The heat index (see chart below) is the "feels like", or apparent, temperature. As relative humidity increases, the air seems warmer than it actually is because the body is less able to cool itself via evaporation of perspiration.
 
As the heat index rises, so do health risks. When the heat index is 90°-105°F, heat exhaustion is possible. When it is above 105°F, it is probable. Heatstroke is possible when the heat index is above 105°F, and very likely when it is 130°F and above. Physical activity and prolonged exposure to the heat increase the risks.
 
Heat exhaustion occurs when the body is dehydrated.

• Symptoms: headache, nausea, dizziness, cool and clammy skin, pale face, cramps, weakness, profuse perspiration
• First aid: move to a cooler spot, drink water with a small amount of salt added (one teaspoon per quart)
• It can lead to: collapse and heatstroke.

Heatstroke occurs when perspiration cannot occur & body overheats.

• Symptoms: headache, nausea, face flushed, hot and dry skin, no perspiration, body temperature over 101°F, chills, rapid pulse
• First aid: cool person immediately, move to shade or indoors, wrap in a cool, wet sheet, get medical assistance
• It can lead to: confusion, coma, and death.