A municipal worker who was exposed to chlorine gas in Corner Brook is already well enough to return to work, the mayor said Friday.

The worker and a colleague were at the city's water treatment plant on Thursday afternoon when gas leaked from a cylinder.

The incident was serious enough to prompt police to close nearby roads in western Newfoundland temporarily, and for a boil-water advisory to be issued Thursday evening.

Mayor Charles Pender said the two workers were able to get out of the building as soon as they realized there was a problem.

"The worker who was brought to hospital was seen to yesterday evening. He was released after some tests were done, and is expected to be back at work this morning," Pender told CBC News.

"So [they were] very lucky, but also very a good response to the training that they received, and they did exactly what they were supposed to do."

Pender said it appears that a faulty valve gasket caused the leak.

Crews are starting to put chlorine back into its water system but it could take up to a week for it to spread throughout Corner Brook.

The boil-water advisory remains in effect.