E2A: Extreme Heat

Evidence

Extreme heat is associated with increased deaths, emergency department (ED) visits, hospitalizations, heat exhaustion, heat stroke, cardiovascular events, respiratory illness, kidney injury, and worsening of chronic disease. Health Canada reported 38,398 ED visits with at least one diagnosis of heat-related illness in Canada between 2004 and 2023.¹ Heat stroke is the most recognized danger, but most excess deaths during heat waves are caused by added strain on the heart and worsening of existing conditions. Heat also increases the risk of workplace injuries in both outdoor and indoor jobs, and heat exposure during pregnancy is linked to preterm birth and stillbirth.

The 2021 western Canadian heat dome showed how serious extreme heat can be. In British Columbia, 619 deaths were attributed to the heat dome. Most deaths occurred among older adults, people with chronic disease, and people living alone, and 98% occurred indoors.² Many of those who died were in homes without air conditioning, highlighting the importance of indoor overheating and access to cooling.

Reviews suggest that heat alert and response systems, public education, cooling strategies, and targeted supports for vulnerable populations can reduce heat-related harms. Public Health Ontario found that multi-component heat alert systems were associated with reductions in mortality, morbidity, ED visits, and hospital admissions. ³ For primary care, this means helping patients prepare before heat events occur: knowing their risk, reviewing medications, identifying cooling options, recognizing early symptoms, and creating a heat plan.

1. Plan Ahead

Knowing when heat is coming and having a plan in place before summer helps patients act early. Encourage patients to:

• Sign up for heat warnings through the WeatherCAN app or weather.gc.ca, and follow local Alberta heat warnings.⁴
• Create a personalized Heat Action Plan that identifies their risk factors, their coolest room, a nearby cooling location, and what to do before and during a heat event.
• Get the home ready before hot weather: check that the air conditioner or heat pump works and keep a thermometer indoors to track indoor temperature.
• Arrange a “heat buddy” who will check in at least daily during heat events, especially for people who live alone.
• Review medications with their doctor or pharmacist. Some medications make it harder for the body to cool itself or increase the risk of dehydration or kidney injury (see Additional Context for Physicians).

2. Reduce Exposure:

Reducing the frequency, intensity, and duration of heat exposure is one of the most effective ways to prevent heat-related illness.⁵ Patients are advised to:

• Stay in cool indoor spaces, especially during the hottest part of the day (typically 11 AM to 4 PM). Health Canada recommends keeping indoor temperatures below 26°C during heat events.
• Keep the home cool: close blinds, curtains, or awnings during the day; open windows at night when it is cooler outside (ideally on opposite sides of the home to create a cross-breeze); avoid using the oven; and use air conditioning or a heat pump if available.
• Go somewhere cool if home cooling is unavailable, such as a library, shopping centre, or community cooling centre. Cool showers or baths also help.
• Cool yourself down with water: “Use a spray bottle for misting. Apply ice packs, or cool down with wet towels or damp clothing. Immersing your forearms in cool water can help cool your core temperature” ⁶
• Drink water often, even before feeling thirsty. Limit alcohol, sugary drinks, and excess caffeine, and eat water-rich fruits and vegetables.
• Plan activities around the heat: move strenuous outdoor activity or work to cooler parts of the day and take frequent breaks in the shade or a cool space.
• Dress for the heat in lightweight, loose-fitting, light-coloured clothing, and use shade whenever possible.
• Use fans wisely. Fans move air but do not cool it. They can help at moderate temperatures, especially placed in or near a window to bring in cooler outdoor air, but they are not a substitute for air conditioning during extreme heat. Do not use a fan to blow very hot air on the body (e.g., indoor temperatures above about 35°C).
• If experiencing homelessness or spending long periods outdoors, seek shaded or air-conditioned spaces, avoid direct sun during the hottest part of the day, and use cooling centres or shelters when available. If sleeping outside during the day, choose shade and remember that shade moves with the sun.
• Check on others, especially older relatives, neighbours, people who live alone, and people spending long periods outdoors. If someone is confused, collapses, or becomes unresponsive, call 911: these may be signs of heat stroke.
• Never leave anyone in a parked vehicle. Infants, children, older adults, and pets should never be left in a parked vehicle, even for a few minutes. Vehicle temperatures rise rapidly, even on mild days, and opening the windows provides little protection.
• When wildfire smoke and extreme heat occur together, limit time outdoors. If you have to choose between opening windows to cool down and keeping them closed because of smoke, staying cool is the priority for most people. Use a portable air cleaner or a good-quality filter in the ventilation system if available.

3. Recognize the Signs and Symptoms:

Heat exhaustion develops when the body loses too much water and salt through sweating and can no longer cool itself effectively. Without treatment, it can progress to heat stroke, a medical emergency in which the body can no longer regulate its temperature. The table below summarizes the key differences:

Ref: Government of Alberta – Watch for signs of heat illness (poster)

4. Prioritize Sensitive Populations:

Extreme heat does not affect everyone equally. Groups at increased risk include older adults, infants and young children, pregnant people, people with pre-existing medical or mental health conditions (e.g., cardiovascular, respiratory, renal, neurologic, or psychiatric conditions), people living alone or who are socially isolated, and those without adequate indoor cooling. Additional high-risk groups include outdoor workers, people experiencing homelessness, people with low income, and those taking medications or using substances that impair thermoregulation or increase the risk of dehydration. Risk is highest when these factors overlap, such as an older adult with chronic disease living alone without air conditioning. ⁷, ⁸
These patients are the priority for proactive outreach before summer and during heat warnings, and for completing a Heat Action Plan.

Adapted from: World Health Organization. Climate Change, Heat and Health

Limitations of available evidence:

Most evidence on the health impacts of extreme heat comes from observational studies, administrative health data, mortality reviews, and natural experiments rather than randomized controlled trials. Interventions such as heat alert systems are usually evaluated as bundled programs, making it hard to know which parts work best. Heat-related deaths are also likely under-counted, because heat often kills by worsening existing conditions and is rarely recorded as the cause of death.