Officials at the Dow Corning chemical plant in Barry in the Vale of Glamorgan have confirmed there was a leak early on Saturday.
Hydrogen chloride leaked close to the plant’s boundary but did not cross it, a spokesperson said.
He described the leak as small, adding the site has now been given the all clear.
A warning siren was sounded at 06:30 BST for 25 minutes as a precaution to warn the local community.
Source: BBC News