Heat Wave Risk in the US: How FEMA Scores It and How to Prepare
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.
Sources
- NWS heat safety — HeatRisk forecasts and warning definitions
- FEMA NRI heat wave methodology
- CDC extreme heat guidance
- LIHEAP energy assistance