Authorities say around 230 workers were evacuated because of an ammonia leak early Tuesday at a Tyson pork plant in Storm Lake.
Firefighters and police were dispatched to the plant, which had already been emptied around 3:10 a.m.
Tyson staffers told firefighters the ammonia flow had been shut off but plenty of gas vapor remained in the plant.
The plant air eventually cleared up.
No injuries have been reported.
Storm Lake authorities say a fan on a refrigeration unit had broken.
It struck a condenser coil in the refrigeration unit, causing the ammonia leak.