If you hold more than a handful of buildings, the first real question before the 30 January 2027 federal radon limit is not "how do we test" but "which buildings first."
That question has a regulatory edge to it. The Labour Program has indicated that where testing an entire untested portfolio in one heating season is not realistic, a phased approach can be a recognised path forward — but it is not automatic, it must be raised and agreed proactively, and it requires a documented plan rather than an explanation. A risk-ranked building inventory is the first element of that plan.
TL;DR: Rank on five factors: regional radon potential, ground-contact area and foundation type, occupancy pattern, which floors people actually work on, and building age and construction. The Canadian evidence supports all five, though most of it comes from housing. A ranking sequences your testing and demonstrates reasoning — it never tells you a building is below the guideline. Only measurement does that.
Start with what a ranking cannot do
This has to come first, because getting it wrong is worse than not ranking at all.
The 2024 Cross-Canada Survey of Radon Exposure states plainly that its results "should not be used as a tool to determine personal radon risk potential, or to decide whether to test a specific household," because radon levels are influenced by both building features and the behaviour of the people occupying the building — and that the only way to know a building's level is to test it.
The same logic governs any ranking you build. Two buildings on the same street, of the same age and construction, routinely test very differently. A low rank is a scheduling decision, not a conclusion. Every building in scope still gets tested; the ranking determines the order.
Used that way, ranking is defensible and useful. Used as a filter to decide which buildings never get tested, it is neither.
The five factors, and the evidence for each
1. Regional radon potential — the strongest predictor
Radon comes from uranium decay in soil and rock, so geology dominates. Concentrations are higher where there is more uranium in the underlying rock and soil, and soil gas is the main source for Canadian buildings. Research on geologic predictors has found bedrock type to be a significant factor: dwellings on carbonate bedrock showed indoor radon values roughly 104 Bq/m³ higher on average than those on siliciclastic rock, and near-soil radon levels were associated with elevated indoor results.
This is also the only factor you can assess before visiting a building. Measured results in the surrounding area, census-division and postal-area statistics, and underlying geology can all be applied from an address list.
Weight it highest. You can look up measured results for any Canadian region on our Canada radon map.
2. Ground contact and foundation type
Radon enters wherever a building meets the ground: cracks in foundation walls and floor slabs, construction joints, gaps around service pipes and support posts, floor drains, sumps and wall cavities. More contact with the ground means more opportunity for entry, and studies use "area in contact with the ground" as a standard variable for exactly that reason.
Canada's cross-Canada survey found that radon concentrations are, on average, significantly higher in buildings with a basement. Slab-on-grade construction, buildings over crawl spaces, and structures sitting above utility tunnels or underground parking are all in direct soil-gas contact.
What to capture: foundation type, approximate ground-contact footprint, presence of a sump, and whether occupied space sits below grade.
3. Occupancy pattern
Radon risk is concentration multiplied by time. A building occupied continuously accumulates far more exposure at a given concentration than one used intermittently, and this factor is about consequence rather than likelihood — it doesn't make radon more likely to be present, it makes the same concentration matter more.
This is why Health Canada's public-buildings guide treats hospitals, care facilities and correctional centres as dwellings for testing purposes: occupancy is continuous. Weight a 24-hour facility above a nine-to-five office at equal geological risk.
What to capture: hours of occupancy, headcount, and whether anyone is present overnight.
4. Which floors people actually work on
Radon concentrations fall with height above ground, and the Canadian evidence is specific. A study of detached houses in Halifax and Winnipeg found that measurements taken on a main or upper floor were associated with roughly half the likelihood of exceeding the geometric mean radon concentration compared with measurements in the basement.
For a portfolio this cuts both ways. A ten-storey building with two occupied ground-contact rooms may rank below a single-storey building of the same footprint, because far fewer people are working in the zone where concentrations are highest. Conversely, a building where staff work in a below-grade records room or plant room should rank up regardless of its height.
What to capture: how many occupied rooms are in ground contact, and how many people work in them.
5. Building age and construction — real, but not in the direction most people assume
The intuition is that older buildings are leakier and therefore worse. The evidence points the other way, and it is worth being precise because this is commonly stated too confidently in both directions.
Internationally, building age is consistently identified as a governing factor in indoor radon, generally understood as a proxy for the airtightness of the foundation and for building design. In Canada, a study of indoor radon trends across a building code change found that detached houses in Halifax built after 2011 had an odds ratio of 1.91 (95% CI 1.04–3.50) of exceeding the geometric mean radon concentration — nearly double the likelihood. In Winnipeg, the same analysis found no evidence of significant change after 2011.
C-NRPP's technical bulletin on radon and energy efficiency states the mechanism plainly: radon levels can be affected by any work that renders a building more airtight, and if the fresh air leaking in is reduced, indoor radon is likely to increase. That applies to retrofits as much as to new construction.
So: newer or recently air-sealed construction can be associated with higher indoor radon, but the effect is not uniform across the country. Proposed mechanisms include increased airtightness reducing air exchange, larger floor slabs producing greater concrete shrinkage and therefore wider floor-to-foundation gaps, and taller building profiles generating stronger stack effect and more negative pressure at the lowest level.
How to use it: as a moderate-weight factor, not a decisive one, and do not assume an older building is lower risk. If your portfolio is concentrated in one region, check whether local data shows an age effect at all before weighting it heavily.
Turning factors into a ranking
You do not need a sophisticated model. A transparent one beats a clever one, because the point is to be able to explain it.
| Factor | Suggested weight | Data source |
|---|---|---|
| Regional radon potential | Highest | Measured results by region; geology. Applied from the address list. |
| Ground contact and foundation | High | Building records or a walk-through |
| Occupied ground-contact rooms and headcount | High | Floor plans; the same count drives your detector requirement |
| Occupancy pattern (hours, overnight) | Moderate | Operations |
| Age and construction type | Moderate | Building records |
Score each factor on a simple scale, multiply by weight, sum, and sort. Record the scoring rules alongside the output — a ranking whose logic cannot be reconstructed is not evidence of anything.
Three tie-breakers worth applying
- Known history. Any building with a previous elevated result, or an adjacent building with one, goes to the top regardless of score.
- Vulnerable occupants. Childcare, care facilities and buildings with continuously present occupants who cannot leave.
- Change since last measurement. Renovation, energy retrofit or air sealing, altered HVAC, changed airflow, or a change in how ground-contact rooms are used all reset the question. See when retesting is required.
What the ranking is actually for
Two things, and they are different.
Sequencing. With one heating season available and a deployment window running roughly 1 October to the end of January, a large portfolio cannot be fully measured before the duty takes effect. Ranking decides what goes first.
Evidence of reasoning. This is the part that matters to the regulator. A defensible phased programme contains a risk-ranked inventory, a documented testing schedule with clear phase sequencing, a framework showing how full compliance will be reached, evidence of ongoing progress, and formal engagement with the Labour Program on record. The ranking is the foundation the rest sits on — it converts "we could not test everything" into a decision you made for stated reasons.
A worked sequence
- Build the inventory. Every building you occupy, with address. Buildings you have forgotten cannot be ranked.
- Apply regional radon potential from the address list. This can be done in an afternoon and gives you a first cut.
- Add building characteristics — foundation, ground contact, age — from building records.
- Add occupancy — occupied ground-contact rooms, headcount, hours. This same count is your detector requirement, so the work is not wasted.
- Score, sort, and apply tie-breakers.
- Write down the method and the resulting schedule, and raise it with the Labour Program before the season, not after.
Common questions
Can a risk ranking be used to decide not to test a building?
No. Ranking determines order, not inclusion. Health Canada's own survey work warns that regional data must not be used to decide whether a specific building needs testing, because levels depend on the individual structure and how it is used.
What is the single most useful factor?
Regional radon potential, because it is the strongest predictor and the only one you can apply to an entire address list without visiting anything.
Are newer buildings lower risk?
Not necessarily, and possibly the reverse. Canadian research found detached houses built after 2011 in Halifax roughly twice as likely to exceed the geometric mean radon concentration, while the same analysis found no significant change in Winnipeg. Treat age as a moderate factor and do not assume newer means lower.
Does this evidence apply to commercial buildings?
The published evidence is overwhelmingly residential. The physical mechanisms — soil gas entry through ground contact, stack effect, dilution with height — apply to any building. The specific effect sizes do not transfer directly, which is a further reason to treat a ranking as sequencing rather than prediction.
How many buildings before ranking is worth doing?
If you can comfortably test everything in one heating season, rank informally and get on with it. Beyond that, the ranking is what makes the phased conversation with the regulator possible.
Read next
Phased compliance for large portfolios · The compliance timeline · How commercial radon testing works · What a compliant radon record contains · 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 and research information and is not legal advice.
Sources
Health Canada, Guide for Radon Measurements in Public Buildings; Health Canada, Canadian radon guideline; "A Summary of Residential Radon Surveys and the Influence of Housing Characteristics on Indoor Radon Levels in Canada," Health Physics; "Indoor radon trends with building code change in two Canadian cities" (Halifax and Winnipeg), National Research Council Canada; "Comprehensive survey of household radon gas levels and risk factors in southern Alberta," CMAJ Open; "Geologic, seasonal, and atmospheric predictors of indoor home radon values"; 2024 Cross-Canada Survey of Radon Exposure in the Residential Buildings of Urban and Rural Communities; Canada Gazette, Part II, Vol. 160, No. 3 (SOR/2026-10).