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Extreme heat warning

An extreme heat warning is an official alert issued by meteorological and public health authorities to inform the public of imminent or ongoing periods of dangerously high temperatures and humidity that threaten human health, prompting immediate protective actions to prevent heat-related illnesses and deaths.^[1] These warnings are part of broader heat health early warning systems (HHEWS) designed to reduce morbidity and mortality by raising awareness and enabling timely responses, such as staying hydrated, seeking air-conditioned environments, and limiting outdoor exertion.^[2] In the United States, the National Weather Service (NWS) issues an Extreme Heat Warning—renamed from Excessive Heat Warning in March 2025—when local criteria for extreme danger are met, typically a heat index (a combined measure of temperature and humidity) of at least 105°F (41°C) for more than three hours per day over two consecutive days, or 110°F (43°C) for any duration, though thresholds vary by region to account for acclimatization and vulnerability.^[1]^[3]^[4] The warning is declared 12 to 24 hours in advance of the event or during its occurrence, often alongside an Extreme Heat Watch issued 24 to 72 hours prior when conditions are favorable.^[5] Internationally, similar systems operate under frameworks like those recommended by the World Health Organization, with many countries implementing HHEWS tailored to local climates, such as Europe's Meteoalarm network or Australia's Bureau of Meteorology alerts, which use impact-based thresholds to forecast risks to vulnerable populations including the elderly, children, and outdoor workers.^[2]^[6]^[7] Extreme heat events necessitating these warnings represent the deadliest weather-related hazard globally, causing approximately 489,000 deaths annually between 2000 and 2019, with Asia and Europe accounting for over 80% of cases, often exacerbated by urban heat islands and climate change.^[2] In the US, extreme heat claims an average of 702 lives each year (2004–2018), surpassing fatalities from floods, tornadoes, and hurricanes combined, highlighting the warnings' role in saving lives through public education and emergency preparedness.^[8]^[9] As global temperatures rise, the frequency and intensity of these events are increasing, underscoring the need for enhanced warning systems integrated with climate adaptation strategies.^[2]

Overview

Definition

An extreme heat warning is a severe weather alert issued by meteorological agencies, such as the U.S. National Weather Service (NWS), when conditions are expected to meet local criteria for extreme heat danger, typically involving a heat index (a measure combining temperature and humidity) of at least 105°F (41°C) for more than three hours per day for two or more consecutive days, or a heat index of 110–115°F (43–46°C) for any duration, though exact thresholds vary by region to reflect local acclimatization and vulnerabilities.^[1]^[3] This alert signifies the highest level of heat danger, surpassing heat advisories and watches, and is designed to communicate the urgent need for protective measures during periods of prolonged or intense high temperatures combined with humidity.^[1] The primary purpose of an extreme heat warning is to alert the public to conditions that can cause heat-related illnesses, such as heat exhaustion and heatstroke, and potentially lead to fatalities, thereby encouraging immediate actions like seeking air-conditioned spaces and limiting outdoor exertion.^[1] By providing timely notifications, these warnings aim to reduce health impacts in communities exposed to such hazards, particularly during heat waves exacerbated by climate change.^[10] Key components of an extreme heat warning include specifications on the expected duration and geographic areas affected, along with basic safety recommendations integrated into the alert message, such as staying hydrated and avoiding strenuous activities.^[1] These alerts are disseminated through official channels like the Emergency Alert System and local media to ensure broad reach.^[1] On March 4, 2025, the NWS renamed the previous "Excessive Heat Warning" to "Extreme Heat Warning" as part of its Hazard Simplification Project, following evaluations including a 2018 public survey that highlighted the need for terminology emphasizing the severity of heat risks to improve public comprehension and response.^[10]

History and Evolution

The origins of extreme heat warnings in the United States date to the early 20th century, when devastating heat waves exposed the need for public alerts amid limited forecasting capabilities. The 1936 North American heat wave, exacerbated by drought and poor soil management, caused over 5,000 deaths across the Midwest and Plains states, prompting the U.S. Weather Bureau—the NWS predecessor—to issue initial informal heat forecasts and advisories through newspapers and local broadcasts to urge precautions like seeking shade and hydration.^[11]^[12] Key milestones in the formalization of these warnings emerged in the late 20th century. The NWS adopted the Heat Index in 1979, a metric developed by physicist Robert G. Steadman to quantify the physiological stress from combined high temperatures and humidity, providing a scientific basis for heat risk assessment.^[13] Building on this, the NWS introduced standardized Excessive Heat Warnings in the 1990s, following mid-decade research that refined criteria to account for regional vulnerabilities and nighttime temperatures, marking a shift from ad hoc alerts to proactive, nationwide protocols.^[14] By the 2000s, these systems expanded with the integration into broader severe weather frameworks, including the rollout of Heat-Health Warning Systems in over 20 U.S. metropolitan areas by 2007 to link meteorological data with public health coordination.^[15] Parallel developments occurred globally, particularly in Europe after the 2003 heat wave that killed about 70,000 people, primarily from heat-related illnesses in urban areas.^[16] This catastrophe accelerated the creation of early warning systems, such as the UK's Heat-Health Alert framework launched in 2004, which uses a tiered alert structure (yellow, amber, red) to trigger targeted interventions like cooling centers and health service escalations.^[17]

Issuance Process

Criteria for Issuance

The National Weather Service (NWS) issues an Extreme Heat Warning (formerly known as an Excessive Heat Warning)—commonly referred to as an extreme heat warning—when forecast conditions meet specific thresholds indicating dangerous heat stress, primarily based on the heat index, which accounts for the combined effects of temperature and humidity on the human body. In March 2025, the NWS renamed the Excessive Heat Warning to Extreme Heat Warning to enhance clarity in public messaging.^[10] The standard national criteria require a heat index of at least 105°F (41°C) for more than three hours per day over at least two consecutive days, or a heat index of 115°F (46°C) or higher at any time, with warnings typically issued when these conditions are expected within 12 to 24 hours. These thresholds are designed to signal widespread risk of heat-related illnesses, and local NWS offices may adjust them based on regional climate acclimation and historical impacts.^[18]^[19] The heat index is calculated using a formula developed by the NWS to estimate apparent temperature, incorporating air temperature (T in °F) and relative humidity (RH in percent):

\text{HI} = -42.379 + 2.04901523T + 10.14333127\text{RH} - 0.22475541T \cdot \text{RH} - 6.83783 \times 10^{-3}T^2 - 5.481717 \times 10^{-2}\text{RH}^2 + 1.22874 \times 10^{-3}T^2 \cdot \text{RH} + 8.5282 \times 10^{-4}T \cdot \text{RH}^2 - 1.99 \times 10^{-6}T^2 \cdot \text{RH}^2

This equation, derived from a multi-regression model of human heat perception studies, applies primarily to shaded conditions at low wind speeds and is adjusted for direct sunlight or higher winds in operational use. For temperatures below 80°F or humidity above 85%, simplified adjustments are applied to avoid overestimation.^[20]^[21] Regional NWS offices tailor criteria to local conditions, recognizing that acclimation and environmental factors influence heat vulnerability. In the arid Southwest deserts, such as around Phoenix, Arizona, where low humidity reduces the heat index's relevance, warnings often focus on dry-bulb air temperature thresholds, such as daytime highs of 110°F (43°C) or more for at least two consecutive days, supplemented by the HeatRisk tool that integrates temperature, duration, and overnight recovery. In contrast, the humid Southeast, including areas like Florida and Georgia, typically maintains or slightly elevates heat index thresholds—such as 108–112°F (42–44°C) for warnings—due to higher population acclimation to steamy conditions, ensuring alerts align with elevated health risks from muggier air. These variations are informed by ongoing assessments of local heat event outcomes.^[22]^[23]^[24] The issuance process begins with numerical weather prediction models, including the North American Mesoscale (NAM) model, which provides high-resolution forecasts of temperature and humidity up to 84 hours ahead, combined with global models like the Global Forecast System (GFS) for longer-range guidance. Forecasters at local NWS offices analyze these outputs alongside observational data from weather stations and satellites to project heat index values, then coordinate with state and local emergency management agencies to confirm impacts and public response readiness. Warnings are issued with a lead time of 24 to 48 hours when confidence exceeds 80%, allowing time for protective actions.^[25]^[26] Post-issuance, the NWS conducts verification through event reviews and service assessments, comparing forecasted conditions against observed data to evaluate alert accuracy and refine criteria based on actual health, infrastructure, and environmental impacts. For instance, analyses of heat alerts from 2010–2019 have led to updates like the 2021 HeatRisk tool, which better incorporates acclimation and vulnerability factors to improve future thresholds.^[14]^[27] Extreme heat warnings differ from heat advisories primarily in severity and immediacy of threat. A heat advisory is issued when conditions pose a significant but not life-threatening risk, typically for heat index values between 100°F and 104°F (38°C to 40°C) persisting over two or more days, or between 105°F and 109°F (41°C to 43°C) for at least three hours in a single day, emphasizing caution and preventive measures like hydration and limiting outdoor exertion.^[1]^[28] In contrast, an extreme heat warning signals imminent or ongoing extremely dangerous conditions, with heat index values meeting or exceeding 105°F (41°C) for more than three hours per day over two consecutive days, or 115°F (46°C) or higher at any time, where prolonged exposure can lead to severe heat-related illnesses requiring urgent protective actions such as avoiding all outdoor activities.^[1]^[10] Unlike an excessive heat outlook, which provides a probabilistic long-range forecast of potential extreme heat events 3 to 7 days in advance to allow for planning, an extreme heat warning is a short-term alert issued within 12 to 24 hours of the onset, focusing on immediate response rather than anticipation.^[5]^[29] Extreme heat warnings also stand apart from other weather warnings, such as those for tornadoes or severe storms, as they address non-convective, sustained thermal stress over hours or days rather than sudden, convective events demanding rapid evacuation or sheltering.^[1] No evacuation is typically required for heat warnings; instead, responses center on staying indoors, using cooling centers, and monitoring vulnerable individuals.^[30] Within alert systems, heat advisories can escalate to extreme heat warnings if forecasted conditions intensify, creating a tiered framework for escalating urgency.^[1] Internationally, equivalents like Australia's extreme heat warnings under the Australian Warning System employ similar tiering—ranging from advice to emergency levels—to communicate escalating risks based on heat severity and duration.^[31]^[32]

Health and Environmental Risks

Human Health Dangers

Extreme heat warnings signal conditions that pose severe risks to human health, primarily through heat-related illnesses that overwhelm the body's ability to regulate temperature. These dangers arise when ambient temperatures, often combined with high humidity, exceed the physiological limits of thermoregulation, leading to a cascade of acute effects. The most critical conditions include heat exhaustion and heat stroke, which can progress rapidly if not addressed.^[33] Heat stroke represents the most severe form of heat-related illness, characterized by a core body temperature exceeding 104°F (40°C), resulting from the failure of the body's cooling mechanisms. Symptoms include confusion, seizures, and potential death if untreated, as the elevated temperature causes cellular damage and multi-organ dysfunction. Heat exhaustion, a precursor condition, manifests with symptoms such as dizziness, nausea, and a rapid pulse, often due to excessive fluid and electrolyte loss from sweating. These illnesses impair thermoregulation, where the hypothalamus normally maintains core temperature through sweating and vasodilation; under extreme heat, this process falters, leading to hyperthermia. Dehydration exacerbates the issue by reducing blood volume, which can precipitate organ failure, while high humidity hinders sweat evaporation—the primary method of heat dissipation—trapping moisture on the skin and intensifying heat retention.^[34]^[33]^[35]^[36]^[37]^[38] In the short term, extreme heat imposes significant cardiovascular strain by increasing heart rate and blood pressure to support skin blood flow for cooling, which can trigger heart attacks, arrhythmias, or strokes in susceptible individuals.^[39] Respiratory issues also arise, as heat stress worsens breathing difficulties and exacerbates conditions like asthma through dehydration of airways and reduced lung function. In the United States, extreme heat contributes to approximately 1,220 deaths annually, based on underlying or contributing causes listed on death certificates.^[40] In 2023, the US saw a record high of more than 2,300 heat-related deaths, reflecting increasing frequency.^[41] Globally, heat-related deaths are estimated at around 489,000 per year (2000–2019), underscoring the widespread immediate threat during extreme heat events.^[2]

Impacts on Vulnerable Populations

Extreme heat warnings pose heightened risks to specific demographic and socioeconomic groups due to physiological vulnerabilities and limited access to protective resources. Older adults, particularly those aged 65 and above, experience reduced sweating efficiency and impaired thermoregulation, making them less able to dissipate heat during prolonged exposure.^[42] Children and infants, with their higher metabolic rates and lower body surface area relative to volume, generate and retain more internal heat, increasing susceptibility to dehydration and heat exhaustion.^[43] Outdoor workers, such as agricultural laborers and construction crews, face prolonged direct exposure to intense sunlight and high temperatures, often without adequate breaks or hydration, elevating risks of heat stroke and occupational injuries.^[44] Similarly, individuals experiencing homelessness encounter barriers like limited access to air-conditioned spaces and potable water, exacerbating heat-related illnesses during warnings.^[45] Socioeconomic factors further amplify these disparities, as low-income communities often reside in poorly insulated housing without air conditioning, trapping heat and prolonging exposure. Urban heat islands, where concrete and asphalt absorb and radiate heat, can make city neighborhoods up to 10°F (5.5°C) warmer than surrounding rural areas, disproportionately affecting densely populated, lower-income urban zones.^[46] These environmental inequities compound vulnerabilities for marginalized groups, including racial and ethnic minorities. In the United States, studies indicate that Black and Hispanic populations face 2-3 times higher heat-related mortality rates compared to white populations, driven by factors like residential segregation and occupational exposure.^[47] Globally, extreme heat exposure affects billions, with the World Bank estimating 1.2 billion people at high risk from climate hazards including heatwaves as of 2024.^[48] Indirect consequences include mental health strain, such as heightened anxiety and sleep disruption from heat stress, which can worsen existing conditions.^[49] During the 2023 U.S. heat waves, heat-related emergency calls surged by approximately 30% in affected areas like Phoenix, overwhelming response systems.^[50]

Mitigation and Response

Preparation Strategies

Individuals preparing for extreme heat warnings should prioritize hydration by drinking plenty of fluids throughout the day, even if not feeling thirsty, while avoiding alcohol, caffeine, and high-sugar beverages that can exacerbate dehydration.^[51] Wearing lightweight, loose-fitting, light-colored clothing helps reflect heat and promote air circulation around the body.^[51] To minimize exposure, individuals can schedule outdoor activities or strenuous tasks for cooler times, such as early morning or late evening, and seek air-conditioned environments like homes or public cooling centers whenever possible.^[52] Home preparation involves simple modifications to reduce indoor temperatures, such as installing window shades, reflectors, or drapes to block sunlight, weather-stripping doors and windows for better insulation, and using fans or attic ventilators to circulate air.^[51] Residents should also check on neighbors, especially older adults and those with chronic conditions, to ensure their well-being.^[52] Stocking non-perishable foods, at least one gallon of water per person per day, and other essentials in an emergency kit prepares for potential power outages that could disable cooling systems.^[53] At the community level, schools and workplaces can develop heat safety plans that include regular hydration breaks, shaded rest areas, and acclimatization schedules allowing new or returning workers to gradually increase exposure over 7-14 days.^[54] The World Health Organization and World Meteorological Organization's 2025 guidance emphasizes protections for outdoor workers, including rest breaks and hydration to mitigate heat stress risks.^[55] Public awareness efforts, such as the National Weather Service's heat safety resources and OSHA's Heat Illness Prevention Campaign, educate residents on proactive steps like identifying local cooling centers and monitoring vulnerable groups.^[52]^[56] Useful tools for preparation include mobile applications like the FEMA App, which delivers real-time weather alerts and emergency notifications, enabling timely adjustments to plans.^[53] The NOAA/CDC HeatRisk tool, available since 2023 and updated ongoing, provides location-specific heat risk forecasts to help individuals assess daily threats and plan accordingly.^[57] The CDC's Heat and Health Initiative, launched in 2024, promotes actions to stay cool, hydrated, and aware of symptoms.^[58] Emergency kits should incorporate electrolyte replacement packets or sports drinks to support hydration during prolonged heat, alongside basic supplies like water and non-perishables.^[54] The U.S. National Heat Strategy (2024-2030) outlines broader efforts to build heat resilience through planning and capacity building.^[59]

Response Measures

During an extreme heat warning, individuals are advised to limit strenuous outdoor activities, particularly between noon and 4 p.m., when temperatures peak, to reduce the risk of heat-related illnesses.^[52] Seeking air-conditioned environments, such as homes, malls, or libraries, is recommended to stay cool, while staying hydrated with water and avoiding alcohol or caffeine helps maintain body temperature regulation.^[51] People should monitor for symptoms of heat exhaustion or stroke, including dizziness, nausea, rapid heartbeat, or confusion, and call emergency services immediately if severe signs appear; for heat stroke, first aid includes moving the person to a cool area, applying cool wet cloths to the skin, and fanning to promote evaporation until professional help arrives.^[60]^[33] Local governments activate cooling centers—such as community centers, libraries, and recreation facilities—during extreme heat warnings to provide free, accessible relief from high temperatures, often extending hours and offering water and snacks.^[61] Hospitals prepare for potential surges in heat-related cases by stocking intravenous fluids for dehydration treatment, implementing rapid cooling protocols like cold saline infusions and ice packs, and coordinating with emergency services to manage increased patient loads.^[62] Following the event, authorities conduct damage assessments for heat-stressed infrastructure, including power grids that may experience overloads or failures from heightened demand for air conditioning, leading to outages that exacerbate risks.^[63] Public health agencies perform surveillance to track spikes in heat-related illnesses, using systems like emergency department data and syndromic reporting to evaluate impacts and inform future responses.^[64] For multi-day events, response coordination integrates federal support through the Federal Emergency Management Agency (FEMA), which provides technical assistance and resources to states and localities, though full disaster declarations for heat events remain rare due to limited infrastructure damage criteria.^[65] In the 2021 Pacific Northwest heat dome, state emergency management offices led coordination with local health departments and cooling center activations, highlighting the need for enhanced federal integration in prolonged heat crises.^[66]

Notable Examples

Historical Events

The 1936 North American heat wave, one of the deadliest weather events in United States history, affected much of the Midwest and Northeast from late June through August, with temperatures exceeding 100°F (38°C) in many areas for weeks amid severe drought conditions. This prolonged episode resulted in approximately 5,000 deaths nationwide, primarily from heatstroke and related illnesses, overwhelming public health systems and infrastructure at the time.^[67]^[11] The event underscored the lethal potential of extreme heat in urban and rural settings, marking a pivotal moment in public awareness of heat-related risks during an era of limited meteorological tools. The 2003 European heat wave, spanning June to August, brought record-breaking temperatures across the continent, with France experiencing daytime highs up to 40°C (104°F) for extended periods. It caused an estimated 70,000 excess deaths, disproportionately affecting older adults and those in urban areas due to poor ventilation and high humidity.^[2] This catastrophe exposed gaps in preparedness, including inadequate early warning mechanisms, and directly spurred the development of heat-health warning systems throughout the European Union.^[68] In the summer of 2010, western Russia endured an unprecedented heat wave from late June to mid-August, with Moscow recording average temperatures 7–10°C above normal and peaks over 38°C (100°F). The event led to about 55,000 excess deaths, many in urban centers like Moscow where air quality deteriorated from widespread wildfires that burned over 500 sites and released massive smoke plumes.^[69]^[70] It highlighted urban vulnerabilities, such as heat-trapping concrete environments and strained emergency services, exacerbating mortality from cardiovascular and respiratory issues amid the smoke and heat. These historical events catalyzed key advancements in heat management. Following the 1930s heat waves, including 1936, meteorological agencies improved forecasting capabilities through better observational networks, such as the introduction of radiosondes for upper-air data, enabling more reliable predictions of prolonged hot spells.^[71] The 2003 European crisis prompted policy reforms, notably France's establishment of the National Heatwave Plan (Plan National Canicule) in 2004, which integrated early warnings, public alerts, and healthcare coordination to mitigate future impacts.^[72] These responses emphasized proactive alerting and vulnerability reduction, shaping modern extreme heat warning frameworks.

Recent Incidents

In June 2021, the Pacific Northwest experienced a severe heat dome that brought temperatures exceeding 110°F (43°C) across parts of the United States and Canada, marking an early precursor to formalized extreme heat warning systems.^[73] The event shattered numerous all-time records, with locations like Portland, Oregon, reaching 112°F (44°C) and Vancouver, British Columbia, hitting 111°F (44°C), prompting the National Weather Service (NWS) to issue widespread heat warnings emphasizing the life-threatening conditions.^[74] It resulted in over 600 deaths across the region, including more than 250 in the U.S. and 619 in British Columbia alone, highlighting the dangers of prolonged extreme heat in typically temperate areas.^[75]^[76] The 2023 heat wave in the U.S. Southwest saw extreme heat warnings issued for Phoenix, Arizona, where sustained temperatures around 113°F (45°C) fueled record-breaking streaks of 110°F (43°C) or higher for 31 consecutive days.^[77] This event strained infrastructure, driving unprecedented energy demands as air conditioning usage peaked, with Arizona's grid operator reporting all-time highs in electricity consumption to meet cooling needs.^[78] NWS alerts urged residents to limit outdoor activities during peak heat, contributing to the year's total of 54 days above 110°F in Phoenix.^[79] In late June 2025, a widespread heat wave affected much of the central and eastern United States, impacting over 100 million people with record-setting temperatures and prompting extreme heat warnings across multiple states. Highs exceeded 100°F (38°C) in areas from the Midwest to the Northeast, with cities like Chicago and New York issuing alerts for heat indices over 105°F (41°C), leading to increased emergency visits and highlighting ongoing climate-driven risks.^[80] On May 13, 2025, the NWS issued an extreme heat warning for San Antonio, Texas, forecasting highs of 103°F to 106°F (39°C to 41°C) amid an early-season heat wave that broke monthly records.^[81]^[82] The advisory stressed hydration, rest in cooled areas, and avoidance of strenuous activity, with actual highs reaching 103°F (39°C) initially and climbing to 106°F (41°C) by mid-week.^[83] No major fatalities were reported, thanks in part to proactive public health measures.^[84] Overall, extreme heat warnings have become more frequent since 2000, with NOAA data indicating roughly twice as many heat events requiring alerts compared to earlier decades, driven by rising baseline temperatures.^[85]

Global Variations

International Standards

International standards for extreme heat warnings vary significantly across regions, reflecting differences in climate, infrastructure, and health vulnerabilities. In Europe, particularly the United Kingdom, the Heat-Health Alert (HHA) system, operated jointly by the UK Health Security Agency (UKHSA) and the Met Office, employs a tiered alerting framework based on forecasted maximum daytime and minimum nighttime temperatures. Alerts are issued from June to September, with thresholds tailored to regional climates: for London, a yellow alert triggers at daytime temperatures of 28.0–31.9°C or nighttime temperatures at or above the 95th percentile of historical summer minima; amber alerts at 32.0–39.9°C daytime; and red alerts at 40.0°C or higher daytime. For other regions, thresholds are slightly lower, with yellow at 27.0–29.9°C daytime, amber at 30.0–37.0°C, and red at 38.0°C or above. This impact-based system emphasizes coordination across health and social care sectors, with red alerts signaling risks to life for the general population and requiring emergency responses.^[86] In Australia, the Bureau of Meteorology's heatwave warning system categorizes events as low, severe, or extreme using the Excess Heat Factor (EHF), a metric that compares forecasted temperatures to the 95th percentile of historical distributions for the location and time of year, integrated over at least three consecutive days. Warnings for severe or extreme heatwaves are issued up to four days in advance when temperatures exceed these percentiles across at least 10% of a weather district, highlighting heightened risks to health and infrastructure, including elevated bushfire danger through linkage with fire weather forecasts. This percentile-based approach accounts for local climatology, ensuring relevance in diverse environments from urban centers to arid interiors.^[87] India's India Meteorological Department (IMD) issues heat wave warnings based on absolute temperature thresholds adjusted for regional norms, declaring a heat wave in plains regions when the maximum temperature reaches or exceeds 40°C, accompanied by a departure of at least 4.5°C from the long-term average; severe conditions apply above 47°C. In coastal areas, warnings are triggered when the maximum temperature reaches 37°C or more with a departure of at least 4.5°C from the normal maximum temperature, accounting for high humidity's role in exacerbating heat stress; these alerts often emphasize impacts on rural agriculture, such as crop wilting and livestock losses. The system requires criteria to be met at two or more stations in a meteorological subdivision for at least two consecutive days.^[88] Key differences in these systems include variations in lead times and terminology, with developing nations like India often providing shorter forecast horizons—typically 1–2 days—due to forecasting challenges, compared to 3–5 days in developed regions like the UK and Australia. The World Health Organization (WHO) advocates for universal guidelines to standardize responses, recommending thresholds like a wet-bulb globe temperature (WBGT) exceeding 35°C as a point beyond which sustained human activity becomes untenable without intervention, serving as a global benchmark for unlivable humid heat conditions despite national adaptations.^[89]

Climate Change Implications

Climate change, driven primarily by human activities such as greenhouse gas emissions, has unequivocally warmed the planet, with global surface temperatures having risen by approximately 1.2°C above pre-industrial levels (1850–1900) as of 2024.^[90] This warming has significantly increased the frequency and intensity of heatwaves, making extreme heat events more likely and severe than in a pre-industrial climate. According to the Intergovernmental Panel on Climate Change (IPCC), human influence has warmed the climate at an unprecedented rate, leading to more frequent compound extreme events, including heatwaves that now occur with higher probability due to elevated baseline temperatures.^[91] Projections indicate that these trends will intensify without substantial mitigation. In the United States, the number of extreme heat days—defined as those exceeding the 99th percentile of historical temperatures—could more than double or triple in many regions by 2050 under moderate emissions scenarios, according to analyses from the National Oceanic and Atmospheric Administration (NOAA). Globally, particularly in the tropics, rising temperatures combined with humidity are pushing wet-bulb temperatures—the metric combining heat and moisture that determines human survivability—closer to critical thresholds. Studies project that under continued warming, parts of the humid tropics could regularly approach or exceed 31–35°C wet-bulb temperatures, beyond which humans cannot effectively cool through sweating, posing lethal risks during prolonged exposure.^[92] The year 2024 exemplified this escalation, marking the warmest on record globally with temperatures exceeding 1.5°C above pre-industrial levels for the first time in a full calendar year, resulting in unprecedented heat stress across Europe and increased issuance of heat warnings.^[93] Addressing these implications requires integrating extreme heat adaptation into international frameworks like the Paris Agreement, which emphasizes limiting warming to well below 2°C while enhancing resilience through Nationally Determined Contributions (NDCs). The Agreement's Global Goal on Adaptation supports strategies to bolster heat vulnerability in vulnerable regions, potentially averting dozens of additional extreme hot days per year if emissions pathways align with 1.5–2°C targets.^[94] A key recommended measure is urban greening, such as expanding tree canopies and green roofs, which can mitigate urban heat island effects by 0.5–2°C through shading and evaporative cooling, thereby reducing the need for heat warnings in densely populated areas. These adaptations, when scaled, could significantly offset the health and economic burdens of escalating heat extremes.

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When the heat index reaches 105° F, or higher, conditions can become dangerous for both people and pets. Another tool for indication of potential heat stress is ...<|control11|><|separator|>
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Heatstroke is a severe heat-related illness involving an elevation in body temperature, typically but not always greater than 40 C.
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practice heat safety at work, home, in your vehicle and outdoors · pregnant women, newborns, children, elderly, those with chronic illnesses are all especially.Understanding Heat Alerts · Heat Forecast Tools · Heat Cramps, Exhaustion, StrokeMissing: strategies | Show results with:strategies
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