The source of the gas leak at Riviera Produce is believed to be from a forklift truck.
Workers in the vegetable packing area at the fresh produce distributor began to complain of feeling unwell around noon, reported local media.
The site was subsequently evacuated but was up and running again hours later.
Two ambulance staff were among those taken to hospital and checked before being released.
Carbon monoxide is a colourless and odourless gas which can be fatal when inhaled by reducing the blood's ability to carry oxygen.
Cornwall-based Riviera Produce employs around 40 people, preparing and packing cabbages and cauliflowers for supermarkets across the UK.