THE HEAT INDEX, OR HEAT STRESS INDEX, MEASURES THE COMBINED EFFECTS OF AIR TEMPERATURE AND HUMIDITY, AND TYPICALLY IS INCLUDED IN WEATHER REPORTS. THE HIGHER THE INDEX, THE GREATER THE CHANCE OF

80 degrees to 90 degrees, fatigue from prolonged sun exposure or physical activity is possible.
90 degrees to 105 degrees, heat cramps and heat exhaustion are possible after prolonged exposure or physical activity.
105 degrees to 130 degrees, heat cramps and heat exhaustion are likely and heat stroke is possible from prolonged exposure or physical activity.
130 degrees and higher, heat stroke is highly likely from continued exposure.
Source: National Weather Service, Senior Health (http://seniorhealth.about.com)