The A&E for kids had to be shut down after pepper spray got into the air conditioning at Stepping Hill hospital, in Greater Manchester.
The spray caused 12 members of staff to fall ill.
The incident took place after the family of a child with an eye irritation visited the children’s A&E department, bringing in a sample of the substance.
But the substance – which turned out to be pepper spray – got into the hospital’s air conditioning system and caused staff to develop sore throats.
Greater Manchester Fire and Rescue Service was called to the hospital at 4.20pm on Saturday, and a hazardous materials team evacuated staff.
The A&E department was closed while the incident was investigated and children were transferred to a different ward.
The substance was identified by the hazardous materials team and was made from oleoresin capsicum, the naturally occurring chemical that makes chilli peppers hot.
Often used a personal security aid, pepper spray causes temporary blindness, coughing and skin irritation.
None of the staff will suffer long-term harm and are now recovering.
The children’s A&E department has now been reopened.