In July 1995, Chicago went through an unforgiving multi-day bout of heat and humidity that took an estimated 739 lives. Ten refrigerated semitrailers were brought in to handle overflow at the city morgue – an image familiar to many who have seen the same as a result of COVID-19 in New York City and El Paso, Texas.
Those felled by heat in Chicago were disproportionately Black, many of them older people who lived alone, either lacking air conditioning or the money to use it. Some closed and locked their windows out of fear of crime, trapping heat indoors.
Heat-wave deaths are “isolated, lonely, painful deaths,” said sociologist Eric Klinenberg, now at New York University and the author of Heat Wave: A Social Autopsy of Disaster in Chicago. “If you look closely at the police reports, or the medical autopsies, they’re just horrific.”
A race with a warming climate
That 1995 Chicago experience helped galvanize the nation into tackling heat waves as a public health threat as serious as tornadoes or hurricanes – a sensible move, given that heat claims more lives in the long run than any other weather disaster.
Cities amped up safety plans using the latest science with steps such as regular check-ups on vulnerable residents. The most innovative plans nowadays mandate free transportation to cooling centers and prevent utilities from disconnecting power during a heat wave. Europe took similar steps after its own deadly heat wave in 2003 killed tens of thousands across the continent.
Since 1995, the United States has not had an urban heat catastrophe comparable to that in Chicago, even though the population has increased about 25% since then and the average summer temperature for the 48 contiguous states has climbed more than 1°F.
Environmental scientist and heat expert David Hondula of Arizona State University is encouraged by some of the signals from national and international heat-death statistics. “There are some reasonable arguments to be made that the broad community of practitioners working on heat resilience have had success in reducing risk,” said Hondula.
But problems persist, according to Hondula and other researchers. Recent years in Phoenix have each seen close to 200 heat deaths, and new research suggests the annual U.S. toll runs into the thousands, far above official numbers. Globally, tropical countries have had a sobering increase in deaths of outdoor workers during extreme heat waves. Also, alarming research in recent years has highlighted the emergence of brief periods of heat and humidity near the threshold of human survivability in pockets of the Middle East and South Asia, along with the potential for such events to become more frequent and widespread with global warming.
A total of 42 studies published over the last decade in the Bulletin of the American Meteorological Society have confirmed links between human-caused climate change and heat episodes. Computer models project more extreme heat waves as Earth warms. The Fourth National Climate Assessment warns that unless greenhouse gas emissions are curbed, the U.S. could average more than 9,300 additional premature deaths per year by the end of the century, even accounting for a reduction in cold-related deaths.
Pollution – including smoke from wildfires increasingly exacerbated by climate change – also contributes to respiratory and cardiovascular deaths in the stagnant air of many heat waves.
Heat deaths are preventable
“Every heat-wave-related death is preventable,” said Kristie Ebi, an epidemiologist and professor of global health at the University of Washington. “If we don’t try to prevent people dying now, it’s immoral on some level.”
Yet heat response has not gotten the attention it deserves. Federal research is scant, for one thing. According to Ebi, a mere 0.04% of the budget of the National Institutes of Medicine goes to topics related to climate and health.
“In D.C., there’s a phrase – ‘budget dust’,” said Ebi. “0.04% doesn’t even rise to the level of dust.”
As studies of heat and mortality become more comprehensive, they’re tending to find that more deaths are related to heat than once realized. In a paper published this year in Environmental Epidemiology, Kate Weinberger of the University of British Columbia and colleagues found evidence that the actual heat toll is far higher than official numbers indicate.
A National Weather Service website shows an average of 138 people in the U.S. felled by heat per year from 1990 to 2019, with a slight improvement to 103 per year over the past decade (2010–2019).
But this tally counts only heat deaths in locations where weather conditions ended up meeting the local thresholds for an NWS heat advisory or warning, according to an agency spokesperson.
A more troubling picture comes in more comprehensive numbers from the U.S. Centers for Disease Control and Prevention (CDC), the nation’s official source of mortality data. The table at right includes data compiled by CDC on the number of heat-related deaths in the U.S. between 2004 and 2018; the numbers show stark year-to-year variability, with more than a threefold difference between the lowest and highest years.
The CDC numbers are drawn from three codes commonly used to signify heat on death certificates: exposure to excessive natural heat; environmental hypothermia of a newborn; or effects of heat and light. One of these three codes is listed as the underlying cause of death in about 60% of cases. The others involve other underlying causes, from drug overdoses to heart attacks to diabetic crises, with one of the three heat-related codes listed as a contributing rather than underlying factor.
Beyond the death certificate
There is still no well-defined nationwide process for determining if and by how much heat contributed to a particular death. One doctor might list a heat wave as a contributing cause to a cardiovascular death, while another doesn’t.
In their recent study, Weinberger and colleagues looked at excess deaths – in this case, the number of fatalities of all types that would have been expected in the absence of dangerous heat.
Each state has its own system for producing death certificates, and it can take months or years for researchers to obtain and analyze the data. The team led by Weinberger examined 297 counties, each with at least 100,000 residents that had full mortality data available throughout the period 1997-2006. Using localized data on daily average temperature, the team came up with a statistical temperature-mortality relationship for each county, then pooled the numbers across the 10-year study period.
The results were eye-popping. In the counties studied, one out of every 227 deaths is estimated to be heat-related. Many of those deaths occurred at “only moderately hot temperatures,” the authors noted.
“If you just use what’s on the death certificate, you lose many people who died of heat,” said Ebi.
Communities at most risk
According to CDC, heat-related death rates in 2004-2018 were about twice as high for Black people, and roughly three times as high for Indigenous people, as for Whites and Hispanics.
For a 2019 investigation by the Guardian newspaper, 24 of the nation’s 30 largest cities or their counties provided heat-response plans. Yet only 10 of those plans specifically targeted vulnerable populations such as homeless people.
Jalonne L. White-Newsome, now a senior program officer at the Kresge Foundation who focuses on climate, health, and equity issues, studied heat vulnerability in Chicago and Detroit, and found that much of the heat vulnerability there has roots in systemic racism and other inequities. These and other communities are laden with poorly ventilated housing, underserved by public health access, and largely bereft of the cooling benefits of green spaces.
Traditionally underserved populations are generally most likely to be affected by excessive heat
Those dying at higher rates from heat in Chicago and Detroit included “the poor, the houseless, people of color, people over the age of 65, and those with pre-existing medical conditions like asthma and respiratory concerns,” said White-Newsome. “As a caregiver for my grandparents, it was also painfully obvious that in these same communities, the systems were failing those that it needed to protect.”
Attention to heat and health has ramped up sharply in Arizona’s Maricopa County, including greater Phoenix, a booming area and one of the hottest urban areas on Earth. Phoenix has been heating up even more quickly than the national average, and 2020 was especially brutal, with a record total of 144 days hitting 100°F. This total includes 53 days that reached 110°F, and 14 days that made it to 115°F. The city went nearly a month this summer without once dipping below 90°F.
Maricopa County’s meticulous tracking of heat-associated deaths, refined over more than a decade, goes beyond the CDC approach and is more fine-grained than excess-death calculations. As of October 31, 2020, the county had confirmed a record 207 heat-associated deaths, with 134 more under investigation. The preceding three years saw consecutive record fatality counts of 179 in 2017, 182 in 2018, and 197 in 2019.
Soaring rates of homelessness in Phoenix are an increasing cofactor. In an in-depth report, the Arizona Republic newspaper confirmed that almost half of heat-associated deaths in 2019 were among people who had no home. Some were found with burns from lying on hot pavement.
The global picture
The worst of Europe’s prolonged 2003 heat wave struck in early August as millions of Europeans traveled on vacation, and many elders were in city housing not built to accommodate the unprecedented heat. Studies estimated that heat wave took more than 70,000 lives prematurely. Ebi noted that only one of the nations affected – Portugal – had a heat-wave early warning system in place.
Since then, many European cities and nations have adopted the kinds of interventions pioneered in the United States. A significant heatwave in 2006 saw much lower mortality. “This was attributed to awareness, warning, and probably increased use of air conditioning,” Ebi said.
Studies now under way seek to assess how many Europeans died in 2019 from two blistering heat waves. The second one sent temperatures to all-time record highs in hundreds of towns and cities, including 108.7°F in Paris, France, and 97.7°F in Oxford, England.
Reports so far – including an excess-death estimate of 2,500-plus released in May by the Centre for Research on the Epidemiology of Disasters – suggest the impact in 2019 was well short of what happened in 2003.
“Adaptation measures could have played a significant role,” said a team of experts in a recent study of the event, noting a range of life-saving measures now in place. However, they added, “Europe is still highly vulnerable to heat extremes.”
Tropical nations such as India are expected to see some of the globe’s most dangerous increases in heat. Outdoor workers in the tropics are especially vulnerable. A puzzling outbreak of kidney disease among sugar-cane workers in Central America has been linked to heat exposure and dehydration, among other factors. The syndrome has ushered in “a new era of climate medicine,” according to a 2019 overview in the New England Journal of Medicine.
Among India’s deadliest heat waves in recent years was a 2015 disaster that took at least 2,300 lives. As of this year, 23 Indian states and more than 100 cities and districts were developing and implementing heat action plans. The nation’s first such urban plan, implemented in Ahmedabad in 2013, prevented 1,190 heat deaths over the following two summers, according to one study.
The relentless toll of heat
Heat need not be extreme to threaten life and health, even in the womb. More than a dozen papers have now shown significant links between heat waves and the rate of preterm births – in some cases with heat that falls below heat-advisory thresholds. An even broader review this year, examining 57 studies of U.S. birth outcomes, found that air pollution and heat were correlated with problems from low birth rates to stillbirths, with a disproportionately high toll among Black mothers and babies.
Other heat-related issues continue to surface. A powerful study spanning 58 nations and 12,000 U.S. school districts found rising heat during the school year is making it harder for students to learn, especially minority and lower-income students in the U.S., who are less likely to have air conditioning at school or at home.
Hondula emphasizes the broad impact of heat: “Emergency department visits, hospitalizations, and other formal medical treatment and services accumulate significant costs and can strain the health care system. More broadly, heat causes stress, worry, anxiety, and general reductions in quality of life for many people in hot cities.”
As Ebi sees it, society must mitigate immense harm from extreme heat even as it works to reduce fossil-fuel emissions that make it worse. According to the U.S. National Climate Assessment, more than half of the increase in heat-related deaths projected by late this century could be averted if emissions begin a sustained drop over the next couple of decades.
Whether to adapt to extreme heat or mitigate it “is not a dichotomy,” Ebi said. “We do both. If we don’t, we suffer.”