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Emily Beers and Craig Patterson cool off in a pool set up in front of Patterson's home during the scorching weather in Vancouver on June 27, 2021.JENNIFER GAUTHIER/Reuters

At the height of British Columbia’s deadly heat wave, Sarah Henderson fielded a flurry of questions from health officials across the province wanting to know more about what exactly had happened. The extreme weather in June, 2021, would soon be known as the deadliest such event in Canadian history. But as it was unfolding, there were many unanswered questions – and only an uncomfortable suspicion that the death toll would be significant.

Dr. Henderson, the scientific director in environmental health at the BC Centre for Disease Control, began poring over the emerging information with her team, working to understand what had transpired and how to prevent such a disaster from happening again. She started with official government reports, but it was an unexpected, commonplace source that would ultimately reveal the most intriguing data.

Analyzing data from Environment Canada, B.C.’s Vital Statistics Agency and an in-house algorithm used to detect anomalies in expected values, Dr. Henderson’s team found that temperatures during the heat wave had soared 10 to 15 degrees above what is normally expected. The heat built up and peaked on June 28, and was followed by a spike in deaths the next day – a pattern common in extreme hot weather events.

On any given day, an average of 110 people die in British Columbia, Dr. Henderson and her team would later write in an article for the BC Medical Journal. On June 29, a total of 380 people died across the province. In the eight days from June 25 to July 2, the researchers found that there were a total of 1,630 deaths – about 740 more than would be expected in a normal summer, according to their statistical models.

(The BC Coroners Service, which has produced different numbers based on deaths that were reported to it, found that there were 595 heat-related deaths between June 18 and August 12.)

Where these additional deaths took place was also startling. On average, 70 deaths a day in B.C. happen in the community (as opposed to in a health care facility). On June 29 alone, there were close to 275. The BC Coroners Service would later release similar findings, reporting that 96 per cent of heat-related deaths from June 25 to July 1 occurred inside a residence.

People died during B.C.’s heat dome not because it was hot outside, Dr. Henderson would soon report to health officials, but because it was hot inside. But how hot?

From her home in the Fraser Valley, Dr. Henderson knew first-hand just how uncomfortable it became indoors during the heat wave, particularly in a province where few people have air conditioning. But there were limited data on indoor temperatures available to the public to show this in any sort of meaningful way.

“That kind of triggered my interest in the temperatures,” Dr. Henderson said in an interview. “So then the next question is: How do I get this information? We have outdoor weather stations all over the place measuring temperatures – I can access those easily – but how do I start getting information about what’s going on in people’s homes?”

Her mind went to Google Nest, a line of smart home products that includes a thermostat that records a home’s temperatures in part to find energy savings. But she hit a dead end when she contacted the company to inquire whether data were available.

She phoned a few acquaintances who she knew had the internet-connected thermostats and they agreed to share their personal data, with one person suggesting she also try ecobee, a Canadian company with a similar line of products. Notably, the company has a program through which more than 80,000 customers – including more than 500 in B.C. – had specifically opted in to share their anonymized thermostat data with scientists.

Within a few days, Dr. Henderson had temperature data from about 300 relevant homes across the province. At her acquaintance’s home in East Vancouver, heat on the main floor had started building on June 25, reaching daytime highs of 34, peaking at about 8 p.m. The overnight indoor lows at the height of the heat dome, from June 27 to 29, exceeded 30 degrees, even though outdoor overnight lows were several degrees cooler. The findings matched that of the other homes for which ecobee data were provided.

“We don’t build to dissipate heat in Canada. We build to retain it. So what you see is this day-over-day increase in indoor temperature, following the day-over-day increase in outdoor temperature,” Dr. Henderson said.

“I think there’s this expectation that a home will normalize to the outdoor temperature overnight but that absolutely does not happen. It’s only able to dissipate a few degrees of temperature, and then it gets even hotter the next day because it starts that day at a very high point and climbs more.”

Dr. Henderson said the network of physical objects that are connected to the internet – the “internet of things” – contains a wealth of valuable data that scientists can tap into to inform public health and engineering decisions as jurisdictions work to adapt to climate change.

“It would be really cool to get out there and see how I would access all the information from a Bluetooth Dyson air cleaner, or one of the fancy refrigerators that people have,” she said. “There have got to be a whole bunch of different things out there that are taking this information, but we’re not getting it and putting it to use.”

Other data sources also produced illuminating findings. By linking anonymized health data from vital statistics, chronic disease registries and other sources, the team found that people with schizophrenia and alcoholism were more likely to die in the heat wave, presenting opportunities to engage with groups such as the BC Schizophrenia Society or Alcoholics Anonymous to better warn these populations in the future.

And by studying the built environment, they found that variables such as greenspace and proximity to large bodies of water also factored into the deaths, as did material and social privilege.

“I think, with a fair degree of confidence, that this was a combination of hot indoor temperatures and social isolation,” Dr. Henderson said. “So how do we build social safety structures around the people who are most at risk, rather than just implementing air conditioning across all housing in B.C.? How do we ensure that we’re reaching out to those people who are really at risk during these events, and giving that kind of social connection that can help to protect them?

Dr. Henderson and her team are expected to publish a paper about the neighbourhood factors associated with community deaths in Greater Vancouver during the heat wave in coming weeks.

The BC Coroners Service expects to complete individual investigations for each of the 595 heat-related deaths it identified by early 2022, at which point it will convene a death review panel of subject-matter experts to make recommendations intended to prevent similar deaths. Those recommendations are expected to be released in the spring.