It's called the Mean Heat Index. What it does is give up to seven days' notice of when the weather is likely to get too hot to be healthy. It's the brainchild of the U.S. National Weather Service. Meteorologist Ed Danaher is chief of development and training branch of the NOAA Hydrometeorological Prediction Center.

"Hot days here and there are no problem, but a heat wave over several days in an urban area is real trouble," Danaher tells WebMD. "The problem is a lot more dangerous than many people think. One of the biggest killers is heat. In 1995, for example, a heat wave killed hundreds of people in Chicago."

A heat wave isn't just the heat -- it's also the humidity. The old heat index combined heat and humidity to give a snapshot of just how hot it feels at the hottest point of the day. The trouble with this is that the effects of heat build up over time. Some very hot days cool off at night. Those days may have a high heat index, but they aren't as dangerous as days when it stays hot.

Enter the Mean Heat Index. "Mean" here has a double meaning. Mean as in nasty. And mean as in average. The measure averages the heat index for the hottest and coldest times of the day. When used with weather predictions, it gives a much better picture of how bad the heat is going to be.

The number to watch out for is a mean heat index of 85 degrees. That's when the heat becomes a problem. When the mean heat index hits 90 and 95 degrees, many more problems can be expected.

You may already be protected by the new measure, which went into effect this summer. Emergency responders in many cities are keeping an eye on it. You can do it, too: Check out the NOAA web site and click on the "mean heat index" or "heat wave" buttons. If the probability of an 85-degree heat index is 60% or more, make plans to beat the heat.

"If you air conditioner is on the fritz, get it fixed. Or if your mother-in-law lives in an apartment without air conditioning, invite her over to stay with you for a few days," Danaher says. "High heat has a cumulative effect and after several days is when you start to see problems."

Here are some other tips on how to handle too much heat:

• Slow down. If you must work or work out, do it before 7 a.m.
• Dress for summer. Lightweight, light-colored clothing reflects heat and helps your body maintain a normal temperature.
• Eat less. Too much food -- especially protein -- adds fuel to your inner fire.
• Drink plenty of water.
• Avoid beverages that contain alcohol or caffeine.
• Don't take salt tablets unless your doctor tells you to.
• Elderly people should seek cool places to spend the day, such as air-conditioned public places. Even spending part of the day in air-conditioned places can help a lot.
• Get out of the sun. Avoid sunburn -- sunburned skin makes it much harder for you to stay cool.