In Canada, the humidex represents the effects that high humidity and high temperatures can have on the body. The higher the humidex, the harder it is for sweat to evaporate and cool the body.

The humidex combines the temperature and humidity into one number to reflect the perceived temperature. Because it takes into account the two most important factors that affect summer comfort, it can be a better measure of how stifling the air feels than either temperature or humidity alone.

General recommendations for high humidex ratings:

• Humidex of 35 to 39: Certain types of outdoor exercise should be toned down or modified, depending on the age and health of the individual, physical shape, the type of clothes worn, and other weather conditions.
• Humidex of 40 and over (extremely high): All unnecessary activity should be limited.

If being outdoors is an absolute necessity, drink plenty of liquids and take frequent rest breaks. In hot, humid conditions, there is a considerable risk of heat stroke and sun stroke.