Heat index

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.

The National Weather Service lists a heat index 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.

A heat index of:

  • 80°F (26.7°C) to 89°F (31.7°C) can cause fatigue.
  • 90°F (32.2°C) to 104°F (40°C) can cause heat cramps or heat exhaustion.
  • 105°F (40.6°C) to 129°F (53.9°C) is likely to cause heat cramps or heat exhaustion, and heatstroke is possible.
  • 130°F (54.4°C) or higher is likely to 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 Updated: October 7, 2008

Author: Jan Nissl, RN, BS

Medical Review: William M. Green, MD - Emergency Medicine & H. Michael O'Connor, MD - Emergency Medicine

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