A leading human rights group says authorities in four Chinese provinces are hiding test results and denying care to children suffering the effects of industrial lead pollution.

In a 75-page report released Wednesday, Human Rights Watch says parents, journalists and others who speak out about the problem have been “detained, harassed and ultimately silenced.”

The U.S.-based group says central government officials have shown growing concern about the effects of lead, which can cause permanently impair physical and mental abilities when ingested by children.

But it says the study of contaminated villages in Henan, Yunnan, Shaanxi and Hunan provinces found local officials are largely ignoring the problem.

The report says officials have arbitrarily limited who will be tested for lead poisoning and, when tests are done, withheld the results from the victims and their parents. It says children in need of treatment have been sent back to their contaminated homes and merely advised to eat certain foods such as garlic and eggs.

The rights group urges the Chinese government to make sure that all affected children receive treatment and are not re-exposed to toxic levels of lead.