Extreme Heat

During a heat wave, everyone is at risk. Extreme heat can lead to adverse health effects such as heat stroke. Most heat illnesses occur because the victim has been overexposed to heat or has over-exercised for his or her age and physical condition. Older adults, young children and those who are sick or overweight are more likely to succumb to extreme heat.

What is a heat warning/extended heat warning and when is it issued?

Environment Canada will issue the following warnings in Northern Ontario:

  • Heat Warning - When forecast temperatures are expected to be at least 29°C and overnight temperatures are above 18°C for 2 days or the humidex is at least 36 for 2 days.
  • Extended Heat Warning - When forecast temperatures are expected to be at least 29°C and overnight temperatures are above 18°C for at least 3 days or humidex is at least 36 for at least 3 days.

What are heat illnesses?

  • Heat illness occurs when a person's body temperature rises quickly and sweating is not enough to cool the body properly.
  • High body temperature can lead to heat stroke, heat exhaustion, heat fainting, heat cramps (muscle cramps), heat rash and heat edema (swelling of hands, feet and ankles).
  • Heat illnesses are preventable.

What are symptoms of heat illness?

  • Dizziness or fainting
  • Nausea or vomiting
  • Headache
  • Rapid breathing and heartbeat
  • Extreme thirst
  • Decreased urination with unusually dark yellow urine
  • If you experience any of these symptoms during extreme heat, immediately move to a cool place and drink liquids. Water is best.

Before Extreme Heat

  • Ensure your air conditioning unit is functioning properly. If you have window-based air conditioners, ensure they are installed correctly.
  • If you do not have air conditioning, install temporary window reflectors (i.e. aluminum foil-covered cardboard) between your drapes and the window to reflect heat back outside.
  • Weather-strip doors and sills to keep cool air in.
  • Cover windows that receive morning or afternoon sun with drapes, shades, awnings, or louvers.

Protect yourself and help others during hot weather

  • Drink plenty of water and avoid alcohol and caffeine
  • Avoid heavy outdoor activity
  • Wear a hat, light and loose-fitting clothing, sunscreen and sunglasses when outside
  • Cool off in an air-conditioned space when available
  • Cool off in the shade or at a park or greenspace.
  • Use a fan and mist your skin with water
  • Take cool baths and showers as often as needed or soak hands and/or feet in cool water
  • Breastfeed according to your child’s cues and drink plenty of water if you are breastfeeding. 
  • Keep your home cool by closing blinds and curtains  on any windows facing the sun 
  • Open windows at night once the outdoor air is cooler than the indoor air; close windows in the morning before hotter air comes in
  • Use fans at night to help exhaust warm indoor air and bring in cool outdoor air
  • Talk to your doctor, nurse or pharmacist if you are taking medications as some can make it harder to control body temperature.
  • Stay connected with people in your community who have a difficult time coping with hot weather and those who live alone. Check on them regularly. 

Hot weather concerns and face masks

Wearing a mask is important in any indoor setting where it may be difficult to maintain at least two-metre distancing or if the room or corridor is small. Wearing a mask may not be necessary outdoors (where higher temperatures may be more of a concern) if distances can be maintained. 

Masks do become more uncomfortable in hot temperatures, but they will still work. The general public should plan outdoor outings for the coolest times of the day and take breaks in the shade or a cool environment if they are finding a face mask uncomfortable in the heat.   

For people undertaking physical exertion in heat, a mask can make the effort more difficult. Decreasing intensity/volume of work, more frequent rests, and more cooling breaks may be necessary. 


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