The U.N. urged Nigeria on Friday to clean up polluted villages and limit ore-processing after hundreds of children died last year from mass acute lead poisoning linked to illegal mining for gold by residents.

High levels of lead pollution were found in soil and mercury levels in the air were nearly 500 times the acceptable limit, according to the U.N. Office for the Coordination of Humanitarian Affairs and the U.N. Environment Program.

“Measures should be taken…to prevent further ore processing activities from taking place at sensitive sites – such as water sources from which humans and livestock drink,” its report said.

It urged authorities to clean up polluted villages as soon as possible so affected children could return for recovery and follow-up care.