What Heat Index Is Considered Dangerous? The Four NWS Bands
The NWS uses four named risk bands for heat index. Here is exactly what each one means, when it is issued, and what actions are recommended.
The National Weather Service classifies heat index values into four risk bands. Each one corresponds to a different physiological response and a different recommended action.
Caution: 80 to 90 F heat index. Fatigue possible with prolonged exposure or physical activity. Most healthy adults barely notice this band. Risk is concentrated in elderly, infants, people on medication that affects thermoregulation (diuretics, beta blockers, antihistamines), and outdoor workers in their first week.
Extreme Caution: 90 to 103 F heat index. Heat cramps and heat exhaustion possible with prolonged exposure or physical activity. The NWS may issue a Heat Advisory if forecast HI stays in this range for 2 or more days, with a typical threshold of HI 100 to 105 F for the issuance depending on local climate. Heat advisories trigger most US municipal cooling center activations.
Danger: 103 to 124 F heat index. Heat cramps and heat exhaustion are likely. Heat stroke is possible with continued physical activity. The NWS typically issues an Excessive Heat Warning at HI 105 to 110 F sustained. School districts in heat-prone states (Texas, Arizona, Florida) suspend outdoor athletic practice at this level. OSHA recommends mandatory work-rest cycles for outdoor labor.
Extreme Danger: 125 F and above heat index. Heat stroke is highly likely. This range is rare and usually only reached in the desert Southwest during severe heat waves or in tropical climates during peak summer. When forecast, the NWS issues an Excessive Heat Warning with strong advisories to stay indoors.
A few important nuances. The chart assumes shade and a light wind. Direct sun adds about 15 F to the effective heat index. So 95 F in the shade with HI 99 becomes effectively HI 114 in direct sun, Danger band.
Indoor heat index in homes without AC tracks outdoor temperatures with a 4 to 8 hour lag and a small reduction. A heat wave outdoor HI of 100 F translates to indoor HI of about 92 F by late afternoon in an unconditioned home. Most US heat deaths happen indoors in this exact pattern.
When in doubt, the safest rule is: outdoor activity below HI 90 F is fine for most people, between HI 90 and 103 F should be limited or moved to morning/evening, and above HI 103 F should be avoided unless trained, hydrated, and acclimatized.