Progress Toward and Next Steps in Characterizing the Health Risks of Extreme Heat Events (EHEs) in Canada

1. Introduction

Since 1948 Canada has warmed more than double the global mean. The temperature and duration warning thresholds for Extreme Heat Events (EHEs), sometimes called heat waves in Canada differ regionally. There are overall increases in the frequency and duration and other characteristics (e.g. intensity; geographic range; length of heat season) of EHEs in Canada. For example, from 1948 to 2016 summer mean temperatures across Canada have increased by 1.5 ºC. Under a high emission scenario (RCP 8.5) the mean annual highest daily maximum temperature is projected to increase by 5.7 ºC and the annual highest daily minimum temperature by 11.2 ºC by 2100. Depending on the RCP, the number of very hot days (≥30 ºC) in a year is expected to more than double in some parts of Canada by 2050 and triple by 2080. Under RCP8.5, the annual highest daily temperature once every 10 years currently, on average, will become a once in two-year event by 2050 — a 500% increase in frequency. The annual highest daily temperature that occurs currently once every 50 years is projected to become a once in five-year event by 2050 — a 1000% increase in frequency. Recently, several temperature records were broken across Canada including the highest temperature ever recorded in Alert, Nunavut (globally the northernmost location with a permanent population) in 2019 and 86 deaths in Quebec were attributed to a 2018 EHE.

Currently, there are many data gaps and uncertainties regarding health outcomes and effect thresholds in Canada. However there is good evidence of increasing ambient exposure to EHEs. Some locations in Canada have not experienced EHEs yet and therefore are less likely to be aware of the risk along with having the capacity to effectively adapt. There is also evidence that populations living in colder climates are more sensitive to heat than more southern populations and even small increases in temperatures may result in negative health impacts.

Health Canada (HC) is working to better characterize the risk of extreme heat events by evaluating the existing evidence and qualitatively estimating the risk of EHEs across Canada to inform appropriate public health and emergency management responses.

2. Methods

This study reviews the applicable existing evidence from available peer-reviewed and grey literature, and Health Canada’s research to characterize the risk of EHEs to Canadians including:

1. Hazard identification: an overview of health outcomes associated with EHEs including direct impacts such as heat- related illness; mortality, morbidity, and indirect impacts such as mental health; food availability and safety; accidents; etc. Health impacts analysis of analogous climate and population scenarios will be extrapolated as appropriate.
2. Current and future modelled exposure to EHEs across Canada for relevant indicators of heat exposure using available datasets.
3. Dose-response relationships for different outcome types and subpopulations
4. Risk characterization including a qualitative estimation for key population centres across Canada and when the risk is likely to be the greatest.

3. Results

The types of health impacts associated with moderate and extreme heat exposure in Canada are similar to those experienced globally. However due to relative novelty of EHEs in some parts of Canada, it is possible that unique and unanticipated health impacts may occur or be more pronounced due to differences in environmental and/or population health status.

There is good evidence of recent and projected future heterogeneous increases in EHEs across Canada. Due to the small and dispersed populations, there is a lack of health outcome datasets well-matched to exposure and it is not yet possible to ascertain dose-response relationships in some locations. Also, evidence suggests the current burden of heat associated morbidity and mortality is underestimated due to limitations in the current system of outcome classifications of administrative health datasources. Therefore there are only a few health impact analyses of EHEs in Canada, particularly in northern regions. However we have extrapolated evidence from other northern climates to characterize these impacts in Canada.

There is a high likelihood that Canada will experience increases in other climate change hazards and this should be considered when characterizing the risk of EHEs. For example, the risk of wildfires and smoke has increased and is projected to continue to increase across Canada. There is evidence that the risk to human health increases during concurrent EHEs and periods of poor air quality. Also, it has been observed that the health impacts for many hazards tend to be the highest the first time communities experience the hazard, so the novelty of hazard increases the risk. While larger populations centre are likely to have the highest absolute health impacts; smaller more isolated communities may be at greater relative risk due to lower capacity to effectively respond.

4. Discussion

Due to the current relative rarity of and long duration between EHEs in some parts of Canada, it is unlikely that full population acclimatization to EHEs will occur. Effective and appropriate structural adaptation measures will take time to implement and personal behaviour adaptations will require extensive health promotion efforts.

HC is currently working on and supporting several projects to characterize the risk to Canadians of EHEs including: establishing the incidence of heat related illness; temperature - mortality curves across Canada; past and projected future heat associated morbidity and mortality; mortality risk factor analyses; and safe maximum indoor temperatures. However there research gaps due to the paucity of data that are challenging to address: dose - response relationships for northern and Indigenous populations; mental health impacts; indirect effects of EHEs, etc.

HC works closely with the Meteorological Service of Canada, Environment and Climate Change Canada (ECCC) to advance ECCC’s Heat Warning and Information System (HWIS) and with health authorities to implement Heat Alert and Response Systems (HARS), and health promotion and outreach activities across Canada. It is critical to ensure that HARS and HWIS are based on the best available public health evidence and therefore HC is exploring options to address these gaps.