Addyston, Ohio - The growl of air-monitoring equipment has replaced the chatter of children at Meredith Hitchens Elementary School in this Cincinnati suburb along the Ohio River.

    School district officials pulled all students from Hitchens three years ago, after air samples outside the building showed high levels of chemicals coming from the plastics plant across the street. The levels were so dangerous that the Ohio EPA concluded the risk of getting cancer there was 50 times higher than what the state considers acceptable.

    The air outside 435 other schools - from Maine to California - appears to be even worse, and the threats to the health of students at those locations may be even greater.