Cities are often a few degrees warmer than nearby rural areas. This "urban heat island" effect is usually attributed to cities' dark, paved surfaces and lack of trees and grass. But according to Larry Kalkstein, of the University of Delaware, it's possible that city-dwellers' use of air conditioners could also make things hotter...

First of all the equipment itself produces heat. Any machine that you run just like a car engine produces heat. And number two is that the hot air that was in the building has to go somewhere, so it goes outside.

Kalkstein says when a heat wave hits a city like Philadelphia or New York, air conditioners get cranked to their highest -- and could actually raise outdoor temperatures by several degrees. Kalkstein says that makes air conditioning a mixed blessing.

Air Conditioning is a great benefit to human health if you are in it. That is if you are elderly or are prone to heat problems you should be in air conditioning. But if you don't have access to it or live adjacent to the big city, it might make things worse for those people.

Kalkstein says widespread use of air conditioning in downtown office buildings became common in the 1970s. At the same time, he says, city temperatures jumped up about two degrees.

More information's available on the web at GreenWorks.tv. I'm Brad Linder.