Photo: Ken Lund from Reno, Nevada, USA · CC BY-SA 2.0St. Thomas is on Canada’s flood inventory: 1 listed, led by Kettle Creek Conservation Authority
Wildfire barely registers for St. Thomas, at 0% of the land within 25 km since 1986 (0.000% a year). Seismic risk in St. Thomas sits near the middle: $0.023 of expected damage per $1,000 of buildings a year, the typical town $0.030. Radon around St. Thomas is ordinary for Canada: 7.5% of tested homes over the guideline (6.6% typical). Hazard risk gives St. Thomas a grade of B+, from earthquakes and radon alone.
What stands out about St. Thomas’s hazards
Four national sources, each read for St. Thomas: earthquakes, radon, past fires and flood studies.
What St. Thomas faces
Each hazard against its own yardstick; meters run from low to high on that yardstick.
Wildfire
Earthquakes
Radon
Flood study
Hazard risk in St. Thomas: B+
For St. Thomas, Risk will be the average of earthquakes and radon, with wildfire added only if it makes the average worse.
Its place: #486 of 866 in Ontario and #1,069 of 2,589 Canadian towns.
How sure: Earthquake losses are a model’s expectation over many years, not a forecast for any one. Small fires and grass fires are largely missing from the national burned-area record.
Few fires near St. Thomas: 0% within 5 km, 0% within 25 km
Fires mapped since 1986 around where St. Thomas’ residents live, and the fuel next to their homes.
St. Thomas burns at 0.000% a year, low against 0.000% for the typical Canadian town.
Around St. Thomas the forest is thin or far off: 0.6% of the land within 5 km and 0.5% within 25 km is forest fuel.
Shares of land within 5 and 25 km of populated blocks: burned at least once in 1986–2025 (CWFIS burned-area composite), forest fuel and grass (fire behaviour fuel types, 2026, 250 m grid).
Burn rate a year, towns in Ontario · highest of 871
To three decimals the burn rate around St. Thomas is zero, as for 638 of the 871 towns in Ontario, while Sioux Lookout reaches 0.16%.
Each bar: the yearly average share of land within 25 km of a town’s homes that burned, 1986–2025 (CWFIS); the seven highest towns of 2,500 people or more.
St. Thomas shares the low band with 92% of Canada’s 2,610 towns, its burn rate rounding to nothing; 7.5% burn faster.
Bands of the yearly burn rate: low under 0.05%, moderate under 0.25%, high under 1%, very high 1% or more; towns of 2,500 people or more.
Fire danger and active fires (CWFIS)
Canadian Wildland Fire Information System: National Burned Area Composite 1986–2025 and fire behaviour fuel types (2026), Natural Resources Canada. Measured within 25 km and 5 km of each populated dissemination area; past fire, not a forecast.
Kettle Creek Conservation Authority has mapped flooding for St. Thomas (1 study)
Flood studies whose area touches St. Thomas, from NRCan’s inventory, each linked to its owner.
One study on the inventory touches St. Thomas, Kettle Creek Conservation Authority’s from 2024, and its area can be large, so it says a map exists, not that the town floods.
| Study | By | Year | Status |
|---|---|---|---|
| Dodd Creek Enhanced Riverine Floodplain Mapping | Kettle Creek Conservation Authority | 2024 | final |
Towns with a flood study on file
St. Thomas is one of the 69% of towns in Ontario with a flood study on file; across Canada it is 58%.
NRCan Canada Flood Map Inventory: a study footprint touching the town’s boundary; towns of 2,500 people or more.
The grade leaves flood out, since a study area says nothing about how many of St. Thomas’ homes are at risk.
The inventory’s own caution (NRCan): “The Canada Flood Map (CFM) does not include every flood hazard map in Canada and may be incomplete or contain inaccuracies. It should not be relied upon as the official source of flood mapping information. Furthermore, if a map appears to be missing, this does not necessarily mean that the area has never been mapped. For the official version of a map, updated information, or confirmation of applicable regulations, please contact the local authority responsible for flood mapping.”
Natural Resources Canada, Canada Flood Map Inventory; links go to each study’s owner.
Expected shaking damage in St. Thomas: $0.023 a year per $1,000
NRCan’s seismic risk model: the damage shaking is expected to do to buildings each year, on average over many years.
0.8 times the typical $0.030: St. Thomas’ $0.023 is ordinary.
Shaking damage a year per $1,000 of building value
At $0.023, St. Thomas is below the typical town in Ontario ($0.043); Alexandria is highest at $0.21.
NRCan seismic risk model (2022): mean yearly building loss per $1,000 of value; medians of towns of 2,500 people or more.
Around half of Canada’s 2,611 towns expect more shaking damage than St. Thomas (54%).
Each column: the share of Canada’s towns of 2,500 people or more in that range of yearly loss per $1,000 (NRCan, 2022).
The loss is to St. Thomas’ buildings alone, from ground shaking; infrastructure and secondary hazards are left out.
Natural Resources Canada, Canada’s Seismic Risk Model (2022), by census subdivision (2016 boundaries; a few 2021 subdivisions take their residents’ mean).
Radon about average around St. Thomas: 7.5% over the guideline
Health Canada’s survey of long-term home tests: the share above the 200 Bq/m³ guideline. · The radon tests numbers here cover the Elgin-St. Thomas Health Unit
St. Thomas’ area sits close to the typical Canadian town: 7.5% over the guideline, against 6.6%.
Tested homes over 200 Bq/m³
Over 600 Bq/m³
Around St. Thomas, 7.5% of tests were over 200 Bq/m³ and 1.1% over 600, against a median of 4% in Ontario.
Health Canada radon survey (2009–2011), by 2007 health region; medians of towns of 2,500 people or more.
37% of Canada’s 2,611 towns share St. Thomas’ band of 5 to 10%; 26% sit higher.
Each bar: the share of Canada’s towns of 2,500 people or more whose survey area had that share of tests over 200 Bq/m³.
The survey sampled homes around St. Thomas; it can’t tell you your basement’s level.
Health Canada, Cross-Canada Survey of Radon Concentrations in Homes (2009–2011).
Weather and fire alerts for St. Thomas
Environment and Climate Change Canada issues the weather alerts for St. Thomas; Alert Ready carries the urgent ones to phones, radio and TV.
| Alert | What it means |
|---|---|
| Warning | Severe weather is happening or about to: act now. |
| Watch | Conditions favour severe weather: be ready. |
| Advisory | Weather that is less severe but could disrupt the day. |
| Special weather statement | Unusual weather worth knowing about. |
| Special air quality statement | Poor air, often wildfire smoke. |
In fire season, check CWFIS for danger and active fires around St. Thomas, and listen to local authorities for orders.
Weather alerts (ECCC) · Alert Ready · Fire danger and active fires (CWFIS)
Hazards in the places around St. Thomas
The same four hazards for the places next door.
| Town | Wildfire | Quake $/1,000 | Radon | Flood study |
|---|---|---|---|---|
| St. Thomas | Low | $0.023 | 7.5% | Yes |
| Central Elgin 15,230 · 2.0 km | Low | $0.020 | 7.5% | Yes |
| Southwold 5,473 · 13 km | Low | $0.021 | 7.5% | Yes |
| Aylmer 8,371 · 15 km | Low | $0.025 | 7.5% | None listed |
| Malahide 9,995 · 17 km | Low | $0.022 | 7.5% | Yes |
| Lambeth 2,609 · 18 km | Low | $0.020 | 1.8% | None listed |
| White Oaks 21,108 · 19 km | Low | $0.020 | 1.8% | None listed |
The neighbour with the most fire is Central Elgin (low), still no more than St. Thomas (low).
Radon is the survey area’s figure, so neighbours in one area share it.
Get ready for what St. Thomas faces
Practical steps for St. Thomas’ hazards, most pressing first.
- Test for three months in winter.
- Over 200 Bq/m³: fix it.
- Kits are cheap; pros test too.
- Never drive through floodwater.
- Move papers and medicines upstairs.
- Ask whether your street is mapped.
- Clear needles and wood nearby.
- Keep a grab-and-go bag ready.
- In smoke, stay in; filter air.
- Has it had a radon test?
- Check the address on the flood map.
- Are flood and wildfire insured?
What this data can and can’t tell you
- Wildfire counts one way. The wildfire figure is the share of land within 25 km of St. Thomas’ homes burned each year from 1986 to 2025, past fire rather than a forecast, and it can only lower Risk.
- Radon is a survey. Health Canada’s survey tested some 14,000 homes over three months; until postal-area boundaries are loaded, St. Thomas takes its 2007 health region’s share.
- Earthquakes are modelled. Shaking damage is modelled from the ground, the buildings and the odds of each quake, and published for whole subdivisions such as St. Thomas’.
- Flood studies aren’t flood zones. The flood inventory points to maps; it is incomplete and not the official source, so ask the municipality about any street in St. Thomas.
Questions about flooding and hazards in St. Thomas
Is St. Thomas at risk of wildfire?
Hardly at all on record: 0% of the land within 25 km of St. Thomas since 1986, a low band, though small grass fires aren’t mapped.
How big is the earthquake risk in St. Thomas?
About $0.023 a year per $1,000 of building value, NRCan’s long-run expectation for St. Thomas.
Is radon a problem in St. Thomas?
The area’s survey found 7.5% of 93 tested homes above Health Canada’s guideline, so a three-month test is worth doing in St. Thomas.
Does St. Thomas flood?
The inventory lists a study touching St. Thomas; whether a given street floods is on that map, not in the grade.
How is St. Thomas’s Hazard risk Grade worked out?
Hazard risk gives St. Thomas B+, weighing earthquakes and radon and counting wildfire only against it.
Seismic risk: Natural Resources Canada. Radon: Health Canada. Wildfire: Canadian Wildland Fire Information System. Flood studies: Natural Resources Canada, Canada Flood Map Inventory. Contains information licensed under the Open Government Licence – Canada.
Sources: NRCan, Canada’s Seismic Risk Model 2022 · Health Canada, Cross-Canada Survey of Radon Concentrations in Homes · CWFIS, National Burned Area Composite and FBP fuel types · NRCan, Canada Flood Map Inventory. Under the Statistics Canada Open Licence.
Adapted from Statistics Canada (Census of Population 2021, population estimates 2025, 2021 census boundaries) under the Statistics Canada Open Licence; this does not constitute an endorsement by Statistics Canada. Neighbourhood names in some cities © OpenStreetMap contributors (ODbL).