A school has been evacuated in Dorset due to the presence of a respiratory irritant, the fire service has said.
About 420 children and 180 staff and adults were forced to leave Kings Park Primary School in Bournemouth after it was closed at lunchtime.
The fire service said a number of asthma sufferers had been treated for breathing problems.
Gas tests by the fire service have so far proved negative. Further tests are yet to be carried out.
Dorset Fire and Rescue Service added that no dangerous levels of natural gases or carbon monoxide had been detected.
Ashley Road has been closed, with the public told to stay away from the area for the "foreseeable future".
The fire service said about 30 pupils, who have had respiratory history such as asthma, had been checked over as a precaution by the ambulance service.
Mike Cox, incident commander at the fire service, said symptoms had first been reported on Thursday.
He added: "We are still trying to establish where this airborne irritant has come from.
"There have been four cases of asthma sufferers having problems over the last two days."
The Health Protection Agency, local hospitals and NHS Direct are dealing with the incident.
Parents were advised to pick up their children, while those that were left were taken to the Kings Park cricket pavilion.
All children have now been reunited with their parents and no-one has been taken to hospital.
Another gas test will be carried out later with a final test at 07:00 BST on Monday to ensure that the school is fume free for staff and children to return safely, the fire service said.