What is the Heat Index?
The heat index, also known as the "apparent temperature," is a measure of how hot it really feels when relative humidity is combined with the air temperature. When the air is humid, your sweat cannot evaporate as quickly, which prevents your body from cooling itself efficiently. This is why a hot, humid day feels much more oppressive and dangerous than a hot, dry day with the same temperature.
The Dangers of High Heat: Heat-Related Illnesses
Understanding the heat index is critical because high values can lead to serious health issues. When your body cannot cool itself properly, you are at risk for heat-related illnesses, which can range from mild to life-threatening.
- Heat Cramps: Muscle pains or spasms, usually in the abdomen, arms, or legs.
- Heat Exhaustion: A more severe condition that can include heavy sweating, weakness, cold and clammy skin, a weak pulse, and fainting.
- Heat Stroke: A medical emergency. Symptoms include a high body temperature (above 103°F), hot, red, dry or damp skin, a rapid pulse, and possible unconsciousness. Call 911 immediately.
NWS Heat Index Chart: Risk Levels
| Heat Index | Risk Level | Potential Health Effects |
|---|---|---|
| 80°F - 90°F | Caution | Fatigue possible with prolonged exposure and/or physical activity. |
| 90°F - 103°F | Extreme Caution | Heat cramps, heat exhaustion possible. |
| 103°F - 124°F | Danger | Heat cramps or heat exhaustion likely; heat stroke possible. |
| 125°F or higher | Extreme Danger | Heat stroke highly likely with continued exposure. |
How to Stay Safe in Extreme Heat
Preparation is key to preventing heat-related illness. Monitoring your activity and health is important, and our other Fitness & Health calculators can help you stay on track.
- Stay Hydrated: Drink plenty of fluids, especially water. Don't wait until you're thirsty to drink. Avoid sugary drinks and alcohol.
- Seek Air-Conditioning: Spend as much time as possible in air-conditioned environments. If you don't have AC at home, go to a public library, shopping mall, or community cooling center.
- Wear Appropriate Clothing: Choose lightweight, light-colored, and loose-fitting clothing.
- Limit Outdoor Activity: If you must be outside, try to do so during the cooler parts of the day, like early morning or late evening. Pace yourself and take frequent breaks in the shade.
- Never Leave Anyone in a Parked Car: Not even for a minute. The temperature inside a car can rise to deadly levels very quickly.