Expert Commentary

How extreme heat affects human health: A research roundup

Studies show that extreme heat can affect most people, particularly vulnerable populations like children, older adults and outdoor workers. We round up recent studies that shed light on how warming temperatures due to climate change are affecting various populations.

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July was the Earth’s hottest month on record, according to the Copernicus Climate Change Service, a program of the European Union.

Extreme heat not only strains water, energy and transportation infrastructure, and affects crops and livestock, it also impacts human health. Many people don’t take heat warnings seriously, as highlighted in the findings of a 2007 telephone survey of 908 people in North America, published in the International Journal of Biometeorology. But studies show extreme heat can affect most people, particularly vulnerable populations like children, older adults and outdoors workers.

“It’s pretty clear that human-caused climate change is causing average temperatures to increase and it’s pretty clear that heat waves globally — but specifically in some regions of the world — are becoming longer, more frequent and more severe,” says Dr. Catharina Giudice, a practicing emergency physician and Climate and Human Health Fellow at Harvard University’s Center for Climate, Health, and the Global Environment. “And we’re certainly seeing the repercussions of that on the health of communities that, unfortunately, are not used to the heat, or not set up to deal with the heat.”

In the U.S., during the first seven months of 2023, there were 15 separate weather and climate disasters in which overall costs and damages reached or exceeded $1 billion, according to the National Oceanic and Atmospheric Administration. And the devastating August wildfire in Maui, Hawaii, which has been declared the deadliest U.S. wildfire in a century, has a current estimated cost of nearly $10 billion.

Heat is the leading cause of weather-related deaths in the U.S., outranking hurricanes, tornadoes and floods, according to the latest data available from the National Weather Service. Between 2004 and 2018, an average of 702 heat-related deaths occurred in the U.S. each year, according to a 2020 study in Morbidity and Mortality Weekly Report. About 70% of those deaths were between May and September.

“With continued warming, cold-related deaths are projected to decrease and heat-related deaths are projected to increase; in most regions, increases in heat-related deaths are expected to outpace reductions in cold-related deaths,” according to the U.S. Global Change Research Program’s Fourth National Climate Assessment, a detailed analysis of how climate change is affecting the United States and its people.

In July, President Biden asked the Department of Labor to issue the first-ever Hazard Alert for heat to protect workers, including farmworkers, farmers, firefighters and construction workers from the dangers of extreme heat. Since 2011, more than 400 workers have died due to heat exposure, according to the White House.

Who’s at risk?

Extreme heat can affect anyone, but certain populations are at a higher risk. They include children, older adults, outdoor workers, pregnant people, people with chronic diseases, people with disabilities, people experiencing homelessness, and incarcerated people. People of color, especially American Indian and Alaska Natives, and Black people have higher rates of heat-related deaths, according to data from the Centers for Disease Control and Prevention.

In addition, “communities that didn’t experience heat waves are now experiencing heat waves and extreme heat and some don’t have the societal infrastructure,” Giudice says. “There are apartments that don’t have air conditioning, because in the past that hasn’t been necessary. So when heat hits the community by surprise, it can be extremely dangerous because the infrastructure is not there.”

Also, certain medications put people at a higher risk. One example is diuretics, which stimulate the kidneys to remove water, and are commonly used to treat high blood pressure or for people who have too much fluid collecting in their bodies.

“So those people are more prone to dehydration and more prone to the effects of extreme heat,” Giudice says.

People who have substance use disorders are also at higher risk, since anything that alters consciousness is going to decrease the ability to respond to changing temperature, she says. “If you’re too intoxicated and you don’t realize that you’re in an extremely hot environment, obviously that’s going to affect your ability to remove yourself and seek shade and shelter.”

What are the solutions?

“I think the conversation several years ago was let’s prevent climate change from happening,” Giudice says. “And unfortunately, we are at a place where we’re already seeing climate change happening. So, we need to prevent future worsening and we need to cut our carbon emissions or usage of fossil fuels as fast as possible.”

The more immediate solutions are adapting to the changing climate at the individual, community and national levels, she says.

“At the individual level, it’s important for people to have a plan. If you don’t have air conditioning, where are going to go? Where’s the heat shelter? Some communities have specific cooling shelters,” Giudice says.

Also, “cities need to think about how to make sure that their electrical grids are able to withstand the surges in electrical usage during heat waves,” she adds.

Lastly, “it’s very important for people to have a community sense during heat waves and check on their neighbors, families and friends, especially if they have a loved one who is elderly and lives by themselves,” Giudice says.

Research roundup

To help journalists better explain the serious impact of extreme heat on physical and mental health, we’ve selected seven recent studies that explore the impact of extreme heat on human health. They address topics including heat strokes, heat waves, impacts on people with diabetes, outdoor workers, people who experience homelessness and the use of ambulance services. We’ve selected each study as a prompt for a news story or additional context for heat-related stories. They’re organized by publication date.

High Ambient Temperatures Associations with Children and Young Adult Injury Emergency Department Visits in NYC
Blean Girma, et al. Environmental Research, Health, September 2023.

The study:Researchers collected data on all daily emergency room visits in New York City from 2005 to 2011 from the New York Statewide Planning and Research Cooperative System, an administrative database of inpatient and emergency room visits in New York. They focused on emergency room visits in hotter months of May through September, and children and young adults from newborn to 25 years old. Heat-related injuries among kids include dehydration, heat-related illness, diarrhea and digestion disorders and infectious diseases. “When it is hot outside, exposed individuals may have impaired response abilities, which may in turn, result in unintentional injury,” the authors explain.

The findings: There was a 30% higher odds of emergency room visit for an injury during the warmest temperatures compared with coolest temperatures during the warm season. Young males between ages 5 and 9 were most likely to go emergency room with injuries during warmer weather, mostly due to increased activity or lack of adult supervision.

Quote from the study: “Our results underscore the importance of considering variation in the health effects of heat, and consequences for healthcare utilization, given the growing impact of climate change. This research raises awareness of the multifaceted ways by which climate change and the urban climate impact children’s health and well-being.”

Also from the study: “We suggest that local, city, and state government consider the vulnerability of children and young adults in their extreme heat mitigation strategies, which could also include highlighting activities, resources and tools community members can use to alleviate social and mental health stressors during warmer periods.”

Heat, Heatwaves, and Ambulance Service Use: A Systematic Review and Meta-Analysis of Epidemiological Evidence
Zhiwei Xu, et al. International Journal of Biometeorology, July 2023.

The study: In this systematic review and meta-analysis, researchers include 48 English-language papers published between 2011 and 2022 about the association between heat or heat waves and the risk of ambulance dispatches.

The findings: There’s an association between ambulance dispatches and heat or heat waves. Each 5 degrees Centigrade increase in average temperature was associated with 7% increased risk of ambulance dispatches for any cause and 2% increased risk of ambulance dispatches for heart disease.

More about ambulance activity: Ambulance data has been used as the bellwether of health service use, including hospitals and emergency rooms, during hot days. You can use this tool to find emergency medical services responses to heat-related illnesses in each county across the country.

Classic Heat Stroke in a Desert Climate: A Systematic Review of 2632 Cases
Saber Yezli, et al. Journal of Internal Medicine, March 2023.

The study: Researchers included 47 English language studies, including 2,632 patients, carried out between 1962 and 2019 and before April 2022 on heatstroke cases during the Muslim pilgrimage (Hajj) in the desert climate of Mecca, Saudi Arabia. About two million people perform the Hajj each year. The authors note that their systematic review includes one of the largest cohorts of heatstrokes reported to date, allowing them the review the natural course of heatstrokes from the earliest signs.

The findings: The early signs and symptoms of heatstroke can affect the nervous, digestive and cardiovascular systems, suggesting that it is a systemic illness. Heatstrokes induce widespread early tissue injury and multiple organ dysfunction and failure. Also, nearly 40% of people who had a heatstroke were overweight or obese, suggesting that this population is particularly vulnerable. Overweight and obese adults also have 3.5 times risk of death from heatstroke. People with high blood pressure and diabetes were also at a higher risk of heatstroke, which, the authors note, agree with previous reports that show an increase risk of heatstroke in people with most chronic conditions and medications used to manage them.

Quote from the study: “The findings of this review indicate that [heatstroke] is a severe medical condition with widespread early multi-organ injury. They suggest that early diagnosis and therapy reduce the progression to more organ damage resulting in lower mortality compared to previous studies in other settings. Importantly, they indicate that in the best possible setting, including early diagnosis and therapy, cooling and supportive therapy are not sufficient, and thus, novel interventions are urgently needed to further reduce morbidity and mortality.”

More about heatstrokes: Heatstroke, also written heat stroke, is the most serious heat-related illness, according to the CDC. It happens when the body can no longer control its temperature and can rise to 106 degrees Fahrenheit or higher within 10 to 15 minutes. Heatstroke can cause permanent disability or death if the person isn’t treated quickly. Early signs include confusion, slurred speech and profuse sweating.

The Impact of Heat Waves on the Mortality of Chinese Population: A Systematic Review and Meta-Analysis
Ranran Pan, et al. Medicine (Baltimore), March 2023.

The study: In this systematic review and meta-analysis, researchers analyzed the findings of 15 studies published before November 2022 on the impact of heat waves on the death of people in China. The authors note heat waves may pose a greater threat to China due to an aging population, which is more vulnerable to extreme heat.

The findings: Heat waves were associated with an increased risk of death in the Chinese population. There was a 19% increase in risk of death due to heat waves in China, which is higher than developed nations such as United States (11%) and Sweden (10%). However, the risk of heatwave-related deaths is lower in China than in developing countries such as India (43.1%) and Pakistan (166.8%). Also, the risk of death was higher among people who had less than six years of education compared with those who had more education, potentially due to more outdoor work and lack of health literacy.

Quote from the study: “In summary, the present meta-analysis provides strong evidence that heat waves are significantly associated with an increased risk of death in the Chinese population, especially among residents with low education levels. Therefore, high-risk populations should be the focus of public health policies, and prevention strategies should be developed and implemented to effectively reduce the public health harm caused by extreme temperature events.”

More about heat waves: There’s no set definition for heat wave. The National Weather Service defines it as “a period of abnormally hot weather generally lasting more than two days.” Heat waves can last weeks. In 2003, an estimated 70,000 people died in Europe due to a heat wave that spanned from June to August. And in 2010 in the Russian Federation, 56,000 more people died during a 44-day heat wave. From 1998 to 2017, more than 166,000 people died due to heat waves, including more than 70,000 who died during the 2003 heatwave in Europe, according to WHO.

Association Between Extreme Ambient Heat Exposure and Diabetes-Related Hospital Admissions and Emergency Department Visits: A Systematic Review
Donghong Gao, et al. Hygiene and Environmental Health Advances, December 2022.

The study: Researchers summarize the findings of 18 English-language papers, published between 2010 and 2022, which explored the link between extreme heat and diabetes emergency room visits or hospital admissions.

The findings: There’s a significant association between exposure to extreme heat and diabetes-related hospital admission or emergency room visits. Extreme heat was associated with 4.5% increased risk of hospital admission or emergency room visit among people with diabetes and 10% increased risk among older adults with diabetes.

Quote from the study: “The findings from this review will be useful for developing evidence-based interventions to reduce the impact of extreme heat exposure on diabetes, identifying the target population for the interventions (i.e., older adults), and supporting public health initiatives for public education to increase public awareness of the risk of extreme ambient heat exposure on diabetes.”

More about diabetes and extreme heat: Diabetes affects more than 10% of the U.S. population, and people with diabetes are among populations that are more vulnerable to extreme heat. Certain complications from diabetes — both type 1 and type 2 — such as damage to blood vessels and nerves, can affect sweat glands and the body can’t cool effectively, increasing the risk of heat exhaustion and heatstroke, according to the CDC. Also, people with diabetes get dehydrated more quickly.

Heat Waves and Emergency Department Visits Among the Homeless, San Diego, 2012–2019
Lara Schwarz, et al. American Journal of Public Health, December 2021.

The study: Researchers obtained emergency department use data from two hospitals in San Diego between 2012 and 2019, analyzing data for 24,688 visits between May to September by patients who were identified as homeless.

The findings: In total, 94% of the patients were younger than 65; 14% said they needed a psychiatric consultation. The strongest risk of emergency room visits was during daytime heatwaves.

Quote from the study: “Homelessness is rapidly becoming a major social challenge in the United States. Structural inequalities, housing crises, high rental costs, and natural disasters have all contributed to the increasing number of persons experiencing homelessness in recent years. … As the threat of increasingly frequent and more intense heat waves continues to rise in the United States, particularly in California, understanding and prioritizing the needs of this rapidly growing vulnerable population will be a critical action in developing and deploying effective mitigation strategies.”

Extreme Heat and Occupational Injuries in Different Climate Zones: A Systematic Review and Meta-Analysis of Epidemiological Evidence
Syeda Hira Fatima, et al. Environment International, March 2021.

The study: Researchers included 24 English and non-English language studies, published between January 2005 and July 2020, which assessed the effects of extreme heat on work-related injuries among non-military workers. The systematic review represents work-related injuries in six countries — Australia, Canada, China, Italy, Spain and U.S. — and across nine climate zones. Work-related injuries during hot weather include heat stress, inability to work, heat-related illness, loss of productivity and even death.

The findings: All studies suggested that working in hot weather increased the odds of a work-related injury. Also, during heat waves, the risk of work-related injuries increased by 17.4%. Male workers and young workers were at highest risk of a work-related injury during hot weather. Outdoor work involving intensive work, including agriculture and construction were at high risk of injuries during high temperatures. Also, male, young and new workers were at a higher risk of injuries during heat weaves.

Quote from the study: “There is an urgent need to mitigate the impacts of occupational heat stress in the context of climate change and the anticipated rise in environmental heat stress. The risk of [occupational injuries] associated with extreme heat is not evenly distributed and is dependent on underlying climatic conditions, workers’ characteristics, nature of work, and workplace characteristics.”

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