xanadu,

You are mixing dew point and humidity (actually relative humidity). Relative humidity is a percentage. Dew point is a temperature. If the relative humidity is 99% and the temperature is 80 degrees, the dew point is about 79 degrees.

A dew point of 70 degrees means there is a lot of water vapor in the air. It makes the heat penetrating since there is very little evaporative cooling. It also displaces oxygen in the air and can lead to a bit of oxygen deprivation that will cause a head ache. It can be rough on asthmatics.