Extreme Heat and Health: Recognizing Heat Illness and Staying Safe

Why Is Extreme Heat So Dangerous?

The human body maintains a core temperature near 98.6°F (37°C) through thermoregulatory mechanisms including sweating (evaporative cooling), vasodilation of skin blood vessels (radiative and convective cooling), and behavioral responses (seeking shade, reducing activity). When ambient temperature exceeds body temperature or humidity prevents effective sweat evaporation, these mechanisms fail and core temperature rises. A core temperature above 104°F (40°C) constitutes heat stroke, a life-threatening medical emergency with mortality rates of 10–65% depending on the speed of cooling and treatment.

The 2023 heat season was the deadliest on record in the United States, with the CDC documenting over 1,670 heat-related deaths—a figure that public health experts consider a significant undercount because heat deaths are often attributed to the underlying conditions they exacerbate, such as cardiovascular disease and renal failure. Globally, the Lancet Countdown on Health and Climate Change estimated that heat-related mortality in adults over 65 increased by 85% between 2000–2004 and 2018–2022.

Climate change is intensifying heat waves in frequency, duration, and severity. According to NOAA and NASA, the period from 2014–2023 represents the warmest decade in recorded history. The number of days exceeding dangerous heat index thresholds has increased significantly across the United States, particularly in the South, Southwest, and Pacific Northwest. Urban areas face additional risk from the "heat island" effect, where concrete, asphalt, and building density cause temperatures to be 1–12°F higher than surrounding rural areas, with the greatest differential occurring at night when cooling is most critical for recovery.

What Is the Difference Between Heat Exhaustion and Heat Stroke?

Heat-related illnesses exist on a spectrum. Heat cramps are painful muscle spasms typically in the legs or abdomen, caused by electrolyte imbalances from sweating. Heat syncope is fainting or dizziness during prolonged standing or sudden rising in hot environments. Heat exhaustion is a more serious condition characterized by heavy sweating, cool and clammy skin, fast but weak pulse, nausea or vomiting, muscle cramps, tiredness, weakness, dizziness, headache, and possible fainting. Core body temperature may be elevated but remains below 104°F (40°C). Heat exhaustion is reversible with prompt cooling, rest in a cool environment, and oral rehydration.

Heat stroke represents a medical emergency and the most severe form of heat illness. It is characterized by a core body temperature at or above 104°F (40°C) accompanied by central nervous system dysfunction: confusion, altered mental status, slurred speech, irritability, delirium, seizures, or loss of consciousness. Skin may be hot, red, and dry (classic heat stroke) or hot and sweating (exertional heat stroke, common in athletes and outdoor workers). Without rapid cooling, heat stroke causes multi-organ failure including rhabdomyolysis, acute kidney injury, disseminated intravascular coagulation (DIC), hepatic failure, and death.

Immediate treatment for heat stroke involves calling 911 and initiating aggressive cooling: move the person to a cool environment, immerse in ice water if possible (cold water immersion is the gold standard, reducing temperature at approximately 0.20°C per minute), apply ice packs to the neck, armpits, and groin, and fan the person while misting with cool water. Do not give oral fluids if the person has altered mental status. Every minute of delay in cooling increases the risk of permanent organ damage and death. The target is to reduce core temperature to below 102°F (39°C) within 30 minutes of collapse.

Who Is Most Vulnerable to Heat-Related Illness?

Adults aged 65 and older account for a disproportionate share of heat-related deaths due to age-related decline in thermoregulatory function: reduced sweat output, decreased thirst sensation, impaired cardiovascular response to heat stress, and higher prevalence of chronic diseases. Many older adults live alone, lack air conditioning, or take medications that interfere with heat adaptation. Common medications that increase heat vulnerability include diuretics (impair fluid balance), beta-blockers (blunt cardiovascular response), anticholinergics (reduce sweating), and psychotropic medications including antipsychotics and amphetamines.

Outdoor workers in agriculture, construction, landscaping, and delivery services face extreme occupational heat exposure. The Bureau of Labor Statistics reports an average of 36 work-related heat deaths per year in the US (widely considered an undercount). OSHA has proposed a federal heat standard requiring employers to provide water, rest, shade, and acclimatization protocols when the heat index exceeds 80°F. Children under 5 are vulnerable because their body surface area-to-mass ratio is higher, they produce more metabolic heat per unit body mass during physical activity, and their sweating capacity is lower than adults.

Socioeconomic factors create significant disparities in heat vulnerability. Low-income communities and communities of color are disproportionately located in urban heat islands with less green space and tree cover. A 2021 study in Nature Communications found that formerly redlined neighborhoods in 108 US cities are on average 2.6°C (4.7°F) hotter than non-redlined areas. Lack of access to air conditioning, inability to afford electricity costs, and housing quality issues compound the risk. People experiencing homelessness face some of the highest heat exposure with the fewest resources for mitigation.

How Can You Protect Yourself During Extreme Heat?

The most effective protection against heat illness is access to air conditioning. Studies show that air conditioning reduces heat-related mortality by 75–80%. During heat waves, people without home air conditioning should spend time in air-conditioned public spaces such as libraries, malls, and community cooling centers. Electric fans are helpful when temperatures are below body temperature but become ineffective and potentially dangerous above 95°F (35°C) in high humidity, as they can accelerate dehydration without providing effective cooling.

Hydration should be proactive rather than reactive—by the time you feel thirsty, you may already be mildly dehydrated. The general recommendation during heat exposure is to drink water every 15–20 minutes, approximately one cup (8 oz) at a time, even if not thirsty. For prolonged physical activity in heat, electrolyte-containing beverages help replace sodium and potassium lost through sweat. Avoid alcohol, caffeine in excess, and sugary drinks, which can worsen dehydration. Monitor urine color as a hydration indicator: pale yellow indicates adequate hydration, while dark yellow or amber suggests dehydration.

The CDC and NOAA jointly developed the HeatRisk tool (heatrisk.gov), which provides a 7-day heat risk forecast using a color-coded scale from 0 (little or no risk) to 4 (extreme risk for entire population). The tool integrates temperature, humidity, time of year, and duration of heat event to provide location-specific risk assessments, including tailored guidance for different population groups. Additional prevention measures include scheduling outdoor activities for early morning or evening, wearing wide-brimmed hats and light-colored clothing, applying sunscreen (sunburn impairs sweating), never leaving children or pets in parked vehicles (interior temperatures can reach 140°F within minutes), and establishing a buddy system to check on elderly or isolated neighbors during heat waves.