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Middlesex-London Health Unit

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Extreme Heat

Each summer, London and Middlesex County experience periods of extreme heat. The Middlesex-London Health Unit issues the following when one or more of the criteria are met:


Heat Alert

  • Environment Canada issues a forecast indicating a daily Tmax of 31oC or higher, or
  • Environment Canada issues a forecast indicating a daily Humidex of 40 or higher.

Heat Warning

  • Environment Canada issues a forecast indicating a daily Tmax of 31oC or higher with a Tmin of 20oC or higher for two consecutive days, or
  • Environment Canada issues a forecast indicating a daily Humidex of 40 or higher for a duration of two consecutive days.

Extended Heat Warning

  • Environment Canada issues a forecast indicating a daily Tmax of 31oC or higher with a Tmin of 20oC or higher for duration of three consecutive days or longer, or
  • Environment Canada issues a forecast indicating daily Humidex of 40 or higher for a duration of three consecutive days or longer.

*Note: Tmax is the maximum temperature and Tmin is the minimum temperature in a forecast.



Heat Warning Today Sign (PDF)

Download a warning sign that you can post at your facility.

City of London - Cooling Centres

View a list of Cooling Centres in the City of London

Are You at Risk?1

During a heat wave, everyone is at risk, but some groups are more vulnerable than others. They include:

  • Infants (under 1 year of age).
  • People 65 years of age or older.
  • People with chronic medical conditions (heart disease, respiratory conditions, diabetes, etc.).
  • People on certain types of medications (for high blood pressure, depression, insomnia, etc.).
  • People experiencing homeless.
  • People with limited mobility.
  • People with mental impairment.
  • People who exercise vigorously outdoors (play sports, cyclists, gardeners).
  • Outdoor workers (depending upon length or time and exertion levels).
  • People who work in places where heat is emitted through industrial processes (e.g., foundries, bakeries, dry cleaners).

Please see Ontario’s website for further advice about heat stress.

Health Risks of Extreme Heat: Know When to Get Help1

Seek help if you experience any of the following symptoms of heat illness:

  • Heat cramps

    Symptoms/signs include painful muscular cramps, usually in the legs or abdomen.
  • Heat exhaustion

    Symptoms/signs include heavy sweating, weakness, dizziness, nausea, vomiting, fainting.
  • Heat stroke

    Symptoms/signs include headache, dizziness, confusion or other altered mental state, fainting. Skin may be hot and dry, or the individual may be sweating due to high body temperature. This is a potentially life-threatening emergency – call 911 immediately.

Things You Can Do to Protect Yourself1

  • Check the Government of Canada's Weather Information website.
  • Check the Ministry of the Environment and Climate Change's Air Quality in Ontario website for information regarding air quality and the Air Quality Health Index.
  • Drink plenty of water and natural juices throughout the day, even if you don't feel very thirsty. Remember to take sips often and to not guzzle your drink.
  • Avoid drinking alcoholic beverages, or beverages that contain caffeine like coffee, tea and cola.
  • Whenever possible, avoid spending extended periods of time outside during extreme heat. Instead try to spend time in air conditioned rooms. During prolonged periods of extreme heat the City of London designates Cooling Centres, where you can go to cool off. If you have to go outside, try to stay in the shade as much as possible. Plan any necessary outdoor activities in the early morning or evening, when temperatures are lower.
  • Wear a wide-brimmed hat outdoors.
  • Keep window shades or drapes drawn and blinds closed on the sunny side of your home.
  • Keep electric lights off or turned down low.
  • Take a cool bath or shower periodically or cool down with cool, wet towels.
  • Wear loose fitting, light clothing.
  • Avoid eating heavy meals and using your oven.
  • Avoid intense or moderately intense physical activity.
  • Never leave a child or pet in a parked car or sleeping outside in direct sunlight.
  • Use fans to draw in cool air at night, but do not rely on a fan as a primary cooling device during extended periods of excessive heat.
  • Consult your doctor or pharmacist regarding the side effects of your medications which could be triggered by extreme heat.
  • Reduce the use of personal vehicles, stop unnecessary idling; avoid using oil-based paints and glues, pesticides and gas-powered small engines (lawn mowers, weed whackers, etc.).

What can I do to help someone who is suffering from a heat illness?

Friends and relatives can help someone with heat illness by doing the following:

  • Call for help. Call 911, consult a healthcare provider or call Telehealth Ontario (1-866-797-0000 or TTY at 1-877-797-0007).
  • Move the person to a cooler location.
  • Remove excess clothing from the person.
  • Cool the person with lukewarm water, by sponging or bathing.
  • Give the person sips of cool water if they are not nauseated or vomiting. Do not give ice cold water.

What can I do to help make sure tenants do not get a heat illness?

As an owner or operator of a residential building or buildings, a landlord can take the following actions to decrease the risk of heat-related illness to residents:

  • Provide residents access to a cooler spot for several hours at a time, e.g. a common room with air conditioning or a basement area.
  • Keep windows in hallways open slightly to allow air to circulate.
  • Use fans to draw cool air at night, but do not rely on fans as primary cooling devices during extended periods of excessive heat.
  • Provide heat safety information to residents or post the information in common areas (e.g. by the elevator, in the lobby, etc.).
  • Have building staff check on at-risk residents every few hours.
  • Advise residents to drink lots of water and natural fruit juices even if they don't feel thirsty.
  • Ask residents to keep windows open and the drapes drawn.
  • Turn off non-essential lighting.
  • Ask residents not to use stoves or ovens.
  • Suggest residents cool down with cool baths, showers, foot baths or by placing cool, wet towels on their necks or underarms.
  • Suggest tenants avoid midday sun or heat and only go outside in the morning or evening when it is cooler.

Additional Information

For a listing of cooling centres, pools, spray pads and wading pools and their hours of operation visit the City of London websites or call:

  • 519-661-5575

For the addresses of local libraries, please visit the London Public Library website or call:

  • 519-661-4600

Call Telehealth Ontario for free advice from a Health Professional, 24/7, at

  • 1-866-797-0000 or TTY at 1-877-797-0007

For pet care information, please visit the London Animal Care Centre website or call:

  • 519-685-1330


Please note: Where indicated, the source of the information on this web page is the Ministry of Health. © Queen’s Printer for Ontario, 2008.

Date of creation: June 12, 2013
Last modified on: May 30, 2022




1Ontario. Ministry of Health. (2015, July 22). Extreme heat. Retrieved from