This document from the Federal Emergency Management Agency is a fact sheet containing information on extreme heat. From the document: "Heat kills by pushing the human body beyond its limits. In extreme heat and high humidity (which is when the air has moisture and feels sticky), the body must work extra hard to maintain a normal temperature and keep the body comfortable. A heat wave is an extended period of extreme heat, and there is often high humidity. These conditions can be dangerous for people who don't take care of themselves properly."
Federal Emergency Management Agency: http://www.fema.gov/