HVAC systems modified, upgraded, replaced or rebalanced
- Airflow through the building altered for any reason, including envelope work and energy retrofits — air sealing, window replacement and insulation upgrades all reduce uncontrolled air exchange
- Major renovation
- Occupancy or room use changed, particularly on the lowest occupied level
- Anything affecting how the building meets the soil
Where a change triggers a retest, the measurement is a three-month test during the first heating season after the work is finished.
The practical implication for facilities teams: a radon result is tied to the building configuration that produced it. An energy retrofit, a controls upgrade, a rebalance after a tenant fit-out — each of these can invalidate a result that was accurate when taken. Adding "does this change airflow or ground-contact room use?" to your project close-out checklist is the cheapest control available.
Can ventilation fix a radon problem?
It can reduce concentrations, and it is not the recommended solution.
Increasing outdoor air dilutes radon, and pressurising a building relative to the soil reduces entry. Both work in principle. In practice, dilution has to be sustained continuously to be relied upon, costs energy indefinitely, and fails whenever the system is off, rebalanced or in an economiser mode. Pressurisation is difficult to maintain evenly across a real building and interacts with envelope performance and other systems.
Health Canada's recommended remedy above the guideline is remediation by certified mitigation professionals — most commonly active soil depressurisation, which addresses entry at the source rather than diluting what has already entered. Where a heat recovery ventilator or other system is part of a remedy, it is a component of a designed solution rather than a substitute for one, and the result still has to be verified by retesting.
If a result is above 200 Bq/m³, Health Canada recommends action within two years, and within one year above 600.
A short checklist for facilities teams
- Map ground contact. Which occupied rooms sit on or below grade, over crawl spaces, tunnels or parking. That count drives the detector requirement.
- Document the ventilation schedule before testing, not after. You will need it if the occupied-hours question arises.
- Note depressurising equipment — exhaust-only systems, fume hoods, combustion appliances on indoor air.
- Walk the placements against your air distribution, not just the floor plan.
- Test during the heating season, when stack effect is strongest and results are most conservative.
- Add radon to project close-out. Any work touching airflow, the envelope, or lowest-level room use raises the retest question.
Common questions
Does better ventilation lower radon?
Increasing outdoor air dilutes indoor radon and can lower measured concentrations, but it must be sustained to be relied on and it does not address entry. Health Canada's recommended remedy above the guideline is remediation by certified professionals, typically active soil depressurisation.
Our HVAC shuts down at night. Does our result overstate exposure?
Possibly. Health Canada provides a correction method using a seven-day continuous monitor during the heating season to estimate occupied-hours exposure, with explicit limits — it is approximate, applies only where the difference is significant, and must not be used if the monitor shows levels above the guideline during occupied hours.
We rebalanced the system after a fit-out. Do we retest?
Changed airflow is a retest trigger. The measurement is a three-month test during the first heating season after the work is complete.
Can we put detectors anywhere in the room?
No. Placement is 0.5–2 m from the floor, at least 30 cm from an exterior wall, 10 cm clear of objects, and away from HVAC air currents, heat and direct sunlight. A detector in a supply stream does not measure room air.
Why test in the heating season?
Stack effect is strongest when the building is heated and closed, so heating-season measurement is the conservative and representative basis for an annual average.
Read next
How commercial radon testing works · Does a certified professional have to do the testing? · Hospitals, care facilities and correctional centres · Workplace radon FAQ
RadonTest.ca provides testing logistics and laboratory submission. We do not perform radon mitigation, we do not design ventilation systems, 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, nor engineering advice.
Sources
C-NRPP Technical Bulletin, Radon and the Impact of Energy Efficiency — Air Tightness; C-NRPP Technical Bulletin, Radon and HRVs; Health Canada, Guide for Radon Measurements in Public Buildings, including the occupied-hours estimation method; Health Canada, Canadian radon guideline; Canada Occupational Health and Safety Regulations, ss. 10.17–10.18 (ventilation systems) and s. 10.26; Canada Gazette, Part II, Vol. 160, No. 3 (SOR/2026-10); "Comprehensive survey of household radon gas levels and risk factors in southern Alberta," CMAJ Open (stack effect and building height).