1.0 Introduction

Individual extreme heat events (EHEs) can be associated with hundreds to thousands of excess deaths (Kovats & Hajat, 2008), with the most severe example being the approximately 70,000 excess deaths across Europe in the summer of 2003 (Robine et al., 2008). The frequency and intensity of EHEs are expected to increase in the coming decades because of climate change, and global temperature extremes have already become more frequent since the 1950s (Ebi et al., 2021; IPCC, 2021). Consequently, it is essential to understand who is at risk of dying during EHEs to help develop interventions to prevent future heat-related mortality.
Susceptibility to the health effects of high temperatures varies between individuals. For example, older adults have an increased risk of death because thermoregulation and the ability to recognize thermal stimuli deteriorate with age (Ebi et al., 2021; Kenny et al., 2018). Other factors associated with an increased risk include pre-existing cardiovascular disease (Ebi et al., 2021) and mental illness (J. Liu et al., 2021) as well as material deprivation (Kovats & Hajat, 2008) and social isolation (Kenny et al., 2019). Age, cardiovascular disease, respiratory disease, and diabetes are often cited as primary risk factors for heat-related illness or death and are generally referenced in public health messaging (Centers for Disease Control and Prevention, 2017; Ebi et al., 2021; Kenny et al., 2018). However, the risk of heat-related death varies over space and time as community characteristics, adaptation strategies, behaviors, and socio-demographics change. Further, EHEs may also co-occur with other environmental stressors such as high levels of ground-level ozone and particulate matter from wildfires (Rahman et al., 2022). Therefore, it is important to understand factors associated with the risk of death during specific EHEs to develop more targeted strategies to protect the health of susceptible populations in different regions.
Western North America experienced an unprecedented EHE in late June 2021, which was rapidly attributed to climate change (Philip et al., 2021). During this event, there was a 95% increase in population mortality across British Columbia (BC), Canada, equivalent to approximately 740 excess deaths, making it one of the deadliest weather events in Canadian history (Henderson et al., 2021). Early research from the BC Centre for Disease Control (BCCDC) found that community deaths in greater Vancouver were associated with neighborhood deprivation and decreased neighborhood greenness (Henderson, McLean, et al., 2022). In this follow-up study, we aim to assess the relationship between chronic diseases and the risk of death during the 2021 EHE. We compare all adults who died in BC during the EHE with all adults who died on the same dates in the previous nine years to examine differences in the prevalence of 26 chronic diseases between the two groups.