The start of summer 2025 brought a massive heat dome over the United States, subjecting more than 255 million Americans to what meteorologists called “dangerous, life-threatening” conditions of triple-digit temperatures and high humidity.1 Spiking summer temperatures as a result of fossil-fueled climate change could mean that Americans have already experienced the coolest summer of their lifetime—and have yet to experience the hottest.2

All 10 of the warmest years on record occurred between 2014 and 2023, until 2024 shattered global heat records and became the hottest year yet.3 In July 2024, the world experienced its hottest day ever, with numerous cities across 35 states breaking their daily heat records in the span of only two months.4 Record highs occur twice as often as record lows across the continental United States, while the average rate of heat waves in the United States has tripled since the 1960s.5

Today, extreme heat is the nation’s deadliest weather-related threat,6 straining hospitals and power grids; endangering workers; and hitting older adults and low-income, Black, and brown communities the hardest.

These recent, more extreme heat waves are not normal.7 Scientists can differentiate between natural fluctuations in weather and abnormal climate patterns by examining glaciers, ocean sediments, tree rings, and even pollen remains, as well as ice cores from the Earth’s polar regions, which reveal the planet’s conditions 800,000 years back.8 The composition of the results from these studies overwhelmingly show that the greenhouse gas carbon dioxide’s levels are 50 percent higher today than they were in the 19th century, before the industrial revolution.9

The use of fossil fuels for industries including transportation, energy generation, manufacturing, and agriculture is driving this increase in carbon dioxide levels, resulting in long-term global warming.10 While heat waves are a natural phenomenon,11 fossil-fueled climate change makes them more frequent and more prolonged today than they were in the 20th century.12 Average global temperatures have increased faster in the past several decades than at any time in at least the past 2,000 years.13 By 2060, average temperatures in more than 240 major U.S. cities are projected to rise by 3.6 degrees Fahrenheit,14 making summers in places such as Casper, Wyoming,15 and Billings, Montana,16 feel more like today’s summers in Albuquerque, New Mexico. And by 2100, cities such as Austin, Texas,17 could experience summer heat comparable to Dubai.

Extreme heat is the deadliest form of extreme weather18 and, in most years, kills more Americans than floods, tornadoes, and hurricanes combined.19 More than 21,000 deaths in the United States from 1999 to 2023 were recorded as being related to heat,20 with mortality rates starting to increase the most from 2016 onward. Although some states experience extreme heat more frequently than others, nearly 210 million Americans live in counties vulnerable to health threats from unexpectedly high summer temperatures.21 Look no further than the June 2025 heat dome, which subjected more than 255 million Americans to extreme heat, for evidence of the health threats posed by extreme heat waves.22 This heat wave was potentially the cause of death for a woman in St. Ann, Missouri, with an underlying health condition who had her air conditioning (AC) cut off while St. Louis was under a heat advisory in late June 2025. Residents in the St. Louis area experience an average of more than nine extreme heat days in the summer, according to the Natural Resources Defense Council’s analysis, which also found that millions more Americans today are living in areas with more than nine extreme heat days per summer than in the early 2000s.23 Extreme heat will continue to threaten public health and is projected to lead to a cumulative total of about 28,000 more deaths in major U.S. urban areas each year by the 2090s.24

Even though the threat of heat-related illness and death is growing, these deaths and illnesses remain dangerously undercounted.25 Extreme heat compromises the body’s ability to regulate temperature and can cause a person to suffer from heat stroke, exhaustion, or dehydration.26 However, a physician might not recognize these illnesses as being heat related or assign them the proper diagnostic code.27 Patients with chronic diseases are more vulnerable to extreme heat and can experience physical complications that worsen their condition, but diagnostic codes might omit mention of heat-related illness and instead focus on the clinical exacerbations of the underlying illness.28

More frequent extreme heat hits working-class and low-income Americans the hardest

Extreme heat can be deadly and cause illness in anyone, but it is disproportionately severe for vulnerable Americans.29 Those who are most vulnerable to heat-related health effects include working-class, low-income, and majority Black or Latino communities, as well as people with disabilities and chronic illnesses, people who are pregnant, people who work or exercise outside, older adults, and young children.30

The risk of heat-related illness and death is particularly high for those who lack AC or cannot afford to run it, and this risk is escalating with climate change.31 Of the 6.3 million U.S. households without AC, most are low income and in regions with above-average temperatures.32 Underserved communities have less access to AC and other heat reduction strategies and health resources, including equitable access to environmental amenities and green spaces. As a result, low-income neighborhoods can be as much as 12 degrees hotter during a heat wave than nearby wealthier neighborhoods and experience higher rates of extreme heat-related illness.33

The risk of heat-related stress and death is far greater for farm workers than for all other U.S. civilian workers.34 The same workers who are most exposed to extreme heat in industries such as construction, warehouses, and agriculture are disproportionately immigrants, Latino Americans, and low-wage earners. For example, 83 percent of U.S. farmworkers are Latino, and 75 percent are Latino migrants.35 Americans with heightened sensitivity to heat also face greater health risks from high temperatures. Those with chronic illnesses may have trouble breathing, chest pain, and other medical complications.36 Extreme heat exposure for people who are pregnant increases their risk of hospitalization, maternal mortality, near-fatal pregnancy complications, and severe maternal morbidity by 27 percent.37 This risk is even higher for pregnant Black women, who are more likely to live in a concrete, heat-trapping “urban heat island”; less likely to have access to AC; and have double the risk of hospitalization, relative to white women, for every additional day they are exposed to extreme heat in their third trimester.38

The oldest and youngest of Americans are also at high risk of extreme heat-related illness. As the human body ages, it becomes less efficient at adapting to extreme temperatures.43 Americans ages 60 or older account for more than 80 percent of U.S. deaths from extreme heat.44 An analysis found that Virginia ZIP codes with higher percentages of residents aged 65 or older had a 23 percent increased risk of heat-related hospitalizations and emergency room visits during heat events.45 Meanwhile, young children who are smaller and still growing have a harder time thermoregulating.46 Not only can their bodies heat up to five times faster than adults, but they are also more vulnerable to extreme heat since they play outdoors often and potentially spend more time in aging, inefficient school and child care infrastructure.47 Extreme heat can worsen inequities in both health and educational outcomes for children, and it will pose a greater risk to their development than previous generations as fossil-fueled climate change exacerbates extreme weather conditions.48