Photo: Jason Zhang · CC0Hazard risk in Underwood is lower than in 88% of Canadian towns
Wildfire isn’t Underwood’s worry: 0% of the land within 25 km burned since 1986 (0.001% a year). Seismic risk in Underwood sits near the middle: $0.034 of expected damage per $1,000 of buildings a year, the typical town $0.030. Radon is lower than usual: 0% of tested homes in the survey area topped the guideline, against 6.6% for the typical town. Hazard risk gives Underwood a grade of A; its wildfire band pulls it down.
What stands out about Underwood’s hazards
Four national sources, each read for Underwood: earthquakes, radon, past fires and flood studies.
What Underwood faces
Each hazard against its own yardstick; meters run from low to high on that yardstick.
Wildfire
Earthquakes
Radon
Flood study
Hazard risk in Underwood: A
The grade weighs Underwood’s seismic risk and radon equally, and wildfire is one-sided: a high band counts against a town, a low one earns nothing.
Its place: #165 of 866 in Ontario and #484 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 Underwood: 0% within 5 km, 0% within 25 km
Fires mapped since 1986 around where Underwood’s residents live, and the fuel next to their homes.
Next to the typical Canadian town (0.000% a year), Underwood’s 0.001% is in the low band.
The 5 km ring around Underwood’s homes is clear of burns since 1986, the nearest 12 km out.
With 0.5% forest fuel within 5 km and 4.9% within 25 km, the land around Underwood gives a fire little to burn.
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
The land around Underwood burns at 0.001% a year, the 83rd highest rate of 871 towns in Ontario; Sioux Lookout tops it at 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.
Underwood is in the low band with 92% of Canada’s 2,610 towns; 7.5% sit in moderate or higher.
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.
No flood study is listed for Underwood
Flood studies whose area touches Underwood, from NRCan’s inventory, each linked to its owner.
The inventory has nothing for Underwood, but a local or conservation-authority map may still exist.
Towns with a flood study on file
Missing from a well-mapped province: Underwood has no study, where 69% of towns in Ontario have one against 58% nationally.
NRCan Canada Flood Map Inventory: a study footprint touching the town’s boundary; towns of 2,500 people or more.
Listed, not scored: a flood study proves a map exists, not that Underwood floods.
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.”
Ordinary seismic risk in Underwood: $0.034 per $1,000 of buildings a year
NRCan’s seismic risk model: the damage shaking is expected to do to buildings each year, on average over many years.
1.1 times the typical $0.030: Underwood’s $0.034 is ordinary.
Underwood shares its earthquake risk numbers with the rest of the city of Markham.
Shaking damage a year per $1,000 of building value
Earthquake risk in Ontario peaks in Alexandria ($0.21); Underwood, at $0.034, is below the median of $0.043.
NRCan seismic risk model (2022): mean yearly building loss per $1,000 of value; medians of towns of 2,500 people or more.
Underwood sits mid-range among Canada’s 2,611 towns: 46% expect more shaking damage.
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 model counts damage to Underwood’s buildings and their occupants from shaking only: not liquefaction, landslides, tsunami or fires after a quake.
Natural Resources Canada, Canada’s Seismic Risk Model (2022), by census subdivision (2016 boundaries; a few 2021 subdivisions take their residents’ mean).
Little radon around Underwood: 0% of area homes over the guideline
Health Canada’s survey of long-term home tests: the share above the 200 Bq/m³ guideline. · Radon tests for the York Regional Health Unit
At 0% over the guideline, the area around Underwood is under the typical Canadian town’s 6.6%.
Tested homes over 200 Bq/m³
Over 600 Bq/m³
Radon runs lower around Underwood than in most towns in Ontario: 0% against 4%.
Health Canada radon survey (2009–2011), by 2007 health region; medians of towns of 2,500 people or more.
Underwood is in the lowest band, with 37% of Canada’s 2,611 towns: under 5% of tests over the guideline, while 63% have more.
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 share describes the area, not a house: in Underwood as anywhere, test to be sure.
Health Canada, Cross-Canada Survey of Radon Concentrations in Homes (2009–2011).
Weather and fire alerts for Underwood
Alerts for Underwood come from Environment and Climate Change Canada and, for emergencies, Alert Ready.
| 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. |
Fire danger and active fires near Underwood are mapped daily by the Canadian Wildland Fire Information System; evacuation orders come from the municipality or province.
Weather alerts (ECCC) · Alert Ready · Fire danger and active fires (CWFIS)
How Underwood’s neighbours compare on hazards
The same four hazards for the places next door.
| Town | Wildfire | Quake $/1,000 | Radon | Flood study |
|---|---|---|---|---|
| Underwood | Low | $0.034 | 0% | None listed |
| Milliken 12,138 · 0.6 km | Low | $0.034 | 0% | Yes |
| Hagerman's Corners 43,555 · 1.7 km | Low | $0.034 | 0% | Yes |
| Steeles 22,943 · 2.4 km | Low | $0.043 | 2.7% | Yes |
| Milliken 24,431 · 2.5 km | Low | $0.043 | 2.7% | Yes |
| Agincourt North 27,790 · 3.6 km | Low | $0.043 | 2.7% | Yes |
| East L'Amoreaux 21,533 · 3.7 km | Low | $0.043 | 2.7% | Yes |
Milliken (low) has the heaviest fire record of Underwood’s neighbours, and Underwood (low) matches or beats it.
Radon is the survey area’s figure, so neighbours in one area share it.
Get ready for what Underwood faces
What to do about the hazards Underwood faces, and what to ask before you buy.
- 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?
- Ask the municipality for flood maps.
- Are flood and wildfire insured?
What this data can and can’t tell you
- Radon is a survey. About 14,000 homes were tested across Canada in 2009–2011, and the share over the guideline for Underwood is its health region’s, the survey’s design unit.
- Flood studies aren’t flood zones. NRCan’s inventory says where flood mapping exists, not where water goes; the map’s owner is the authority for Underwood.
- Wildfire counts one way. The wildfire figure is the share of land within 25 km of Underwood’s homes burned each year from 1986 to 2025, past fire rather than a forecast, and it can only lower Risk.
- 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 Underwood’s.
Questions about flooding and hazards in Underwood
How big is the earthquake risk in Underwood?
About $0.034 a year per $1,000 of building value, NRCan’s long-run expectation for Underwood.
Is Underwood at risk of wildfire?
Fire has reached 0% of the land around Underwood since 1986 (last in 2001); at low, it neither helps nor hurts the grade.
Does Underwood flood?
With 0 studies on the inventory, Underwood’s flood risk is unmapped nationally; check with the municipality before buying.
How is Underwood’s Hazard risk Grade worked out?
Underwood grades A for hazard risk, from earthquakes and radon, with wildfire counted only where it lowers the grade.
Is radon a problem in Underwood?
Around Underwood, 95 survey homes were tested and 0% came in over 200 Bq/m³, a guide to the area, not to any one home.
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).