The incident happened in the evening due to leakage when chlorine was being mixed in a water tank, Chandrasekharpur inspector in-charge, Rabi Satpathy said.
Altogether 17 persons in the vicinity were taken ill after inhalation of the gas, he said.
The affected people were immediately taken to the Capital Hospital for medical treatment. The condition of four of the affected persons, who were placed under close observation, was stated to be serious, hospital officials said. One of them is in the Intensive Care Unit, they added.
Efforts were on to ascertain the circumstances that led to the incident, the police said.