Women and children have died in the port city of Ludhiana, Punjab. Several people were found unconscious in their homes.
The area has been cordoned off and residents evacuated as many people have reported difficulty breathing. Four people are being treated in hospital.
The cause of the leak has not yet been officially confirmed.
PTI News Agency quoted Ludhiana Deputy Commissioner Surabhi Malik as saying: It is very likely that some chemicals reacted with the methane in the shaft.”
Local MP Rajinder Pal Kaul Chyna told Reuters the incident happened near a milk store. “People who came to buy milk in the morning passed out outside,” she said.
A team of the National Disaster Response Force has been dispatched to the scene in the Giaspra area of Ludhiana. There is a factory nearby.
Industrial gas leaks are not uncommon in India.
Three years ago, at least 12 people were killed in a gas leak at a chemical plant in Visakhapatnam city, Andhra Pradesh.
In 1984, a chemical leak at a pesticide factory in downtown Bhopal killed thousands and is considered the world’s worst industrial disaster.