The effects of heat on the body are known as heat stress.
Many factors contribute to heat stress, but the most important elements influencing heat stress and comfort are temperature and humidity.
As the chart indicates, the combination of high temperatures and humidity greatly increases the threat for heat stress.
The Apparent Temperature or Heat Stress Index assumes a very light breeze and you being in the shade. Of course, how hot "it feels" varies from one person to another, but this index seems to give a good idea of what the hot weather "feels like."
To find out what the air "feels like," use the table below. For example, with the air temperatures of 90 degrees, and a relative humidity of 60%, move down to where the columns meet and find the Heat Stress Index of 100 degrees.