A chemical incident on an industrial estate has left 14 people needing treatment, including three in hospital.
Firefighters were called to Brookfield Foods Ltd at Deeside Industrial Estate, Flintshire, just before 0500 BST.
The workers were exposed to chlorine gas after an "uncontrolled mix of chemicals" was released from a factory.
Brookfield Foods said an investigation into how the incident happened was under way.
Environment Agency officials are ensuring that the clean up and disposal of any hazardous waste is carried out safely.
Nearby firms were warned to close their doors and windows but the incident was not said to be near residential properties.
North Wales fire service said the warning was issued to firms as a precaution.
A spokesman for the Welsh Ambulance Service said two of the casualties were taken to The Countess of Chester Hospital and one was taken to Wrexham Maelor.
All three were treated for eye irritation and chest discomfort. Their condition is not serious.
"When we arrived the building had already been evacuated and a number of staff were waiting outside. We had to set up cordons to limit the number of people coming onto the site as the incident progressed," said fire spokesman Justin Evans.
"There had been a small chemical spillage within the premises which resulted in a small release of chlorine gas.
"There were initially reports of three people treated by ambulance personnel but as time progressed that number went up to 14.
"I believe three people are now in hospital for precautionary checks."
The fire service's environmental unit from Wrexham attended the incident. They followed guidance from Environment Agency officers to ensure safe decontamination and management of pollutants on site.
Mr Evans said the spill had now been cleared.
He added: "We ventilated the property and used some specialist equipment to clear the area out."
Brookfield Foods said in a statement: "An immediate evacuation of the site took place through which all staff were accounted for, although three members of staff were taken to hospital as a precautionary measure.
"Emergency services were called and arrived promptly, they were briefed and took control of the incident.
"After a short time the emergency services were satisfied there was no risk present and staff were allowed back into the building."
Lee Rawlinson, of Environment Agency Wales, said: "Our officers are now working with Brookfield Foods to ensure that the clean up and disposal of any hazardous waste generated by the incident is carried out safely, and in compliance with relevant environmental legislation.
"An investigation into the causes of this incident is continuing."
He added that the spillage had highlighted the need for businesses to be fully aware of their environmental risks and obligations.