Canadian Environmental Health Atlas

Understanding Our Environment is Key to Promoting Health and Preventing Disease

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Heat Waves

Heat Waves

Rising Temperatures
The dramatic effect of heat on human health was revealed during major heat waves in Chicago in 1995, in the American Midwest in 1999, and across Europe in 2003.Together, these heat waves resulted in thousands of excess deaths.123Each year, about 120 heat-related deaths occur in Toronto, 121 in Montreal, and 41 in Ottawa.45 Scores of people experience illnesses and suffer from reduced quality of life when temperatures remain atypically high for several days.

Climate scientists predict that heat waves will increase with climate change and heat-related mortality is expected to double in Canadian cities by 2050 and triple by 2080.6Increasing urbanization and a rapidly aging population are expected to exacerbate the effect of heat waves.

Silent Killers
Heat waves can be thought of as “silent” killers because they are not visible like floods, hurricanes, and other weather-related health hazards, and because intense heat can produce effects on vulnerable people quickly and unexpectedly.

“Heat-related illness” refers to a broad spectrum of clinical conditions that range from mild discomfort to heat exhaustion and heat stroke (Figure 1).7Symptoms of mild illness can include muscle cramps, nausea, fatigue, and weakness. Severe heat-related illnesses can result in complications like renal failure, brain damage and death. 

Heat can also aggravate pre-existing conditions by placing stress on already strained body systems, particularly for people who have chronic cardiovascular and respiratory disorders. Heat-related illness is not the only problem—high outdoor temperatures are also associated with increased violence, suicide, and psychiatric emergencies.

The relationship between extreme cold or heat and mortality, is often described as a V- or U-shaped curve. In projections using historical data and climate simulations for three Quebec cities,8Doyon and colleagues (2008) showed that mortality increased at both low and high temperature extremes (Figure 2).


  • 1. Palecki MA, Changnon SA, Kunkel KE. The nature and impacts of the July 1999 heat wave in the midwestern United States: Learning from the lessons of 1995. Bull. Amer. Metero. Soc. 2001;82(7):1353-1367.
  • 2. Robine JM, Cheung SL, Le Roy S, et al. Death toll exceeded 70,000 in Europe during the summer of 2003. C R Biol. 2008;331(2):171-178.
  • 3. Whitman S, Good G, Donoghue ER, et al. Mortality in Chicago attributed to the July 1995 heat wave. Am J Public Health. 1997;87(9):1515-1518.
  • 4. Cheng C, Campbell M, Pengelly D, et al. Differential and combined impacts of winter and summer weather and air pollution due to global warming on human mortality in south-central Canada. Technical Report. Toronto: Toronto Public Health; 2005.
  • 5. Pengelly LD, Campbell ME, Cheng CS, et al. Anatomy of heat waves and mortality in Toronto: Lessons for public health protection. Can J Public Health. 2007;98(5):364-368.
  • 6. Toronto Public Health. Toronto Staff Report. Combined impact of extreme heat and air pollution on mortality. 27 May 2005.
  • 7. Bassil KL, Cole DC, Moineddin R, et al. Development of a surveillance case definition for heat-related illness using 911 medical dispatch data. Can J Public Health. 2008;99(4):339-343.
  • 8. Doyon B, Belanger D, Gosselin P. The potential impact of climate change on annual and seasonal mortality for three cities in Quebec, Canada. Int J Health Geogr. 2008;7:23.