Extreme Heat on the Rise

Heat-related mortality in the United States is a significant and evolving public health concern, with an average of 702 heat-related deaths annually between 2004 and 2018.Reference Vaidyanathan ³ Notably, heat-related mortality has shown complex temporal trends. Studies found a decline from 41 excess deaths per million in the 1960–70s to just 10.5 per million in the 1990s, likely due to increased air conditioning and other adaptation efforts.Reference Davis ⁴ However, recent research found increasing heat-related deaths in most US regions, particularly in the Southeast and Southwest, suggesting this decreasing mortality trend may be reversing, with growing vulnerability to extreme heat events.Reference Narayanan and Keellings ⁵ Addressing extreme heat in the US requires a multi-faceted approach; however, significant barriers remain, including limited resources, local contextual factors in heat vulnerability, and political will. The next paragraphs will expand key heat-related terms to guide future policy agendas to protect the public and vulnerable populations.

Air temperature, heat index, and wet-bulb temperature are distinct measures of heat stress, each capturing different aspects of environmental thermal conditions. Air temperature measures atmospheric temperature, while heat index (HI) combines temperature and humidity to assess perceived temperature or how hot the body feels.Reference Robinson ⁶ Wet-bulb global temperature (WBGT) incorporates multiple environmental factors affecting human heat stress — including potential for evaporative cooling — accounting for direct sunlight, humidity, and temperature.Reference Baker ⁷

Research shows significant increases in warmer temperatures across the United States. Heatwaves, which are prolonged periods of excessively hot weatherReference Guo ⁸ typically lasting at least three consecutive days, are becoming more frequent.Reference Qian ⁹ Average annual temperatures have risen by 1.2°F from 1986 to 2016 while cold waves have decreased.Reference Vose and Wuebbles ¹⁰ Large ensemble climate models project continued increases in record-breaking summertime temperatures, with high amplitude records expected in the Northwest and central states.Reference Dahl ¹¹ By the mid-21st century, days with heat indices exceeding 100°F and 105°F are projected to double and triple respectively, with late-century scenarios showing four-fold and eight-fold increases under high greenhouse gas (heat-trapping gas) emissions. ¹² High-resolution modeling confirms that extreme warm temperatures will intensify significantly, with 5-day heat waves occurring 5–10 times annually and ≥95°F days increasing by 1–2 months by the end of the 21st century.Reference Zobel ¹³

Rising global temperatures are significantly increasing human exposure to dangerous heat extremes. Historical warming of ~1.8°F has tripled the population exposed to dangerous heat-humidity conditions from 97 million to 275 million people annually.Reference Li ¹⁴ Over one billion people could be exposed to heat and humidity annually if global temperature rises by 2°C or more above pre-industrial levels. ¹⁵ Heat stress disparately affects vulnerable populations including older adults, children, outdoor workers, and those with chronic diseases, not only causing heat exhaustion but also exacerbating various medical conditions.Reference Kravchenko ¹⁶

The WBGT threshold varies significantly with humidity. In high-humidity environments, humans become heat-stressed at lower temperatures, while dry environments allow slightly higher tolerances.Reference Lu and Romps ¹⁷ Healthy wet-bulb temperatures are significantly lower than the previously assumed 95°F limit, with critical thresholds ranging from 68°F to 90°F depending on humidity levels.Reference Wolf ¹⁸ WBGT events — characterized by a dangerous combination of high temperature and humidity that significantly impairs human physiological cooling mechanisms — are significant and becoming more frequent, exposing millions to life-threatening heat stress, particularly in regions with limited cooling infrastructure.Reference Marx ¹⁹ These WBGT events could increase 100- to 250-fold by 2070–80, with 150–750 million person-days of exposure to temperatures exceeding today’s deadliest heatwaves. ²⁰ Key contributing factors that can be targeted to mitigate and slow the progression of heat-related impacts include limiting emissions of greenhouse gases — heat- trapping gases released from burning oil, coal, and natural gas — and building cooling infrastructure for a rapidly growing population.Reference Jiang ²¹

The Health Dangers of Heat Exposure

Heat exposure encompasses environmental and personal dimensions that pose risks to human health and safety. Personal heat exposure is defined as contact between individuals and indoor or outdoor environments, risk-increasing body core temperature, and/or perceived discomfort.Reference Kuras ²² This exposure can be measured directly through wearable monitors or estimated indirectly using time activity and meteorological data. ²³ Ambient temperature has been shown to significantly correlate with personal heat exposure, though factors like time spent indoors, income level, labor intensity, humidity, and body composition influence overall exposure patterns.Reference Hondula ²⁴

Heat-related illnesses include a wide range of medical conditions, such as disrupted thermoregulation during exposure to high heat.Reference Gauer and Meyers ²⁵ These conditions range from mild syndromes, such as heat cramps, to more severe forms including heat exhaustion and life-threatening heat stroke.Reference Lugo-Amador ²⁶ Heat exhaustion is characterized by cardiovascular hypoperfusion and core temperatures up to 104°F without central nervous dysfunction. Heat stroke presents as a medical emergency with core temperatures of 105°F or greater, multiorgan damage, and central nervous system dysfunction.Reference Yokoyama ²⁷ Once core temperature reaches 104°F, cellular damage occurs, potentially leading to organ failure and death.Reference Becker and Stewart ²⁸ Management strategies include removal from heat stress, as well as cooling measures such as cold-water immersion, rehydration, and supportive care. ²⁹ Even rising temperatures that do not meet the criteria for emergency action represent a threat that must be dealt with at the household level to ensure people maintain thermal comfort, particularly because hotter temperatures can disrupt or impede quality sleep.Reference Chevance ³⁰ The implications of diminished or lost sleep resulting from chronic heat exposure are manifold. Workers exposed to high heat on the job that lack access to cooling at home are more prone to heat stroke, injuries, or fatal accidents.Reference Morrissey ³¹

Research identifies multiple contributing factors that influence the severity of extreme heat impacts on human health. Socioeconomic factors including poverty, work location, and limited access to health resources and information increase heat vulnerability.Reference Tran ³² Individual-level factors play a crucial role, with age, gender, and pre-existing medical conditions significantly affecting vulnerability.Reference Lyu and Song ³³ People of color, low-income earners, people with respiratory conditions, seniors, young children, pregnant people, outdoor workers, athletes, unhoused individuals, and individuals incarcerated in facilities without air conditioning are especially vulnerable to the impacts of chronic heat exposure. Higher heat index values directly increase injury severity, while secondary complications like collapses, heart attacks, and falls worsen outcomes. ³⁴

Vulnerable populations face compounded risks, with persons with severe mental illness like schizophrenia, which affects close to 24 million, showing three-fold increased mortality odds during extreme heat events due to physiological, behavioral, and social factors.Reference Kirby ³⁵ People experiencing homelessness are also at risk because they spend a longer period of time outdoors and face limited access to resources, such as cooling spaces.Reference Noor ³⁶ Rising temperatures have shown strong correlation with increased mortality in US state and private prisons from 2001 to 2019, with a 5.2 percent increase in total mortality for every 10°F of temperature rise — and higher vulnerability among those 65 years and older, as well as those incarcerated for less than a year. Reference Skarha ³⁷

Heat Exposure in the Workplace

Heat Exposure Outside the Workplace

State and Local Protections from Extreme Heat

States and cities across the nation are tailoring policies to protect individuals from the health impacts of extreme heat, including policies to protect at-risk individuals such as the elderly, youth exposed to heat during sports, and individuals exposed to extreme heat on the job. At the same time, communities are recognizing the need to ensure individuals have access to cooling equipment at home, and cities have taken steps to enact ordinances to set maximum temperatures for rental units. While there are a wide range of policies aimed at alleviating harm from heat in a variety of different settings, this section will focus first on policies to prevent heat sickness and death at work, and then on policies aimed at protecting individuals from extreme heat at home.

Florida

Central Florida experienced unprecedented heat during the summer of 2025, breaking several long-standing temperature records. Throughout July, heat index values soared between 110°F and 120°F, creating dangerous conditions for residents. Since 2007, there have been fourteen instances of heat or excessive heat events in the region, highlighting a concerning upward trend. On June 23, Tampa reached 100°F for the first time ever. A month later, on July 29, Sanford also hit 100°F, surpassing its previous record of 98°F set in 2010. On that same day, Leesburg also recorded 100°F for the second consecutive day, a record not seen in fifty-seven years. Authorities issued rare Extreme Heat Warnings for parts of Central Florida, an unusual and alarming measure signaling that conditions could become life-threatening without proper precautions. In 2025, HCA Florida Brandon Hospital reported an increase in emergency room visits for heat exhaustion, primarily among landscapers and roofers.Reference Bowen ⁷⁵ Similar observations were reported by Orlando Health Orlando Regional Medical Center, with heat exhaustion being more prevalent among those who were working or exercising outdoors. ⁷⁶

Efforts to establish heat protection measures for outdoor workers in Florida have repeatedly failed over the past several years. In 2022, Senate Bill (SB) 732 was indefinitely postponed and died in the Senate Health Policy Committee, halting early attempts to address heat exposure. ⁷⁷ The following year, in 2023, SB 706 also died in the Commerce & Tourism Committee despite renewed attention after a farmworker’s death in South Florida highlighted the urgent need for stronger worker protections. ⁷⁸ In 2024, worker protections proposed in SB 762 / House Bill (HB) 433 were never enacted. ⁷⁹ Instead, the bill was amended to prohibit local political subdivisions from requiring employers to provide heat protections unless those protections were required by state or federal law. When Governor DeSantis signed HB 433 into law it contained zero protections for workers from extreme heat, and preempted localities from requiring employers to do so. ⁸⁰ Most recently, in 2025, SB 510 / HB 35 died in committee in June, marking yet another missed opportunity to establish meaningful safeguards against heat illness. ⁸¹ Previously, a Tampa Bay Times (2024) article reported that twice as many workers have died across the state of Florida from heat than officials knew.Reference Critchfield and Chavez ⁸²

In Florida, several organizations have launched programs to educate communities about extreme heat risks and prevention. Worker-focused campaigns like ¡Qué Calor! led by WeCount! advocated for life-saving heat protection for outdoor workers in Florida. ⁸³ Health-focused organizations like Florida Clinicians for Climate Action (FCCA) train clinicians on the health impacts of extreme heat. ⁸⁴ The CLEO Institute, an organization building climate resiliency with science based education through collaborations with local partners, trained community members and advocated for extreme-heat-season preparedness. ⁸⁵ These efforts demonstrate a multilevel approach to heat education to raise public awareness, reduce risks, and inform initiatives to protect the public.

New York City

New York City residents have been working to develop policy plans designed to implement protective measures that reduce the health and economic risks associated with extreme heat, and which other jurisdictions experiencing extreme heat could replicate. WE ACT for Environmental Justice (WE ACT) represents the communities of Harlem, Inwood, and Washington Heights in New York City — including over 1,100 members invested in uplifting environmental and climate health for neighborhoods confronting generations of harmful policy decisions. WE ACT’s members are fighting to have their concerns addressed in the City’s climate adaptation planning. The state’s average temperature has warmed 0.6°F degrees per decade since the 1970s.Reference Rosenzweig ⁸⁶ This change is felt more dramatically in New York City. Nearly half the population live in census tracts designated as Disadvantaged Communities (DACs), which means residents face disproportionate environmental burdens, climate change risks, and social and health vulnerabilities that can contribute to more severe adverse effects of climate. ⁸⁷

New York City is one of the nation’s most intense urban heat islands, because its thermal signature is drastically hotter than surrounding areas with more natural landcovers.Reference Declet-Barreto ⁸⁸ Over 7.2 million New York City residents are exposed to temperatures 8°F hotter than surrounding areas due to this phenomenon. ⁸⁹ Notoriously a concrete jungle, the high density of asphalt, glass, concrete, and steel trap heat and leave little room for trees and bodies of water. Trees are one of the most effective nature-based solutions to mitigate the urban heat island effect because they lower temperatures using shade, evapotranspiration, and albedo. Air pollution from traffic and buildings heightens respiratory stress, making it harder to cool down. The New York State Department of Environmental Conservation developed the draft Hottest Hour Map, showing that the heat index routinely exceeds 100 degrees throughout the city. ⁹⁰

Buildings absorb and dissipate heat slowly over long periods of time.Reference Ramamurthy and Bou-Zeid ⁹¹ While the Urban Heat Island effect’s relationship with ambient temperature is well understood, there is comparatively little research about how it impacts indoor temperatures and human health. In 2016, WE ACT collaborated with researchers at Columbia University to conduct the Harlem Heat study. The project used data from temperature and humidity sensors in 30 residences of Northern Manhattan, with variable reporting periods (depending on recruitment and battery life) from early July through late September. The results indicate that indoor temperatures are often elevated above ambient temperatures and remain hotter even at night. Additionally, because of a building’s thermal inertia, residents experience heatwaves indoors on a time scale distinct from outdoor heatwaves, experiencing peak temperatures up to a day later. The study shows that indoor temperatures maintain this pattern, especially in un-air-conditioned apartments.Reference Vant-Hull ⁹²

The Harlem Heat Study offers a glimpse into the building science that informs the gravest impacts of heat health disparities in New York City. Neighborhoods like Harlem, with larger Black and Latino populations, tend to score high on the Heat Vulnerability Index. This means that they face a higher risk of community-level harm due to lower canopy cover, hotter temperatures, lower area median income, and lower percentages of air conditioner ownership. ⁹³ This pattern is a consequence of systemic disinvestment resulting from redlining, a practice by the Home Owner’s Loan Corporation that marked neighborhoods with large populations of Black and immigrant communities as “high risk” for loans, effectively restricting their access to adequate funding for building maintenance, economic development, and high-quality green spaces.Reference Nardone ⁹⁴ Today, the built environment in Harlem and other formerly redlined neighborhoods contributes to a variety of public health vulnerabilities that exacerbate heat stress. For example, childhood asthma rates are the highest in low-income neighborhoods in the Bronx and northern Manhattan, where people are routinely exposed to household hazards — such as mold, cockroaches, and rats — that can trigger asthma attacks.Reference Walters ⁹⁵ Hot, humid weather both increases the risk of exposure to pollutants and tightens the airways, making breathing a laborious task for people living with asthma. ⁹⁶ These structural problems represent an area for significant investment.

However, the most immediate means available to address the impact of heat stress is through mechanical cooling. The presence of a functioning air conditioner in the summer months has been proven to be lifesaving. ⁹⁷ According to the New York City Department of Mental Health and Hygiene’s 2025 Heat Mortality Report, none of the 525 heat-stress decedents who died at home used air conditioning. Forty-seven percent had no air conditioning at home while the other 53 percent had air conditioners that were not in use or not functional. ⁹⁸

Eliminating barriers to cooling is a matter of public health and racial justice. All heat deaths are preventable, yet Black residents in New York City die from heat-exacerbated deaths at twice the rate of any other race. ⁹⁹ Data from the New York City Housing and Vacancy Survey shows the citywide average for air conditioning access is approximately 90 percent, but the lowest income areas have access rates falling into the low 70s. ¹⁰⁰ Per the Climate Leadership and Community Protection Act of 2019 and Local Law 64 of 2017, New York City has claimed equitable climate adaptation and investment as a legislative priority. ¹⁰¹ In keeping with this commitment, the Mayor’s Office of Climate and Environmental Justice listed the development of an indoor maximum temperature standard as a goal in their PlaNYC Sustainability Report. Introduction 994-2024 was heard at a City Council Stated Meeting a year later, a bill that would require landlords to ensure all tenants in buildings without central cooling systems are provided with mechanical cooling devices. ¹⁰² The language of the bill stipulates that when outdoor temperatures reach 82°F at 50 percent relative humidity, landlords are required to provide cooling and dehumidifying devices that can maintain 78°F no later than 4 years after the bill is enacted into law. Landlords are responsible for ensuring that the devices, whether purchased by themselves or their tenants, are installed and maintained properly. The bill is held over in the Housing Committee at the time of writing.

Legislation of this kind is a critical component of heat resilience. Recent reports from the New York City Panel on Climate Change (NPCC) have highlighted an alarming trend that heat mortality rates are increasing primarily at ambient temperatures between 82–90°F, below the heat advisory threshold.Reference Matte ¹⁰³ New Yorkers with cardiovascular disease represented 57 percent of heat decedents who had detailed medical examiner records. ¹⁰⁴ Beyond heat mortality, sleep deprivation lowers immune system function,Reference Mehta ¹⁰⁵ compromises mental health and stress management, and reduces cognitive and physical function.Reference Baniassadi ¹⁰⁶ Overall, cooling at home is conducive to survival and safer interactions with other people and public settings.

However, this approach needs policy changes beyond the scope of city government to reach its full potential to protect residents. The cost of cooling remains a significant deterrent for low-income tenants to cool their homes. At least one in four US households experiences some form of energy insecurity, meaning they are unable “to adequately meet basic household energy needs.”Reference Hernández ¹⁰⁷ The dimension of energy insecurity that can be addressed most rapidly is economic insecurity, otherwise known as energy burden, which is the proportion of energy costs compared to total income. ¹⁰⁸ In order to empower all residents to protect themselves from extreme temperatures and chronic heat exposure, states must prioritize affordability for the lowest-income residents. One strategy to do so is reducing costs of extending and maintaining fossil fuel infrastructure, especially in states like New York that have legally binding mandates to reduce greenhouse gas emissions by 85 percent from 1990 levels by 2050. The New York Home Energy Affordability Act (NY HEAT) was proposed legislation with this objective. If it had passed, it included provisions that would have resulted in substantial savings by ending the ability of utilities to pass-through the cost of extending gas pipeline infrastructure for new developments onto ratepayers state-wide. ¹⁰⁹ For context, the average cost per mile of a gas distribution main installed for New York’s gas utilities is “over $3 million, with a total cost to ratepayers that is closer to $6 million” according to a report by the Building Decarbonization Coalition.Reference Krueger ¹¹⁰ NY HEAT did not pass into law after two legislative cycles. At the time of writing, a concession bill (A.8888/S8417) to remove the requirement in public service law to connect new ratepayers to gas infrastructure at no cost to them if they are within one hundred feet of existing infrastructure has passed the legislature and is awaiting Governor Kathy Hochul’s signature.Reference González ¹¹¹ Additionally, the global emissions trajectory could change because the current US presidential administration is actively reducing investment in renewable energy growth in favor of fossil fuels. Still, without an affordability guarantee, people will be forced to cope with increasingly fatal outcomes.