DDG said high incidents of vitiligo (skin discolouration), shivers, reduced vision, burning sensation in the limbs and impaired language skills were found among the people.
"All these are known to be symptoms of mercury exposure.
Chronic exposure to high levels of mercury can affect the nervous system and cause of memory loss, severe depression, increased excitability, delirium, hallucination and personality change, besides it can also damage the kidneys," he said.
Narain alleged that the fact of presence of high mercury level was already known through the studies carried out by Indian Institute of Toxicology Research and Central Pollution Control Board, but their reports were never made public.
Bhushan said in 2009, Sonbhadra was declared a critically polluted area and a moratorium was imposed on new projects, which was later removed on the basis of action plan by respective pollution control boards of UP and MP.
To check the problem, CSE DG said a moratorium should be imposed again and an impact assessment study should be done.
CSE has also demanded that mercury standards be developed for thermal power plants, coal mines and coal washeries.
"Industries which do not comply with the standards should be shut down till they meet the norms," Narain said.
Bhushan said treated water must be provided to the people.
"An advisory should be issued asking people not to consume fish contaminated with methyl mercury," he said.
Narain said the study would be submitted to the Union Ministry of Environment, Uttar Pradesh government and industries concerned.