Managing director Kevin Keen said that the alarm was raised at 7.45 am when an engineer who was mixing chlorine used to clean borehole water realised that he had made an error with the mixture.
About 60 litres of the chemical were released, and the building was immediately evacuated.
The 20 or so staff in the dairy gathered in the car park.
The engineer managed to escape without being contaminated, although he did go to hospital for a check-up.
