( CNN ) -- As many as 400 children have died of lead poisoning-related illnesses in Nigeria since March , two international aid groups say , and as many as 30,000 people could be affected by lead contamination . The deaths occurred predominantly in children under the age of 5 in the state of Zamfara , according to Lauren Cooney , the emergency manager for Medecins Sans Frontieres . The group is also known by its English name , Doctors without Borders . Cooney told CNN Tuesday that the death toll may be even higher , but said the numbers should n't mask the crisis . `` Rather than focus on specific death toll numbers , because those often are very difficult to obtain , the international community should focus on the harsh reality in Nigeria : It is very clear that a serious and significant number of children have died due to lead poisoning , '' she said . A study conducted by the World Health Organization and the Health Ministry of Zamfara State `` identified 180 villages where children may have been poisoned by lead , which means that up to 30,000 people could be affected , '' said a statement Friday from the United Nations ' Office for the Coordination of Humanitarian Affairs . The report on child deaths comes a few months after the Nigerian government announced in early June the deaths of more than 160 Nigerians , including scores of children , of lead poisoning due to illegal gold mining . `` The lead poisoning is a consequence of villagers practicing small-scale gold extraction from lead-containing ore . The processing of the ore involves crushing and drying , often inside the homes of villagers , resulting in the contamination of soil , '' Doctors without Borders explained in a statement . Since June , Nigerian health officials have cooperated with the World Health Organization to clean up the affected areas . Since the process began , two out the seven villages known to be affected have been decontaminated , Cooney said . `` We already started in June treating patients . We are focusing on treatment of children under 5 but the most important thing is the remediation , or clean-up , so the exposure can stop , '' she added . Remediation is a lengthy process that requires removing all the contaminated soil and replacing it with clean soil . The process was stopped in the month of August due to the beginning of the rainy season but it started up again a few weeks ago . Efforts to clean up contaminated areas are being hampered by the reluctance of villagers to report potential lead exposure , for fear the Nigerian government will prohibit the informal gold mining , OCHA said . Doctors without Borders says it 's still treating hundreds of lead-poisoning cases in Zamfara , but the group warned that the treatment is only effective if patients do not return to contaminated sites .
Doctors Without Borders says the death toll may be even higher . The contamination comes from villagers ' attempts to extract gold from ore . The clean-up involves removing contaminated soil and replacing it with clean soil .