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Q&A: Extreme heat and wildfire smoke combine to pose heightened health risks

With wildfires blazing and temperature records being broken, many Canadians could be experiencing smoky air alongside soaring heat this summer—and according to new UBC research, the combined effects could be uniquely hazardous to human health.

Dr. Sarah Henderson, professor in the UBC School of Population and Public Health and scientific director of Environmental Health Services with the BC Centre for Disease Control, is the senior author on a new study that examined how such double exposures impact health and mortality.

Here's what she told us about the published in npj Clean Air—and what's needed to protect those who are most at risk.

What did your study find?

Our research looked at the combined effects of heat and air pollution, including wildfire smoke, which is expected to become increasingly common due to climate change.

We examined more than 21,000 deaths outside of health care settings in the Greater Vancouver area during 13 wildfire seasons between 2010 and 2022. We found that the risk of death spikes when people are exposed to both elevated levels of fine particulate matter (PM2.5) from wildfire smoke and temperatures above 26 C.

But it's not as simple as more smoke plus more heat means worse outcomes. Mortality risk during extreme heat rises exponentially, while mortality from wildfire smoke rises quickly at moderate exposures and then levels off. So, in terms of risks to your health, extreme heat matters more. On average, being too hot with no smoke exposure is riskier than being exposed to smoke at cooler temperatures.

What happens to your body when you're exposed to both extreme heat and wildfire smoke?

When it's very hot, your body is fighting hard to maintain its core temperature and keep cool, and when it's smoky, your body is fighting hard to reduce the inflammation caused by smoke exposure. When these things happen at the same time, it puts more stress on your body. Our research confirms that together, the impact is greater than the sum of their individual effects.

Who is most susceptible during these heat and smoke events?

In all our analyses of the 2021 heat dome in B.C.—when it was not only very hot, but air quality was also quite poor due to wildfires and ground-level ozone—the population with the highest increased risk was people with schizophrenia. This is because people with schizophrenia can have a hard time recognizing when they are unwell, the illness can have cardiovascular impacts, and anti-psychotic medications can impede thermoregulation, among other factors.

Other populations at increased risk included people receiving income assistance—an indication of living in poverty—as well as older adults with underlying health conditions like heart failure, chronic obstructive pulmonary disease (COPD), kidney disease and substance use disorder. Although not included in our mortality studies, children and people who are pregnant are also at higher risk from extreme heat and wildfire smoke. For example, there is a higher risk of preterm birth on hot days and on smoky days.

What should people do to manage the effects of heat and wildfire smoke?

Respect the exposures and take precautions to reduce your exposure. I often say that you need to listen to your body—if your body's telling you you're not coping, don't ignore that. Move to a good indoor environment with cleaner, cooler air, if you can. And remember that both exposures can have longer-lasting effects, including impacts on cardiovascular and brain health, so you're helping protect your future self by taking these steps.

What's needed to protect everyone during extreme heat and smoke events?

As a society we need to think about what policies and measures will ensure everyone has access to healthful and safe indoor environments during extreme exposure events.

For instance, we have minimum indoor temperature standards, but no limits on the upper end. Ideally, there should be at least one room in everyone's home that can be maintained at a maximum temperature of 26 C, regardless of what's going on outside.

Similarly, we do not have an indoor standard for PM2.5, which is the primary pollutant we're concerned about with wildfire smoke outdoors. Setting some national guardrails would help to protect the health of current and future generations in the changing climate.