The manager of a Whanganui tanning factory says a toxic gas reaction on Friday was caused by a chemical being added to the wrong vat.
Two men, who inhaled hydrogen sulphide fumes, were admitted to hospital in a serious condition following the incident at Tasman Tannin in Castlecliff.
Factory manager Hunter Tait said it appears sulphuric acid was added to the wrong vat at the wrong time, probably as a result of human error.
Mr Tait said the factory was back in full operation on Saturday, and everyone, apart from the two injured men, is back at work.
One of the men is in a stable condition in Whanganui hospital and the other is to be discharged from Wellington hospital on Monday.