Baghdad, Iraq (CNN) -- At least 500 people were sickened Tuesday night by a cloud of chlorine gas that leaked from a water treatment plant over Sadr City in eastern Baghdad, health and police officials said.
A mechanical failure led to an explosion that caused the chlorine to escape. The gas is used to purify water in a water treatment plant in the neighborhood.
Health officials said most of the people who were sickened were taken to area hospitals, treated and released.
Sadr City, home to at least 1.5 million people, is one of Baghdad's poorest neighborhoods.
CNN's Mohammed Tawfeeq contributed to this story.