Emergency services for the Skåne region in southern Sweden say that staff complained of nausea and discomfort in their necks and throats before calling for ambulances.
Gustaf Sandell, from the area's search and rescue service told news agency TT that a ventilation system in a treatment plant on the site had failed.
Ozone is a pale blue gas with a distinctively pungent smell. It is poisonous to inhale and can be linked to lung and heart problems.
Officials at the site say that the leak has now been stopped and the premises are being aired out.