Radon in London, ON (2026): Southwestern Ontario Geology, Testing, and Mitigation

Flat-vector map of Canada with a marker pin at London, ON — radon testing in London, ON

A note before you read. This article is general health and home-testing information for London, Ontario homeowners and renters, drawn from publicly available Health Canada, Middlesex-London Health Unit, CARST, and Canadian Cancer Society materials. It is not medical, legal, or contracting advice. See full disclaimers at the bottom.

Quick answer. London, Ontario sits in a southwestern Ontario region that Health Canada and Take Action on Radon both flag as warranting routine testing. Every London home should be tested with a 3-month (≥91-day) long-term alpha-track test in the lowest lived-in level. If results exceed Health Canada's 200 Bq/m³ action level, mitigation by a C-NRPP-certified contractor typically costs $2,500–$4,500 for sub-slab depressurization. Ontario new builds with an APS signed on or after February 1, 2021 may have radon mitigation coverage of up to $50,000 under the Tarion new-home warranty (qualification rules apply).

Table of Contents

Why London Is on the Radon-Testing List

London sits on a mix of glacial till and sedimentary bedrock typical of southwestern Ontario. While not as visibly "radon-prone" as Canadian Shield regions like Sudbury, southwestern Ontario has consistently shown elevated radon prevalence in regional surveys, and the Middlesex-London Health Unit has historically supported radon awareness programming.

Three factors stack up:

  • Geology. Glacial deposits and sedimentary rock with localized uranium-bearing material can produce radon under London-area homes.
  • Long heating season. Cold Ontario winters mean furnaces run for months, and the resulting stack effect pulls soil gas through foundation cracks, sump openings, and slab penetrations.
  • Building stock. Many London homes — especially in older neighbourhoods like Old North and Wortley Village, plus newer subdivisions in Northwest, South London, and Byron — have full basements used as living space.

The pattern is consistent: any specific home, regardless of neighbourhood or vintage, can have elevated radon. The only way to know is to test.

What London-Area Data Shows

Health Canada's Cross-Canada Radon Survey reports Ontario at roughly the Canadian average prevalence (~7% of homes ≥ 200 Bq/m³), with material variation by region. Southwestern Ontario, including London-Middlesex, has shown elevated prevalence in regional CARST and public-health surveys.

The practical implication for London homeowners: don't rely on Ontario averages — test your specific home.

How to Test Your London Home

Health Canada's recommended test for residential decision-making is a long-term (≥91-day) alpha-track test in the lowest lived-in level, ideally during the heating season (October–April).

For most London homes:

  • Finished basement if used as a bedroom, home office, gym, rec room, or rental suite (Western University area students often rent basement apartments — that's where to test)
  • Lowest sleeping level if the basement is unfinished
  • Main floor only if the home has no basement

Place the kit at breathing height (1–2 metres), away from drafts, exterior walls, windows, and HVAC supply registers. Keep it in place for at least 91 days, then return to the lab.

Order your $89 all-in test kit — designed and analyzed in Canada, ships across Ontario.

Mitigation in London

If your test exceeds 200 Bq/m³, the standard fix is active sub-slab depressurization (SSD). Typical London-area cost: $2,500–$4,500 for a single-family home.

Always use a C-NRPP-certified Mitigation Professional. Verify on the C-NRPP Find a Professional directory, filter by Ontario.

For the full mitigator-selection playbook, see our How to Choose a Licensed Radon Mitigator in Canada guide. After mitigation, run an independent post-mitigation test.

Ontario New-Home Buyers: Tarion Coverage

Ontario new builds with APS signed on or after February 1, 2021 may have up to $50,000 of radon mitigation coverage under Tarion's new-home warranty, within a 7-year window. Meeting qualification criteria does not guarantee a claim will be approved. See our Tarion Radon Warranty Claim Guide.

Real Estate & Disclosure in Ontario

Ontario does not require radon disclosure on the standard SPIS, but the doctrine of latent defects (under common law and Ontario case law) generally creates an obligation to disclose known material latent defects. A confirmed elevated radon test is the kind of information a court might consider material — consult an Ontario real estate lawyer for any specific transaction.

For the full real-estate playbook, see Radon and Real Estate in Canada.

Renters in London

Ontario's Residential Tenancies Act generally requires landlords to maintain rental units in good repair fit for habitation. London renters — especially students renting basement apartments near Western University, Fanshawe College, or older neighbourhoods — should consider testing their unit. See our Radon for Canadian Renters (2026) guide.

Frequently Asked Questions

Is radon a real problem in London, Ontario? Yes. Health Canada and CARST regional data both flag southwestern Ontario, including London-Middlesex, as a region warranting routine residential radon testing. The only way to know your specific home's level is to test.

What's the action level for radon in London? 200 Bq/m³ — the Health Canada residential guideline.

How do I test my London home? Use a 3-month (≥91-day) long-term alpha-track test from a C-NRPP-recognized lab, placed in the lowest lived-in level during the heating season.

How much does radon mitigation cost in London? Typical: $2,500–$4,500 for standard sub-slab depressurization (SSD). Get 2–3 written quotes from C-NRPP-certified contractors.

Does Tarion cover radon mitigation for new homes in London? Up to $50,000 for qualifying new builds with APS signed on or after February 1, 2021, within a 7-year window. Meeting qualification criteria does not guarantee claim approval. See our Tarion claim guide.

Is there a Middlesex-London Health Unit radon program? Middlesex-London Health Unit has historically supported radon awareness programming. Check directly with the health unit for current offerings.

Can London students renting basement apartments test for radon? Yes. Renters in Ontario have the right to test their rental unit. Basement suites are exactly where radon concentrates. See our renters guide.

Should I retest my London home after mitigation? Yes — Health Canada recommends retesting every 2 years after mitigation, or sooner after major renovations.

Do newer London subdivisions have lower radon? Not necessarily. Newer homes can be more airtight, which can concentrate radon if soil-gas barriers and rough-ins weren't installed correctly. Test your specific home regardless of vintage.

What if my London landlord won't act on elevated radon? Document everything in writing and consider escalating to the Ontario Landlord and Tenant Board.

Order Your Test Kit

Order your $89 all-in long-term test kit

Long-term 91-day alpha-track test kit. C-NRPP-listed device. Analysed at Lex Scientific in Guelph, Ontario — Canadian lab, C-NRPP listed, ISO/IEC 17025 accredited by CALA. Tracked Canadian shipping both ways. Written lab report PDF delivered to your inbox.

Ships across southwestern Ontario including London, Strathroy, St. Thomas, and Sarnia.

Disclaimers

Not medical, legal, contracting, or tax advice. Consult qualified Ontario professionals.

Health Canada survey statistics are drawn from Health Canada's Cross-Canada Radon Survey and related publications.

Mitigation cost ranges of $2,500–$4,500 reflect typical Canadian residential pricing as of 2026.

Tarion qualification hedge. Coverage depends on APS date, test type and timing, professional qualifications, and other Tarion-specific rules. Meeting the qualification criteria does not guarantee a claim will be approved.

No diagnosis or treatment claims. RadonTest.ca sells radon test kits. We do not diagnose, treat, or prevent disease.

No warranty as to completeness. RadonTest.ca makes no warranty as to the completeness or accuracy of the information herein and accepts no liability for decisions made in reliance on this article.

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