(AP) -- When it comes to the discomfort and health risks of the current heat wave, it's not just the heat or the humidity - it's both.
The temperature conspires with the amount of moisture in the air to make it hard for the human body to cool itself.
When people get hot, the body tries to cool down by moving extra blood to the skin and by sweating.
Blood in the tiny vessels near the skin can dissipate heat into the air, if the air is cooler than the body. But that doesn't work if the air is as hot as the body or hotter.
Sweat helps, because when water evaporates it removes heat. But the more moisture already in the air - the higher the humidity - the less evaporation can occur.
Those two processes account for more than 90 percent of the body's ability to dissipate heat, and when they aren't working, trouble can come from heat exhaustion, heat stroke and even death.