Chronic Diseases Associated With Mortality in British Columbia, Canada During the 2021 Western North America Extreme Heat Event

Abstract

Western North America experienced an unprecedented extreme heat event (EHE) in 2021, characterized by high temperatures and reduced air quality. There were approximately 740 excess deaths during the EHE in the province of British Columbia, making it one of the deadliest weather events in Canadian history. It is important to understand who is at risk of death during EHEs so that appropriate public health interventions can be developed. This study compares 1,614 deaths from 25 June to 02 July 2021 with 6,524 deaths on the same dates from 2012 to 2020 to examine differences in the prevalence of 26 chronic diseases between the two groups. Conditional logistic regression was used to estimate the odds ratio (OR) for each chronic disease, adjusted for age, sex, and all other diseases, and conditioned on geographic area. The OR [95% confidence interval] for schizophrenia among all EHE deaths was 3.07 [2.39, 3.94], and was larger than the ORs for other conditions. Chronic kidney disease and ischemic heart disease were also significantly increased among all EHE deaths, with ORs of 1.36 [1.18, 1.56] and 1.18 [1.00, 1.38], respectively. Chronic diseases associated with EHE mortality were somewhat different for deaths attributed to extreme heat, deaths with an unknown/pending cause, and non-heat-related deaths. Schizophrenia was the only condition associated with significantly increased odds of EHE mortality in all three subgroups. These results confirm the role of mental illness in EHE risk and provide further impetus for interventions that target specific groups of high-risk individuals based on underlying chronic conditions.

Keywords: administrative data; air quality; chronic disease; extreme heat; mortality; vital statistics.

Figure 1

Daily counts of mortality (bars) and the population‐weighted maximum daily temperature (blue line) for 2021 (top left) and 2012–2020. Bars are colored corresponding to their statistical deviation from the expected daily number of deaths (black line) calculated using the Public Health Intelligence for Disease Outbreak algorithm which is used by the British Columbia Center for Disease Control to detect anomalies. The extreme heat event was defined as the period when there was significant excess mortality (orange and red bars) in the province (25 June–02 July 2021).

Figure 2

Maps of the extreme heat event (EHE) deaths across the 16 health service delivery areas (HSDAs) of British Columbia, Canada. The left (a) shows the crude adult mortality rate per 1,00,000 population. The right (b) shows the percentage of total EHE deaths in each HSDA. Deaths occurred throughout the province but were concentrated in the greater Vancouver area (insets).

Figure 3

Flow chart of extreme heat event (EHE, 25 June–02 July 2021) and typical weather deaths included in the study. The BC Coroners Service (BCCS) investigated hundreds of deaths during the EHE (818 the week of 25 June–01 July and 333 the week of 02–08 July). At the time of study, BCCS had not yet submitted 398 certificates of death to the BC Vital Statistics Agency, and those deaths had the ICD‐10 code R99 (cause unknown or pending) as their underlying cause in the vital statistics data. Of the deaths certificates that BCCS had submitted, 284 were coded as X30 (extreme heat). BCCS has reported 562 heat‐related deaths from 25 June to 02 July 202, but there is no way to assess which 278 deaths will eventually be coded as X30 and which 120 will have non‐heat‐related causes of death. The BCCS certificate of death can take months or years to submit, depending on the investigation. The X30, R99, and non‐heat‐related numbers here differ slightly from the numbers in Table 1 because excluded deaths are shown as the last step.

Figure 4

Chronic diseases and odds of mortality during the 2021 extreme heat event (EHE). Odds ratios (ORs) and 95% confidence intervals, derived from conditional logistic regression, for each chronic disease (adjusted for age, sex, and all other chronic diseases) among all EHE deaths compared with typical weather deaths. The chronic diseases are ordered from top to bottom by the OR point estimates.

Figure 5

Burden of chronic diseases and odds of mortality during the 2021 extreme heat event (EHE). Odds ratios and 95% confidence intervals, derived from conditional logistic regression, for the total number of chronic diseases per person among all EHE deaths compared with typical weather deaths. Those with 0 chronic diseases were used as the reference category.

Figure 6

Subgroup analysis of chronic diseases and odds of mortality during the 2021 extreme heat event (EHE). Odds ratios and 95% confidence intervals, derived from conditional logistic regression, for the heat‐related (X30, red), pending (R99, blue), and non‐heat‐related (not X30 or R99, green) cause of death subgroups. Each subgroup is compared with all typical weather deaths. Panel (a) shows the odds of death during the EHE among those with specific chronic diseases (adjusted for age, sex, and all other chronic diseases). Panel (b) shows the odds of death during the EHE associated with the burden of chronic diseases per person (adjusted for age and sex).