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.
  • 32°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.
  • 54°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.

Current as of: May 27, 2016

Author: Healthwise Staff

Medical Review: William H. Blahd, Jr., MD, FACEP - Emergency Medicine & H. Michael O'Connor, MD - Emergency Medicine