4.0 Discussion
4.1 Summary
When deaths during the 2021 EHE were compared with deaths during
previous years, there was a pronounced increase in odds of death for
those with schizophrenia. This increase was observed among all deaths
and the three EHE subgroups: heat-related (X30) deaths, deaths with
pending cause (R99), and non-heat-related deaths (not X30 or R99). We
also found that odds of EHE mortality were increased for those with
chronic kidney disease and ischemic heart disease among all EHE deaths.
While chronic kidney disease was increased among the heat-related (X30)
and non-heat-related subgroups (not X30 or R99), ischemic heart disease
was only increased in the non-heat-related subgroup (not X30 or R99).
Depression and diabetes were associated with increased odds of EHE
mortality among heat-related deaths (X30), substance use disorder and
chronic obstructive pulmonary disease were associated with increased
odds among deaths with pending cause (R99), and ischemic stroke was
increased among non-heat-related deaths (not X30 or R99). Having 3 or
more chronic diseases was also associated with increased odds of EHE
death, though the odds did not increase with an increasing number of
diseases. Finally, the odds of EHE mortality were significantly lower
among those with angina, hospitalized transient ischemic attack,
dementia, and osteoporosis.
4.2 Chronic diseases and increased odds of death during the
EHE
4.2.1 Specific chronic
diseases
Schizophrenia was the most strongly associated with the odds of EHE
death after adjusting for the other 20 chronic diseases. This aligns
with a growing number of studies reporting an increased risk of death
among those with mental illness during EHEs (J. Liu et al., 2021;
Lõhmus, 2018). For example, Hansen et al. (2008) found that the rate of
deaths attributed to schizophrenia-type disorders was doubled (Incidence
Rate Ratio = 2.08) during heat wave periods (daily max temperature ≥35℃
for at least 3 consecutive days) compared with non-heat wave periods in
Adelaide, Australia from 1993-2004. We report an OR of 3.07 [2.39,
3.94] for administrative diagnosis of schizophrenia among all-cause
EHE deaths in BC. The relationship between increased risk of death and
mental illness is not fully understood and is likely the result of a
complex set of interacting factors. Some people with schizophrenia may
lack insight into their own health status, and thus may not perceive and
respond to overheating (J. Liu et al., 2021). Schizophrenia is also
treated with antipsychotic medications that can affect thermoregulation
(Lõhmus, 2018). Furthermore, the condition is associated with
stigmatization, social isolation, economic marginalization, and
coincident substance use disorder, all of which are independent risk
factors for EHE mortality (Henderson, McLean, et al., 2022; J. Liu et
al., 2021; Raphael et al., 2020; Semenza et al., 1996).
While subgroup analyses showed that people with schizophrenia were at
increased odds of EHE mortality regardless of the cause of death,
cause-specific associations with depression and substance use disorder
may indicate more nuanced links to mental illness. Specifically, the
odds of heat-related death (X30) were increased among those with
depression. Like schizophrenia, depression is treated with medications
that can affect thermoregulation and is associated with stigmatization,
social isolation, and lower-socioeconomic status (Lõhmus, 2018). Along
with the results for schizophrenia, this suggests that mental illness is
an important risk factor for heat-related mortality.
Our results also suggest that substance use disorder was associated with
increased odds of EHE mortality among deaths with pending underlying
cause (R99). Many of these deaths may be due to the ongoing toxic drug
supply crises in BC (BC Coroners Service, 2022b). Substance use disorder
has been previously associated with increased hot weather mortality (J.
Liu et al., 2021; Wilson et al., 2013). Acute intoxication during EHEs
may impair decision-making, and different substances (e.g., cocaine) can
affect physiological susceptibility to heat (Bohnert et al., 2010).
Long-term substance use is associated with health impacts that may
increase the risk of EHE death (Ebi et al., 2021) such as alcoholic
cardiomyopathy (Maisch, 2016). Further, mental illness and substance use
disorder often co-occur, and may overlap to increase heat-related
mortality risks (Brady & Sinha, 2005). For instance, 144 (47%) of all
309 decedents with schizophrenia in this study also had substance use
disorder.
The association between chronic kidney disease and increased odds of EHE
mortality aligns with previous research. For example, in Adelaide,
Australia, hospital admissions for renal disease and acute renal failure
were heightened during heat wave periods (A. L. Hansen et al., 2008),
and the risk of same-day mortality among hemodialysis patients with
end-stage renal disease was increased by 31% [1%, 70%] during
EHEs in three USA cities from 2017 – 2019 (Remigio et al., 2019). This
is quite consistent with our OR estimate of 1.36 [1.18, 1.56].
Associations between EHE mortality and kidney disease may be linked to
increased sweat production, fluid loss, and dehydration during hot
weather. For instance, as the body becomes dehydrated, the blood volume
decreases putting strain on the kidneys, which may increase the risk of
renal damage and failure (Ebi et al., 2021). Although pre-existing
kidney problems may exacerbate this risk, chronic kidney disease is also
associated with other conditions, such as heart disease, which may also
increase the risk of death during EHEs (Ebi et al., 2021; Gansevoort et
al., 2013). Indeed, we report increased odds of EHE mortality among
those with ischemic heart disease and ischemic stroke. This is
unsurprising because cardiovascular conditions are among the most widely
recognized risk factors for heat-related mortality (Ebi et al., 2021)
and are often referenced in public health messaging about extreme heat
(Centers for Disease Control and Prevention, 2017).
Finally, consistent with previous studies, the OR for diabetes was
increased among EHE deaths with a heat-related cause of death (X30). For
example, a recent meta-analysis reported a pooled risk ratio of 1.18
[1.13, 1.25] for diabetes-related death during hot weather across
more than twenty studies (Moon, 2021). This is somewhat lower than the
OR of 1.42 [1.08, 1.86] we report for administrative diagnosis of
diabetes among heat-related EHE deaths. The association between diabetes
and hot weather mortality may be explained by findings that diabetes can
affect the physiological capacity to dissipate heat by decreasing
cutaneous blood flow and sweating (Kenny et al., 2016). However, these
impacts depend on how well the disease is managed, how long someone has
lived with diabetes, and their aerobic fitness (Kenny et al., 2016).
4.2.2 Burden of chronic
disease
The odds of EHE mortality were higher for those with 3 or more chronic
diseases but did not increase markedly for those with a higher number of
chronic diseases. Previous research has also found higher risk among
those with a higher burden of chronic disease. For example, a study of
nearly 200,000 people in Germany found that dementia was associated with
higher risk of death during hot weather, and that risk increased among
those needing more care (Fritze, 2020). Similarly, Foroni et al. (2007)
found that elderly persons living in Modena, Italy during a hot weather
period in 2003 were more likely to die if they; (1) had a higher
comorbidity score, (2) were more likely to use public health services,
(3) took more medications, (4) were more likely to have been admitted to
the hospital in the previous year, and (5) were more likely to have
cognitive impairments or other disability. Together, these results
indicate that individuals with more pre-existing conditions and with
poorer overall health may have been at increased risk of death during
the 2021 EHE in BC.
4.3 Chronic diseases and decreased odds of death during the
EHE
Angina, hospitalized transient ischemic attack, dementia, and
osteoporosis were associated with significantly lower odds of mortality
during the EHE. This was surprising because some of these factors have
been previously linked to increased heat susceptibility (Ebi et al.,
2021; Fritze, 2020). For example, as previously mentioned,
cardiovascular conditions are among the most widely recognized risk
factors for heat-related mortality (Ebi et al., 2021), and dementia was
associated with an 11% increase in the risk of death during periods of
high ambient temperatures between 2004 – 2010 across Germany (Fritze,
2020). Our results may be inconsistent with other studies due to the
case-only design or because these individuals may have received
heightened care during the EHE, such as an increased frequency of health
checks (Kafeety et al., 2020) and increased use of air conditioning.
This is supported by BCCS reports (BC Coroners Service, 2022a) that most
heat-related deaths occurred inside of private residences and not in
health care settings where the frailest individuals may have been
housed. However, more detailed investigations are required to better
interpret these results.
4.4 Non-heat-related deaths
The distribution of chronic diseases among the non-heat-related (not X30
or R99) EHE deaths was similar to the typical weather deaths. However,
the odds of non-heat-related death during the EHE were still increased
for those with schizophrenia, chronic kidney disease, ischemic stroke,
and ischemic heart disease, suggesting that deaths with non-heat-related
causes were still affected by the EHE. For example, decedents with
ischemic heart disease may have been coded with a cardiovascular cause
of death. However, exposure to extreme heat increases the oxygen demand
of the heart as the body cools itself by redistributing blood flow to
the skin. As a result, those with ischemic heart disease are at a
heightened risk of experiencing cardiovascular events during hot weather
because the disease causes a reduction in blood flow and oxygen delivery
to the heart (Ebi et al., 2021). Despite these deaths being related to
heat exposure, they may have been coded with non-heat-related causes.
There may also be unidentified heat-related deaths (X30) included in the
non-heat-related group. Only deaths reported to BCCS had the opportunity
to be attributed to heat exposure, and approximately 50% of deaths
during the EHE were not investigated by BCCS (BC Coroners Service, 2021;
2022a). Furthermore, there were an estimated 740 excess deaths during
the 8-day EHE (Henderson et al., 2021), but BCCS only attributed 562
deaths to extreme heat during this period, suggesting that the
non-heat-related group may include unidentified heat-related deaths.
Under-attribution of heat-related deaths in vital statistics data is a
widely recognized concern (Henderson, Lamothe, et al., 2022).
4.5 Limitations of this
study
This study had important limitations. First, this is a case-only study
comparing individuals who died during the EHE with those who died before
the EHE. The reported effect estimates cannot be interpreted as risk and
protective factors as could be done for a case-control study comparing
EHE deaths with EHE survivors. Instead, an increased odds ratio
indicates that the prevalence of a specific condition was higher among
those who died during the EHE compared with those who died during more
typical summer conditions. Second, typical weather deaths were drawn
from the 2012-2020 period, and may not reflect the deaths that would
have occurred in 2021 in the absence of the EHE. Changes in population
demographics and therapeutics may lead to different point prevalence of
the chronic diseases we examined over time.
Third, we were not able to fully separate deaths from heat-related and
non-heat-related causes during the EHE because of reporting delays
between BCCS and BCVSA. However, most studies of EHE mortality cannot do
any such subgroup analyses because heat-related deaths often go
unrecognized and unattributed in vital statistics data (Henderson,
Lamothe, et al., 2022). In comparison, the health impacts of the 2021
EHE in BC were recognized early in the event, prompting the BCCS to
remind clinicians to report potentially-heat-related deaths. As a
result, many excess deaths during the EHE were attributed to extreme
heat (BC Coroners Service, 2022a). This study represents an important
advancement over prior work because it was able to examine the
prevalence of chronic diseases separately among heat-related deaths,
deaths with pending causes, and non-heat-related deaths.
Finally, we were not able to separate the effects of air pollution from
those of heat. However, BC experienced long periods of very poor air
quality in July 2017 due to extreme wildfire smoke, and there was no
substantive increase in excess mortality (Figure 1; Table S1). In nearby
Washington state, poor air quality from wildfire events has been
associated with a 4.4% increase in all-cause mortality (Y. Liu et al.,
2021), which is substantially lower than the 95% increase in daily
mortality observed during this EHE. However, the effects of air
pollution and heat may also interact to increase the overall risk of
death. For example, approximately 2000 of 11000 excess deaths during a
2010 heat wave in Russia were attributed to an interaction between
wildfire smoke and high temperatures (Shaposhnikov et al., 2014). During
the 2021 EHE in BC, high ground-level O3 concentrations
in greater Vancouver (Henderson, McLean, et al., 2022) and higher
concentrations of PM2.5 throughout the province (Table
S1), likely contributed to the increased deaths with pending cause (R99)
among those with chronic obstructive pulmonary disease. While such
deaths might eventually be coded with a non-heat-related cause, air
pollution concentrations were increased by the hot weather, such that
these deaths were still associated with the overall event (Henderson,
McLean, et al., 2022).
4.6 Conclusion and future
directions
We found that schizophrenia was most strongly associated with the odds
of death during the 2021 EHE in BC. Other chronic diseases associated
with increased odds of death were chronic kidney disease and ischemic
heart disease, as well as a higher overall burden of diseases. The BCCS
reported that most heat-related deaths occurred inside private
residences, often among those who lived alone in multi-unit buildings
without functioning air conditioning (BC Coroners Service, 2022a). While
we do not have individual-level information on socioeconomic status, our
previous study on this EHE showed a doubling of odds in the most
deprived urban communities (Henderson, McLean, et al., 2022).
Together, these details paint a stark picture of those who were likely
at the highest risk: people with mental and physical illnesses and
disabilities who were economically and socially marginalized. These
results, based on relatively rapid case-only analyses, should now be
confirmed and extended with more detailed analyses in a case-control
design. Our future work will compare individuals who died during the EHE
with similar individuals who survived matched on age, sex, geographic
region, setting (e.g., long-term care, emergency room, community), and
other indicators using the same data platform. This will allow further
examination of specific chronic diseases, the burden of chronic disease,
and other potential risk factors such as dispensed pharmaceuticals.
As with our first analysis on this EHE (Henderson, McLean, et al., 2022)
we undertook this rapid case-only study to generate actionable evidence
for public health policy and practice. These studies suggest that some
interventions should be directed to those with severe mental illness,
finding ways to reach susceptible people in their homes. Public health
should partner with organizations that serve such populations to
facilitate outreach before, during, and after EHEs (Kafeety et al.,
2020). In the short-term, targeted interventions should focus on health
checks during hot weather and rapid transition to indoor environments
with safe temperatures. Longer-term interventions are also necessary to
mitigate the future risks of climate change, including changes to
building codes (Crawford, 2022) and strategies that promote passive
cooling through construction and urban design (Kenny et al., 2018).