Since the government lowered the threshold for lead poisoning last year, the number of at-risk children has doubled from previous high estimates. High levels of lead in the blood can cause organ damage, low IQ and death.

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Lead poisoning can occur from drinking tainted water, as well as exposure to paint, dust and soil from old homes or industrial worksites.

More than half a million U.S. children are now believed to have lead poisoning, roughly twice the previous high estimate, health officials reported Thursday.

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