Federal investigators believe hundreds of thousands of children continue to drink water containing lead at school despite laws aimed at reducing lead contamination.

States have failed to enforce laws requiring schools to test drinking water for lead and the government has not provided adequate guidance on what water coolers pose a health risk, according to an Environmental Protection Agency report made available Thursday.Lead is a powerful poison. Even traces can be harmful to children because it can hurt development of the brain and nervous system. Children under 7 are considered particularly vulnerable.

The EPA inspector general's office estimated that, based on a limited survey, fewer than half of the school districts in the country may be testing thoroughly.

An examination of 13 school districts in the mid-Atlantic region revealed that three had not tested for lead while most of the others conducted inadequate tests.

"Our findings confirm that harmful amounts of lead exist in the drinking water provided by schools. Both the EPA and the states must be more aggressive in eliminating the health hazard imposed by lead in drinking water," said the report.

Three of the school districts - Pittsburgh, Richmond, Va., and Charleston, W.Va. - had not tested for lead at the time of the survey. The investigation was finished in April and the report completed in September.

The EPA considers lead concentrations above 20 parts per billion to be hazardous to schoolchildren. The report said that the agency believes every year more than 250,000 children are exposed to lead level in drinking water that are high enough to impair their intellectual and physical development.