Because humidity can greatly decrease your body's ability to maintain its normal temperature, activity...even walking in high humidity can be dangerous.

Humidity makes the temperature feel hotter than it actually is. The heat index tells you the "apparent temperature" -- how hot it feels to the average person -- for various combinations of air temperature and relative humidity.

For example, when the air temperature is 85 degrees Fahrenheit and the relative humidity is 75 percent, it actually feels like it's 95 degrees Fahrenheit outside. You can find out the air temperature and the humidity level (and often the heat index) on any given day from local forecasts.

When the heat index is between 80 and 90 degrees Fahrenheit, you need to use caution when exercising outside. This is especially true if you are just starting a walking program; if you are obese; if you have any serious health problems; if you take medication; or if you are over age 50. Under these conditions, you may need to cut down the amount of time you spend walking to avoid heat illness.

When the heat index reaches 90 to 105 degrees Fahrenheit, heat cramps, heat exhaustion, and heatstroke are possible if you exercise intensely outdoors. You should decrease the intensity and length of your workouts, walk in a shaded area, and be sure to drink plenty of fluids.

When the heat index exceeds 105 degrees Fahrenheit, exercising outdoors is dangerous. Heat cramps, heat exhaustion, and even heatstroke are likely. Move your walking program indoors.

If you take care of your body's needs, it is possible -- and safe -- to walk in hot weather.