Three people fainted after inhaling toxic gas at POSCO’s Pohang Steelworks in North Gyeongsang Province on Thursday.

The accident occurred at around 1:30 p.m. when two contracted cleaners and a POSCO employee inhaled gas and collapsed while removing sludge outside one of the rolling mills at the plant, according to fire authorities.

All three were found unresponsive. Three others have also been hospitalized with minor injuries.

Fire officials believe the workers suffered carbon monoxide poisoning.

The company and fire authorities are looking into the accident, which comes just 15 days after one worker was killed and three others suffered burns at the POSCO plant due to exposure to chemicals that leaked from damaged pipes.