# Radon Levels Across the GTA: Why 19% of Homes Test Above

The Health Canada Cross-Canada Radon Survey (2009-2011) and subsequent regional sampling consistently put roughly 19% of GTA homes above the 200 Bq/m3 guideline. Many Toronto homeowners are surprised by that figure — radon is more associated in the public mind with rural Saskatchewan, the New Brunswick uranium belt, or the Canadian Shield. The data does not match the cliche. This post explains the GTA geology, the building stock, and the neighbourhood patterns behind the 19% number.

The 19% Figure: Where It Comes From

The Cross-Canada Radon Survey was a Health Canada multi-year study that placed long-term alpha-track detectors in over 14,000 homes across Canada. Results for the GTA showed a meaningful share of homes above 200 Bq/m3, with subsequent provincial and Toronto Public Health sampling reinforcing the figure in the high-teens percent range.

Different methodologies produce slightly different numbers — single-municipality studies range from 7% (older Toronto Public Health regional figures used at the city scale) to over 20% (some suburban-municipality studies). The roughly 19% figure is the most commonly cited GTA-wide estimate as of the mid-2020s and is the figure used here.

Two important nuances:

The figure is for the lowest occupied area of the home — typically the basement. Levels on upper floors are generally a fraction of the basement reading.
The figure is an annual average across the heating season. Summer-only short-term tests would show a lower share above 200, but that is a measurement artifact, not a real difference.

Why GTA Homes Accumulate Radon

Three factors interact.

1. Soil Uranium Content

All Ontario soils contain trace uranium. The radon production rate is not zero anywhere in the GTA. While the Canadian Shield north of the GTA produces dramatically more radon, the glacial till and shale-derived clays under Toronto produce enough that homes in poorly ventilated basements accumulate to action-level concentrations.

2. Foundation and Slab Communication With Soil

Homes built directly on a porous gravel sub-base — almost all GTA postwar construction — have efficient pathways for soil gas to enter through cracks, slab seams, sump pits, and plumbing penetrations. Older Toronto homes with stone or brick foundations and earthen-floor cellars are also significant communicators.

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3. Building Tightness

This is the one most people miss. Newer or retrofitted homes with tight envelopes accumulate radon more aggressively than older drafty homes because there is less passive air exchange. A 1950s bungalow with single-pane windows and a leaky rim joist may produce the same radon entry rate as a 2010 retrofit, but the older home dilutes it through air leakage. The retrofitted home accumulates it.

This is the central paradox of energy-efficiency retrofits in Toronto and is the topic of [Radon and Energy-Efficient Homes: The Toronto Irony](/blog/radon-energy-efficient-homes-irony-toronto).

GTA Neighbourhoods That Tend to Test High

The pattern is not random. Specific zones across the GTA show repeatedly elevated test results, driven by a mix of geology and housing stock.

Don Valley Corridor and Ravine Edges

Homes built into Don Valley fill, ravine-side properties along Bayview, the Beach corridor adjacent to ravines, and Leaside on the valley shoulder. The valley fill includes glacial outwash with higher porosity, and ravine-edge homes often have walk-out basements and multiple slab elevations creating multiple radon entry pathways.

Etobicoke Creek-Side and Postwar Bungalow Stock

Homes along Etobicoke Creek and Mimico Creek, plus the broad postwar bungalow inventory throughout south Etobicoke. The combination of crawl-space construction (some areas) and creek-adjacent fill produces a higher share of high-test homes.

North York TTC Tunnel Corridor

Homes along the Yonge subway corridor in North York, particularly the older bungalow and side-split stock from Lawrence to Sheppard. The shale-rich strata in this corridor (well documented from TTC tunnel construction reports) produce higher background radon than the city average.

Scarborough Bluffs and Adjacent Postwar Bungalow Zones

Homes near the Scarborough Bluffs, plus the broad postwar bungalow inventory inland. Crawl-space construction is common in older sections; both factors push the local share of high-test homes upward.

Selected 905 Pockets

Parts of Mississauga along the Credit River, parts of Vaughan and Richmond Hill on the Oak Ridges Moraine, parts of Pickering on creek-adjacent terrain. Suburban municipalities have not been mapped as thoroughly as the City of Toronto, but where studies exist, similar patterns appear.

Important caveat: these are zones where the share of high-test homes is elevated, not zones where every home is high. A neighbour testing at 50 Bq/m3 tells you nothing definitive about your home testing at 350 Bq/m3. Test the house, not the postal code.

Where Radon Comes From in the Home

In a typical GTA detached or semi-detached home, the dominant entry pathways are:

1. Floor-wall slab joints. The seam where the basement slab meets the perimeter foundation wall is the largest single entry pathway in most homes.
2. Cracks in the slab from settling or initial pour shrinkage.
3. Sump pits without sealed lids.
4. Plumbing and electrical penetrations through the slab.
5. Hollow concrete-block foundation walls (common in 1950s-1960s GTA stock).
6. Crawl-space soil in homes with crawl spaces.

Sub-slab depressurization addresses pathways 1-4 in a single intervention; block-wall depressurization addresses pathway 5; sub-membrane depressurization addresses pathway 6. See [Sub-Slab Depressurization Explained for Toronto Homes](/blog/sub-slab-depressurization-radon-toronto).

The Practical Takeaway

The 19% figure is not a reason for panic, and it is not a reason to dismiss the issue. The honest framing is:

About 1 in 5 GTA homes will test above the Health Canada guideline.
The only way to know which homes are which is to test.
For homes that test high, mitigation is among the cheapest, most effective health upgrades available — typically $2,500-$5,000 all-in.
The post-mitigation re-test verifies the system worked.

This is not a problem that requires guesswork. Spend $60 on a long-term alpha-track kit. For where to buy and how to test, see [Radon Testing Toronto: Cost and Where to Buy](/blog/radon-testing-toronto-cost-where-buy).

How RenoHouse Coordinates GTA Radon Work

RenoHouse coordinates C-NRPP-certified radon professionals across the GTA. Our typical pattern is renovation-anchored — homeowners come to us for a basement finish, an underpinning, or a sauna build-out, and we add the radon test and (if needed) mitigation to the project so all the slab and pipe work happens once. For the underpinning interaction, see [Basement Underpinning Toronto: 2026 Complete Guide](/blog/basement-underpinning-toronto-2026-complete-guide). For the sauna interaction, see [Basement Sauna Installation Toronto 2026](/blog/basement-sauna-installation-toronto-2026).