The incident occurred night when workers were conducting an overhaul of the generator boiler at Shougang Shuicheng Iron and Steel Company Limited, local authorities reported.
Authorities said that two people were also poisoned, but the doctors were able to save them, and causes of the accident are under investigation.
The Shaoguan city government said in a post on its microblog that the leak occurred at 3 am at the Shaoguan Iron and Steel company's Songshan plant.
It said the rescue and recovery effort was underway and the cause was being investigated.
In 2015, an explosion traced to improperly stored chemicals killed at least 173 people in the port city of Tianjin, about an hour east of Beijing.
More recently, 10 people were killed in an explosion at a biotech company workshop in the eastern city of Lianyungang on December 9.