LAGOS, Nigeria — An international watchdog said Tuesday it will cost about $4 million to clean up toxic lead and secure mines in northern Nigeria, where activists say “the worst outbreak of lead poisoning in modern history” has taken place.

At least 400 children have died since March 2010, and thousands more continue to be exposed to dangerously high levels of lead, said Human Rights Watch researcher Jane Cohen.

Children are being exposed while processing ore in these informal mines not owned by any company, or when their miner relatives come home covered with lead dust. The children’s food and surroundings also have been tainted when people crush rocks at home to extract the ore.

Nearly two years after the problem began, women are still sorting groundnuts on the ground, exposing their food to contamination.

“People are more aware (of the causes of the child deaths), but there is a … sense that there are no alternatives,” said Cohen.