The heat index provides information about how hot it feels outside in the shade. It is a measure of the air temperature in relation to the relative humidity for a particular day.
A heat index is listed each day in the newspaper to alert people of the risk for a heat-related illness. Direct exposure to the sun can increase the risk for a heat-related illness on days when the heat index is high. Babies, older adults, or anyone with a health condition may have more risk of problems with the heat because of their age and general health.
A heat index of:
- 27°C (80°F) to 32°C (89°F) may cause fatigue.
- 33°C (90°F) to 40°C (104°F) may cause heat cramps or heat exhaustion.
- 41°C (105°F) to 54°C (129°F) may cause heat cramps or heat exhaustion, and heatstroke is possible.
- 55°C (130°F) or higher may cause heatstroke.
Prevention measures during days of high heat index will help reduce the risk of a heat-related illness. When the outdoor humidity is greater than 75%, losing body heat by sweating is not as effective, so other measures to keep cool are needed.
Last Revised: November 2, 2011
Author: Healthwise Staff
Medical Review: William H. Blahd, Jr., MD, FACEP - Emergency Medicine & H. Michael O'Connor, MD - Emergency Medicine
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