Suspected chemical leak Kent England, Forty-five people have been treated.
More than 40 firefighters were sent to a commercial site off Sutton Road, Langley, near Maidstone, just before 15:00 GMT.
Three ambulances, five fire engines, hazardous material officers and a specialist chemical unit are onsite.
The fire service said no-one was in a serious condition but one person was taken to hospital.
Fire crews wearing breathing apparatus and hazardous material officers are working to establish what the chemical is, Kent Fire and Rescue Service said.
It added the incident was contained to the site where 57 people were treated in total.
‘Hazardous materials tent’
A spokeswoman for Maidstone and Tunbridge Wells NHS Trust said Maidstone Hospital was “currently preparing for the arrival of a number of patients who are experiencing breathing difficulties”.
“As a precautionary measure a hazardous materials tent has been erected outside the emergency department to help us manage those patients,” she added.
She said the trust’s emergency departments were “open as normal” but encouraged people only to use them for emergency and life-threatening situations.
A Kent Police spokesman confirmed the force had responded to “reports of a suspected chemical leak” after being contacted by the ambulance service.
“Officers are at the scene along with the Kent Fire and Rescue Service and ambulance crews, and are offering assistance where needed.”