JACKSONVILLE, Fla. -

More than half-million U.S. children are believed to have lead poisoning, roughly twice than previous thought.

Health officials say the increase is the result of the government lowering the threshold for lead poisoning, so now more children are considered at risk.

The Centers for Disease Control and Prevention says about one in 38 young children have lead poisoning. Lead poisoning used to be a much larger concern in the U.S., but now the CDC has made stricter guidelines on paint, gasoline and toys.

With the stricter guidelines, pediatricians say they're going to start testing lead poisoning more often with a simple blood test about once a year.

"We've gotten into the habit lately because we had so few coming up positive of not screening as many people," said Dr. Hilleary Rockwell. "Although now with the new CDC regulations, we are going to start screening almost everybody on a regular basis."

Lead can harm a child's brain, kidneys and other organs. High levels in the blood can cause coma, convulsions and death. Lower levels can reduce intelligence, impair hearing and behavior, and cause other problems.

"Unfortunately, unless you have a toxicity, you probably won't have much in the way of symptoms," Rockwell said.

Often, children who get lead poisoning live in old homes. About 9 to 15 percent of homes in Duval and Bradford counties are at-risk homes -- those built before 1950.

Lead has been banned in household paint since 1978 and was gone from gasoline by the late 1980s.

"You want to be especially careful with children under age 6 when doing renovations with houses that were built before 1970 with old paint, especially doing sanding of the paint where there is a lot of dust in the air," Rockwell said.

Once lead poisoning is diagnosed, doctors often refer parents to local health departments to get their homes checked out to try to find the source of the problem.