Heat Wave Risk in the US: How FEMA Scores It and How to Prepare

Written by Dvir · Data: FEMA National Risk Index v1.20 · Updated August 4, 2026

Extreme heat kills more Americans in a typical year than hurricanes, tornadoes, and floods combined — and it does it quietly, one overheated apartment at a time. The 2021 Pacific Northwest heat dome killed hundreds in a region where most homes lack air conditioning; Maricopa County, Arizona now records several hundred heat deaths every summer. In the National Risk Index, heat waves (hazard code HWAV) account for roughly $7.1 billion in national expected annual loss, spread across 3,062 counties.

What counts as heat wave risk

The NRI’s heat wave hazard covers prolonged periods of abnormally high temperatures — “abnormal” being the operative word. Heat kills relative to what a place is built for: 95°F is a normal Phoenix afternoon and a mass-casualty event in Seattle. That’s why the highest-risk list mixes desert metros with northern cities.

The dangerous mechanics: heat illness escalates from cramps to exhaustion to heat stroke (body temperature above 103°F, confusion, loss of consciousness — a medical emergency with high fatality). Two multipliers dominate the death toll: overnight lows that stay high, which deny the body recovery time, and the urban heat island, which keeps dense neighborhoods 5–15°F hotter than their suburbs after dark. Most victims are elderly people, alone, in homes without functioning air conditioning.

How the NRI measures it

FEMA combines each county’s exposure (population and its economic value, plus agriculture), annualized heat-wave frequency from NOAA records, and historic loss ratios — including the valuation of deaths and injuries, which heat produces disproportionately — into an Expected Annual Loss scored 0–100 relative to all counties.

Where the risk concentrates

Maricopa County, AZ (Phoenix) leads the nation — score 100 and a striking ~$958 million expected annual loss, with heat-wave conditions nearly 20 times a year. But the rest of the top five is telling: Philadelphia County, PA (~$256M), Dallas County, TX (~$288M), Clark County, NV (Las Vegas, ~$295M), and St. Louis County, MO (~$253M). Northern cities appear because their populations and housing stock are less adapted — the pattern behind history’s worst US heat disasters, like Chicago 1995 (over 700 deaths in a week).

How to read your county’s score

A high score means your county regularly sees heat its population and infrastructure struggle with. The personal-risk questions are specific: Does everyone in your household have reliable cooling — and can they afford to run it? (Many heat deaths occur in homes with AC that was off.) Are there elderly neighbors or relatives who live alone? Does anyone take medications that impair heat regulation (diuretics, beta blockers, antipsychotics)? Do you work outdoors — the other major casualty group?

What actually reduces the risk

  • Check on isolated older people daily during heat events. This is the single most effective intervention; most victims die alone indoors over multiple days.
  • Cool the person, not just the house. A few hours a day in air conditioning (mall, library, cooling center) resets heat strain. Fans alone stop helping above roughly 95°F — a cool shower or wet cloths work better.
  • Never leave children or pets in cars. Interiors reach lethal temperatures in minutes even on mild days.
  • Shift outdoor work to mornings, hydrate before thirst, and know the escalation signs: confusion or stopped sweating means call 911 and cool aggressively while waiting.
  • Harden your home cheaply: window coverings on sun-facing glass, a programmable thermostat, weatherization (the same insulation that saves heating costs blocks heat), and a plan for power outages — a heat wave plus a blackout is the nightmare scenario.
  • Use assistance programs. LIHEAP funds cooling in most states; many utilities ban summer shutoffs — nobody should ration AC into a hospital visit.

The heat disasters that taught the lessons

Heat waves do not leave debris fields, so they are remembered through mortality statistics rather than photographs. Three American events changed how public health agencies treat them.

Chicago, July 1995. A five-day heat wave with high overnight temperatures killed more than 700 people. The sociological post-mortem is the most important document in American heat policy: the victims were overwhelmingly elderly, poor, living alone, on upper floors of buildings without air conditioning, in neighborhoods where fear of crime kept windows closed and residents indoors. Neighborhoods with similar demographics but more active street life and social contact had dramatically lower death rates. Chicago’s modern heat response — proactive cooling centers, wellness checks, and emergency declarations triggered by forecast rather than by body count — was built directly from that analysis, and it has been copied nationwide.

The European heat wave of 2003 killed tens of thousands of people across the continent and demonstrated at scale what happens when a temperate, largely un-air-conditioned population meets sustained extreme heat. It is the reference case for what an unadapted region looks like.

The Pacific Northwest heat dome, June 2021. Temperatures reached levels never before recorded in Portland and Seattle — well above 100°F in a region where a large share of homes have no air conditioning at all — and killed hundreds of people in the US and Canada in under a week. It made the central point of this hazard undeniable: heat kills relative to what a place is adapted to. A temperature that is an ordinary Tuesday in Phoenix is a mass-casualty event in Portland.

Maricopa County, Arizona now publishes detailed heat mortality surveillance each year and consistently records several hundred heat-associated deaths per summer. Its data is the best in the country and shows the modern American profile clearly: a large share of victims are unsheltered, and among indoor deaths, a majority occur in homes where air conditioning was present but not functioning or not running — frequently because of cost or a power disconnection.

Season, timing, and who is exposed

  • Peak season runs May through September, but the deadliest events are usually the early-season ones — a June heat wave kills more people than an equivalent August heat wave, because bodies have not yet acclimatized and cooling systems have not been serviced.
  • Duration is the killer, not the peak. A single 105°F afternoon is survivable. Four consecutive days with overnight lows above about 80°F are not, because the body never gets the recovery window that nighttime cooling normally provides. Rising overnight minimums are the most dangerous trend in this hazard.
  • Humidity does the rest. In the Southeast and Midwest, high dew points prevent evaporative cooling, so a 95°F day with 70% humidity is more dangerous than a 105°F dry day in the desert. The NWS heat index and the newer HeatRisk product both account for this.
  • The urban heat island keeps dense, paved, tree-poor neighborhoods 5–15°F hotter than nearby suburbs, especially at night. Within a single city, heat mortality maps closely onto tree canopy and historical disinvestment — the hottest neighborhoods are frequently the ones with the least shade and the oldest housing.

The people who die are predictable: adults over 65 living alone; people with cardiovascular, respiratory, kidney, or psychiatric conditions; people taking diuretics, beta blockers, antihistamines, or antipsychotics, which impair thermoregulation; outdoor workers, especially in the first days on a job; unsheltered people; infants and young children; and anyone left in a parked car. Athletes and outdoor workers account for the acute heat-stroke cases; the elderly account for the bulk of the mortality.

Cooling the building: what works and what it costs

Heat mitigation is unglamorous and cheap compared with every other hazard in this index.

  • Block the sun before it enters the glass. Exterior shading — awnings, shutters, a tree on the west side — is several times more effective than interior blinds, because once sunlight is through the window the heat is already inside. Reflective window film and blackout curtains on south- and west-facing glass are inexpensive retrofits with immediate effect.
  • Insulate and air-seal the attic. The same work that cuts winter heating bills cuts summer cooling load, and it is the highest-value envelope upgrade in most older homes. Weatherization assistance programs fund it for income-qualified households.
  • Service the air conditioning before the season. Filters, coils, and refrigerant charge. A failing unit fails on the hottest day, when every HVAC contractor is booked for a week.
  • Cool roofs and radiant barriers. A light-colored or reflective roof surface measurably lowers attic and interior temperatures in sun-dominated climates, and is a marginal cost at re-roofing time.
  • Create one cool room. If whole-house cooling is not possible, make a single interior room — ideally on the lowest floor, with minimal window exposure — the place the household sleeps during heat events. A window or portable unit sized for one room is far cheaper than central air.
  • Fans have a threshold. Moving air helps by aiding evaporation, but above roughly 95°F a fan pushing hot air over skin stops cooling and can accelerate dehydration. Above that point the intervention is water on skin, a cool shower, or getting to a cooled space.
  • Plant shade. Trees on the west and southwest sides of a house are a decades-long investment that lowers both indoor temperature and neighborhood heat island effect.
  • Plan for the compound event. A heat wave with a grid failure is the worst-case scenario in this hazard, and it is increasingly common because peak load and peak heat coincide. Know where the nearest cooling center is, keep phones charged, and if anyone in the household depends on powered medical equipment, register with your utility’s medical baseline program.

Cost, assistance, and the insurance question

Heat is unusual: there is no “heat insurance,” and the financial exposure is mostly utility cost and lost income rather than property damage.

  • LIHEAP (the Low Income Home Energy Assistance Program) funds cooling as well as heating in most states, including help with bills, emergency repairs, and in some states free window air conditioners. It is chronically underused for summer cooling.
  • Weatherization Assistance Program funds insulation, air sealing, and sometimes cooling equipment for income-qualified households — permanent reductions in both bills and risk.
  • Utility shutoff protections. Many states prohibit summer electricity disconnection during declared heat emergencies or above certain temperatures, and most utilities offer payment plans. Nobody should be rationing air conditioning into a hospital admission; find out what your state’s rule is before you need it.
  • Homeowners insurance and heat. Heat itself is not a covered peril, but the secondary damage often is: a failed HVAC condensate line, spoiled food after a power outage (usually a small sublimit), or a burst pipe. Check your policy’s power-interruption terms.
  • Workers’ protections. Heat illness prevention rules for outdoor workers — water, shade, rest breaks, and acclimatization schedules for new employees — exist at the federal level through OSHA’s general duty clause and in stronger form in several state plans. Acclimatization matters enormously: a disproportionate share of occupational heat deaths happen in a worker’s first week on the job.

Before, during, and after

Before. Identify the vulnerable people in your life and agree on a daily check-in plan for heat events — this is the single highest-leverage action available for this hazard, because most victims die alone indoors over several days. Locate your nearest cooling center, library, or mall. Service the AC, stock electrolyte drinks, and confirm that anyone on heat-sensitive medication has talked to a doctor about it.

During. Watch the NWS HeatRisk forecast rather than the raw temperature — it accounts for humidity, duration, overnight lows, and local acclimatization. Drink before you are thirsty, and avoid alcohol and heavy meals. Shift outdoor work and exercise to before 10 a.m. Spend at least two to three hours a day somewhere air conditioned, even if home is not; that alone substantially reduces heat strain. Never leave a child, older adult, or pet in a parked car — interiors reach lethal temperatures within minutes even on mild days, and cracking a window does not help.

Know the escalation. Heat cramps and heavy sweating are warnings. Heat exhaustion — heavy sweating, weakness, nausea, cold clammy skin, dizziness — means stop, get to a cool place, and take fluids. Heat stroke — body temperature above 103°F, hot skin, confusion, slurred speech, or loss of consciousness, sometimes with sweating stopped — is a medical emergency with a high fatality rate. Call 911 and cool the person aggressively while you wait: cold water immersion if possible, otherwise ice packs at the neck, armpits, and groin, and continuous wet cloths. Minutes matter more here than in almost any other disaster.

After. Heat mortality lags the event — deaths continue for days after temperatures drop, particularly among people with cardiac and kidney conditions. Keep checking on people through the cool-down. Use the aftermath to fix what failed: the AC unit that could not keep up, the room with no shade, the neighbor nobody had a phone number for.

Sources

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