People who dial 911 from their cell phones put themselves at risk if they are unable to give the operator the correct address, U.S. emergency experts said.

The country's 911 systems are set up so that calls from land-line phones are automatically traced, and the address appears immediately on an operator's screen, USA Today reported Tuesday. The problem is a growing number of Americans are getting rid of their land-line phones. The U.S. Centers for Disease Control and Prevention says the number of households with only cell-phone service grew 3 percent to 20.2 percent between 2007 and 2008.

In 2008, Darlene Dukes, who lived in Johns Creek, north of Atlanta, died of a blood clot. Her trip to the hospital was delayed by an hour because the operator sent the ambulance to Wells Street in Atlanta, not 602 Wales Drive, Dukes's address.

"Lots of people are dying each year," says David Aylward, director of Comcare Emergency Response Alliance. "We're sending in responders where they don't know information about the person they are responding to. We're sending them in looking for someone when they should know where they are exactly."

Even the most technologically advanced 911 centers can identify the locations of cell phones only within about 1,000 feet, experts say, and being that precise may take several critical seconds.