Climate experts explain why torrid nights can be a ‘silent killer’

This article was written by Kate Allen and was published in the Toronto Star on July 20, 2023.

As a searing sun rose over Phoenix, Ariz., on Wednesday morning, the city incinerated yet another temperature record — one that generated far fewer headlines than the string of all-time daily highs set the day prior.

The overnight low hovered above 35 C, the hottest night there since record-keeping began.

Huge swaths of the Northern Hemisphere are baking under a punishing heat wave, with scorching daytime temperatures setting historic highs.

But experts are urging the public to pay closer attention to the unrelenting nighttime lows, which have a profound effect on human health — helping extreme heat earn its nickname as the “silent killer.”

The World Meteorological Association warned this week that while “most of the attention focuses on daytime maximum temperatures, it is the overnight temperatures which have the biggest health risks,” especially for the most vulnerable people.

“We need the world to broaden its attention beyond the maximum temperature alone,” said senior WMO extreme heat adviser John Nairn.

As climate change increases the frequency and intensity of extreme heat, experts say that much more can be done to reduce deaths and hospitalizations during these events — starting with a better appreciation of just how dangerous heat can be.

“In general, there is a pretty low awareness that heat kills, and that essentially all heat-related deaths are preventable,” said Kristie Ebi, a professor at the Center for Health and the Global Environment at the University of Washington. “So heat has been framed as a ‘silent killer.’ ”

It can be difficult or impossible to attribute an individual death to heat alone. But when epidemiologists zoom out and look at how many more deaths than normal happen during these events, the lethal toll of heat is clear. According to a study published in Nature Medicine last week, at least 60,000 people died last summer from extreme heat in Europe.

During the 2021 heat dome in British Columbia, when the hottest temperature ever recorded in Canada was measured in Lytton, B.C., the day before the town burned to the ground in a wildfire, public health officials reported a 100 per cent increase in deaths among adults 50 or older. The vast majority occurred inside people’s homes.

Studies that have analyzed the relationship between heat events and mortality have often found a stronger association with hot nights than hot days, said Lara Cushing.

“Nighttime temperatures are important because the night is when your body has a chance to cool off and chill … there’s just not that relief,” said Cushing, a professor of environmental health sciences at the University of California Los Angeles.

Our bodies are very good at maintaining a stable core temperature amid fluctuations in the temperature surrounding us, a process called thermoregulation. But there is a limit to our ability to adapt, and when the body is under heat stress all day and then gets no break at night, problems can come on quickly.

During heat events, “we do know that mortality starts within 24 hours,” Ebi said. “And so one really hot night after a really hot day for somebody who is particularly susceptible can be enough to cause adverse health consequences.”

The very young and the very old, whose bodies aren’t as good at thermoregulating, are especially vulnerable to heat-related health problems. People with underlying health conditions are also at particular risk. Ebi added that big epidemiological studies of excess deaths after heat waves have found that about half of those deaths come from cardiovascular causes, like heart attacks.

Other effects are less well understood. Multiple studies have found a link between extreme heat and a heightened risk of infants being born pre-term. Cushing said that in her own research, she has observed this association after just a single day of high temperatures.

“Within a day, you do see health effects. But obviously the longer it goes on, the worse and worse it is.”

As worrisome as the problem is currently, policies and solutions can eliminate all heat-related deaths, said Tarik Benmarhnia, a professor of epidemiology at the University of California San Diego.

“Heat is a big issue, and probably the most problematic issue in a context of climate change for human health and environmental justice,” he said. “But this is preventable, easily preventable. And we can make a difference.”

For policies to be effective, they have to take into account that heat waves don’t affect every place and every person equally.

Benmarhnia points out that most forms of outreach during heat waves happen during the day: Cooling centres aren’t usually open in the middle of the night, for example, and it’s hard to ask someone to show up at a library at 3 a.m.

Effective solutions need to be more targeted: for example, identifying not only who doesn’t have access to air conditioning at night, but also people who have it and can’t afford to run it.

“There is a lot of literature now showing how even people that have access to air conditioning don’t use it because they can’t afford that extra bill.”

Housing and the design of neighbourhoods can be improved to lower temperatures, too. Cities amplify heat through something called the “urban heat island effect.” Materials like concrete and asphalt absorb heat throughout the day, while higher buildings and other built forms block cooling winds. Air conditioners, vehicles and other human activities add more heat to the environment.

Over the course of a year, a city might be a degree or two hotter than its rural surroundings, said James Voogt, a professor in the department of geography and environment at Western University. But under the perfect conditions for an urban heat island, on a given night in a city like Toronto, it might be 10 degrees hotter than the surrounding area, he said.

“We also know the urban heat island effect is larger at night than it is during the day.”

In humid cities like Toronto, one of the best strategies for combating the urban heat island effect is planting trees, Voogt said: Vegetation helps cool the environment, both through direct shade and through a process called evapotranspiration.

For Cushing, another policy solution is critical: “No. 1, we need to stop burning fossil fuels because this is just getting worse and worse the longer that we do that.”

Nighttime temperatures are important because the night is when your body has a chance to cool off and chill … there’s just not that relief.

LARA CUSHING UNIVERSITY OF CALIFORNIA LOS ANGELES