Heat Index Calculator · Practical guide
Heat Index Danger Levels Explained
The four heat index danger zones, what each one means for your body, and the exact thresholds the National Weather Service uses.
At a glance
The four danger levels
The National Weather Service divides the heat index into four risk categories. Below 90°F is Caution: fatigue is possible with prolonged exposure. From 90 to 103°F is Extreme Caution: heat cramps and heat exhaustion are possible. From 103 to 124°F is Danger: heat cramps or exhaustion are likely and heat stroke is possible. Above 124°F is Extreme Danger: heat stroke is highly likely.
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Heat Index Calculator
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Heat index danger level chart
| Heat Index (°F) | Level | Health Risk |
|---|---|---|
| Below 90 | Caution | Fatigue possible with prolonged exposure and activity |
| 90 - 103 | Extreme Caution | Heat cramps and heat exhaustion possible |
| 103 - 124 | Danger | Heat exhaustion likely; heat stroke possible |
| Above 124 | Extreme Danger | Heat stroke highly likely |
How quickly conditions escalate
Humidity can push a day up a full category without the air temperature changing. At 90°F, humidity of 50% gives a heat index of about 95°F (Extreme Caution), but at 70% humidity the same 90°F day jumps to roughly 106°F — solidly into the Danger zone. At 95°F and 60% humidity the heat index is about 113°F, and at 80% humidity it reaches 134°F, which is Extreme Danger.
What to do at each level
In the Caution zone, drink water and take normal breaks. At Extreme Caution, schedule outdoor work for morning or evening and watch for dizziness or cramps. At Danger, move activities indoors, check on elderly neighbors, and never leave children or pets in cars. At Extreme Danger, avoid all outdoor exertion — heat stroke can develop in minutes, and it is a medical emergency requiring immediate cooling and a 911 call.
Frequently Asked Questions
What heat index is considered dangerous?
A heat index of 103°F or higher is classified as Danger by the National Weather Service. Heat exhaustion becomes likely and heat stroke becomes possible with prolonged exposure.
What is the difference between heat exhaustion and heat stroke?
Heat exhaustion involves heavy sweating, weakness, and nausea, and usually resolves with rest and fluids. Heat stroke means body temperature above 103°F with confusion or loss of consciousness, and it is life-threatening.
Can the heat index be lower than the air temperature?
Yes. In very dry air, sweat evaporates efficiently, so it can feel slightly cooler than the thermometer reading. For example, 90°F at 20% humidity yields a heat index of about 86°F.
Do the danger levels account for direct sunlight?
No. The categories assume shade and light wind. Working in direct sun can add up to 15°F of apparent heat, effectively moving you up a category.