Why heat kills

“Everyone buttons up their apartment at night,” he said. “It is a critical social ritual that people can’t seem to break.”

That ritual may have played a role in the high mortality rate following the horrific 2003 heat wave that killed 15,000 people in France and 70,000 across continental Europe. Researchers, including Keller, have determined that social and behavioral factors can be more predictive of death during extreme heat events than age, gender or physiology — complicating efforts to save lives. Keller, author of Fatal Isolation: The Devastating Paris Heat Wave of 2003, examined the experience of a small subset of victims and helped establish that social isolation is the greatest predictor of death from extreme heat.

Keller holds a doctorate in history, but he doesn’t study the distant past or even the 19th century. As a humanities scholar exploring the ecology of sickness and health in contemporary times, Keller is interested in how people’s physical surroundings, behavior (influenced by beliefs, cultural practices and economic factors), and social networks play a role in health outcomes. He looks at the numbers — and then looks beyond them, asking not only what happened, but why it happened and what it means.

“I attempt to understand a population’s experience of a disaster,” Keller said. “The questions I ask are not so much about the biological factors but rather the social, cultural and environmental factors that shaped what people went through.”

According to Keller, a lack of understanding about heat and its impacts continues to cause unnecessary deaths in France and other countries. His most recent article in the Journal of Disaster Studies reports that an unsettling number of respondents to a 2019 survey did not know that opening windows, drinking water to stay hydrated, and avoiding outdoor work during extreme temperatures could help save their lives during a heat wave, or “canicule.” Public health messaging, developed after the 2003 disaster, has made some impact, Keller said, but not enough. Even though Paris officials created telephone networks as a check-in mechanism for vulnerable individuals, relatively few people signed up. Keller said one reason for that may be that many do not self-identify as “vulnerable.”

The article also critiques the standard method for calculating heat-related deaths, arguing that the numbers do not reflect reality. At issue, Keller said, is a shifting baseline — a problem that arises when the standards used to measure a phenomenon change incrementally over time.

In the article, Keller explained it this way: “The desertification of the Sahel might not appear so dramatic if one were to compare the desert’s reach in 2022 to its reach in 2020. But comparing it to …1980 would reveal a stark transformation.”

It’s the same with dangerous heat waves in France, which were once so uncommon that most buildings still do not have air conditioning. Now heat waves occur with increasing frequency. Since 2003, French officials have calculated heat mortality by averaging the number of deaths occurring during the same period (say, two scorching weeks in July) over the three previous years. They then subtract that average from observed deaths during the crisis period. The difference is the official excess death toll, and those deaths are ascribed to heat.

Keller’s article argued that climate change — and the COVID-19 pandemic — have made this method almost useless.

“Every summer in Europe, now, is getting unbearably hotter,” said Keller. “If you are using excess deaths as a tool for measuring heat mortality, you are looking back three years to get your average. But what happens when those numbers are elevated every year? Excess deaths are baked into the background data, and the baseline shifts.”

Shifting baselines can lead to underestimating the impact of a disaster.

Keller, who chairs the Department of Medical History and Bioethics in the University of Wisconsin School of Medicine and Public Health, works closely with epidemiologists and demographers and relies on their quantitative research to create the big picture.

“I use those numbers as a way into a problem,” he said. “As I start looking at big-picture data, I find I have questions that the data don’t answer.”

For example, a question arose when Keller realized that the COVID-19 pandemic had likely affected the French population’s vulnerability to high heat.

“People can only die once,” Keller pointed out. “COVID-19 took thousands of vulnerable French people who might, had they not died from the virus, have been victims of the record-breaking heat waves in 2022.”

In other words, while the official assessment is that the 2003 heat wave was the worst weather disaster in contemporary European history, it is quite possible that other heat waves since then have been just as deadly. Keller supports a new method for calculating heat mortality that compares daily temperature data with total mortality. This method was proposed by Mathilde Pascal, an epidemiologist at Santé publique France, the country’s public health agency. The method, which doesn’t rely on past averages or on the government’s definition of a “heat wave” (three consecutive days of extreme heat), reveals that heat-related deaths are happening in dramatically higher numbers than excess mortality measures suggest.

Disasters are incredibly complex and their aftermath often leaves survivors worried and confused. Climate change and habitat loss, which are contributing to the rise of animal-borne diseases, make it imperative to understand the human dimension of every crisis. Keller pointed to the Ebola epidemic in Liberia in 2013 as an example of how culturally-trained anthropologists and social scientists established trust and helped change behaviors that contributed to the spread of disease, succeeding where the government had failed.

Keller emphasized that a nuanced understanding of a crisis, often gained by collaborating across disciplines, can help us learn how to survive events like plagues,  floods, heat waves and other climate-related occurrences.

“In order to reduce vulnerability, we are going to need to develop new kinds of tools for social interventions with groups that are notoriously hard to reach,” he said. “We need expertise that will help officials make smart decisions and change people’s behavior. Even just a little understanding goes a long way.”