The values are for people wearing the right amount of clothing doing light work, with very little wind. Few people begin to feel uncomfortable. About half of the people feel uncomfortable. 90 - 104 Sunstroke, heat cramps, and heat exhaustion are possible with prolonged exposure and physical activity. 105 - 129 Sunstroke, heat cramps, or heat exhaustion likely.
Heatstroke is possible with prolonged exposure and physical activity.
When the THI is 90-104, for moderate to heavy activity, workers should be concerned about heat stress and should alternate time working in the heat and time in cooler areas or light work. When the THI is 105 or higher, extreme caution is indicated. Workers should be encouraged to drink plenty of fluids and be on the lookout for signs of heat stress.