In July 2006, California endured one of the worst heat waves in the state's history. Cities reported record highs: Fresno cooked at 110 degrees Fahrenheit for six successive days, and Sacramento baked under 100-degree temperatures for 11 straight days. Never before had central California seen so many consecutive triple-digit temperatures. But mercury wasn't the only thing on the rise during that time: a study in the January 2009 issue of the journal Environmental Health Perspectives found that hospital visits also increased.

By the end of a two-week period, according to the paper, emergency room visits were up by 16,000 statewide compared with non-heat wave reference periods from July and August of the same year; also, county coroners reported some 140 heat-related deaths. Electrolyte imbalances and kidney problems caused by dehydration were at the root of the hospital visits, and those most affected were children below the age of 5 and adults 65 and older, as well as people with diabetes and cardiovascular, respiratory, and kidney diseases, which are worsened by heat.

"The increased severity and frequency of heat waves are at the top of climatologists' lists for effects of continued global warming," says Kim Knowlton, coauthor of the research article and NRDC health and environmental senior scientist, who adds that these extreme temperatures won't be just in California. Governments and individuals can use studies such as this one, which identifies vulnerable populations, to help reduce future illness and mortality.