Extreme Heat

Heatwaves typically result from persistent high-pressure systems aloft — sometimes called heat domes — that compress and warm descending air, suppress cloud formation, and persist for days to weeks. The 2021 Pacific Northwest event was driven by an exceptionally strong heat dome paired with anomalously warm Pacific sea-surface temperatures and dry soils, producing all-time record temperatures in Portland (116°F), Seattle (108°F), and Lytton, BC (121°F).

Other contributors to heat exposure:

• Urban heat island: cities are often 2–10°F warmer than surrounding rural areas, with the largest differences at night when concrete and asphalt re-radiate stored heat. Within a city, low-income neighborhoods often have less tree canopy and run several degrees hotter than wealthier neighborhoods.
• Humidity: high dew points reduce the human body's ability to cool through sweat evaporation. Wet-bulb temperatures above about 95°F (35°C) approach the theoretical limit at which a healthy adult cannot survive prolonged exposure without external cooling, regardless of fitness.
• Climate change: NOAA, the IPCC, and peer-reviewed attribution studies have shown that recent extreme heat events would have been essentially impossible without anthropogenic warming. The frequency of dangerous heat days is increasing in nearly every US region.

The CDC, NOAA, and EPA collaboratively maintain the National Integrated Heat Health Information System (NIHHIS) to support state and local heat-health planning.

Most US extreme-heat events occur June through September, but seasonality varies. The Desert Southwest (Phoenix, Las Vegas, Palm Springs) experiences sustained dangerous heat from mid-May through mid-September. The Southern Plains and Lower Mississippi Valley peak July–August. The Pacific Northwest's record-breaking events have occurred late June through early August. Florida sees year-round heat exposure, with the deadly combination of high dew point and limited acclimatization driving cooling-system failures from May through October.

Early-season heatwaves are disproportionately dangerous. Bodies that have not yet acclimatized to summer temperatures can experience life-threatening heat illness at temperatures that will feel routine by August. The 2022 May heatwave in the Pacific Northwest produced excess mortality at temperatures that would not have produced excess mortality two months later.

Heat preparedness is overwhelmingly about ensuring access to cooling.

Cooling capacity

• Confirm your air conditioning operates correctly before the season. Service the system in spring; replace filters monthly during heavy use.
• If you do not have AC, identify the coolest space in your home (typically below-grade or a north-facing room with closed blinds). Battery-powered fans, evaporative coolers (in low-humidity regions only), and ice packs can extend tolerance.
• Identify your community's cooling centers — public libraries, community centers, shopping malls, and dedicated cooling centers activated during heatwaves.
• A window AC unit in even one room can dramatically reduce health risk during heatwaves. Energy-assistance programs (LIHEAP) help income-qualified households purchase cooling equipment in many states.

Health preparation

• Review medications. Many common medications — diuretics, beta-blockers, antihistamines, antipsychotics, lithium — impair heat tolerance. Your prescriber may adjust dosing during heatwaves.
• Know the signs of heat exhaustion (heavy sweating, weakness, cool moist skin, fast pulse) and heat stroke (high body temperature, hot dry skin or profuse sweating, confusion, loss of consciousness).
• If you have an outdoor job, talk with your supervisor about OSHA heat-illness prevention practices before a heatwave.

Vulnerable populations

• Adults 65 and older are at sharply elevated risk, especially those who live alone, have chronic illness, or take multiple medications.
• Infants and young children cannot regulate body temperature as effectively as adults.
• People experiencing homelessness face the highest exposure and the fewest options. Counties with homelessness response teams should plan heat-event outreach in advance.
• Identify neighbors who fit any of these categories and plan to check on them by phone or in person during heatwaves.

Pets and vehicles

• Never leave a child or pet in a parked vehicle, even briefly, even with windows cracked. Interior temperatures can exceed 130°F within 30 minutes on an 85°F day.
• Walk pets during the cooler parts of the day. Asphalt temperatures can exceed 140°F when air temperatures are 90°F, burning paws.

When a Heat Advisory or Excessive Heat Warning is in effect:

• Stay in air-conditioned spaces during the hottest part of the day, typically 10 a.m. to 6 p.m.
• Drink water steadily, even before you feel thirsty. Avoid alcohol and large amounts of caffeine during heatwaves.
• Wear lightweight, light-colored, loose-fitting clothing.
• Reschedule outdoor activity for early morning or evening.
• Take cool showers or baths. Cooling the skin and especially the head and neck dramatically improves tolerance.
• Check on at-risk neighbors at least once per day in person or by phone.

Recognize and respond to heat illness:

• Heat exhaustion is treated by moving to a cool environment, drinking water, applying cool wet cloths, and resting. If symptoms persist more than an hour or worsen, seek medical care.
• Heat stroke is a medical emergency. Call 911 immediately. While waiting, move the person to a cool place, remove unnecessary clothing, apply cool water or ice packs to the neck, armpits, and groin, and fan the skin to promote evaporative cooling. Do not give fluids to an unconscious person.

During multi-day heat events, focus especially on overnight cooling. Nighttime is when the body recovers from daytime heat stress; persistently high overnight low temperatures (above about 75°F) are a strong predictor of excess heat mortality.

Recovery from a heat event differs from other disasters: there is rarely a federal declaration, and damage tends to be biomedical rather than structural.

1. If you or a family member required emergency care during a heatwave, follow up with your primary care provider. Heat illness can have lasting cardiovascular and renal consequences that benefit from monitoring.
2. If you experienced air-conditioning failure, repair or replace the system before the next heatwave. Many states offer rebates for high-efficiency systems and heat-pump conversions.
3. Document any heat-related financial losses (spoiled food, hospitalization, equipment damage). If a federal or state declaration is later issued, DisasterAssistance.gov and your state emergency-management agency may have applicable assistance programs.
4. Talk through the experience. The SAMHSA Disaster Distress Helpline (1-800-985-5990) is available 24/7. Prolonged confinement during heatwaves, especially for people living alone, produces real psychological strain.
5. If you lost a family member or neighbor during a heatwave, the local public health department often partners with the medical examiner to provide grief support and information about cause of death.