More than 30 Metropolitan Fire Brigade firefighters entered Ace Heat Treaters in Murphy Street in Richmond about 1pm Wednesday after hydrochloric acid was accidentally mixed with sulphuric acid creating a chlorine cloud in the factory.

MFB commander Barry Gray said fire crews rushed to the scene at about 11am, and evacuated more than 25 workers from surrounding factories.

He said firefighters wearing encapsulated gas suits had gone into the building to try and dilute the chlorine, which had spilled onto the factory floor, by flooding it with fresh water.

“We're going to need to try and get it all to a safe concentration,” he said.

“We suspect it's only small doses, but even small quantities can be fatal.”

Police blocked off River and Burnley streets from Bridge Road.

Paramedics were on standby but no one was injured by the accident.

Commander Gray said Ace Heat Treaters had evacuated its employees by the time emergency serviced had arrived at the scene.

“Without the evacuation, it's likely employees could have been seriously affected by the chlorine."

He expected the operation to dilute the chemicals and make the building safe would run until 4pm.

The Department of Health advises anyone experiencing symptoms that may be due to chemical exposure to contact Nurse on Call on 1300 606 024.

WorkSafe is investigating the incident.