Asbestos found at Bloomfield Collegiate School building
Damaged asbestos has been found in buildings belonging to Bloomfield Collegiate in east Belfast.
A private nursery had been using the rooms which were once part of the grammar school's prep department.
The Health and Safety Executive started the investigation following tests showing what are described as "elevated levels" of asbestos in the air.
The Public Health Agency confirmed work carried out in 2001 on cupboards in the building may have exposed the asbestos.
The prep school on the Upper Newtownards Road closed in June last year.
The Astoria Kindergarten rented rooms while staff and some senior pupils also continued to use that part of the building.
The building is separate from the main school.
According to a briefing to the Health Minister Edwin Poots and seen by the BBC, the Health and Safety Executive (HSE) conducted a survey and found evidence of the presence of asbestos in the fabric of the building and elevated levels in the air.
The Public Health Agency said there was no immediate risk to public health as the building has been closed and it is providing advice to teachers and parents.
The PHA is working with the HSE after the discovery.
The presence of the asbestos was discovered in May.
The school has plans to renovate the building and in preparation for this work a survey was carried out by a specialist company. This firm advised the school that there was asbestos in some areas of the building.
The school's headmaster Dr Darrin Barr said steps were taken straight away to ensure the safety of the nursery children.
"We took the decision to move the children out immediately, so the children were not in the building after it was discovered," he said.
He said after HSE tests found elevated levels of asbestos "we completely sealed the building and it has not been used, in fact people have not been in it since".
Dr Barr said the school then commissioned two independent tests over two weeks.
"The surveyors conducted 30 air tests in each of the seven rooms and central hall and they could not find any measurable levels of asbestos."
Dr Anne Wilson of the PHA said. "It's important to remember that not everybody who is exposed to asbestos becomes unwell.
"As much as we can say is there's the possibility that the children and the staff might have been exposed to these higher levels.
"What we know about asbestos is that those who are exposed to it for longer periods of time to higher levels are more at risk.
"So the studies mainly focus on people who worked with it."
Asbestos, a naturally occurring, affordable material with good heat resistant properties, was widely used in industry and construction for much of the 20th century.
It was only after decades of use in a wide variety of ways that people noted the link between exposure to asbestos fibres and a number of different lung diseases - particularly the aggressive chest cancer mesothelioma.
Asbestos fibres can lie dormant in people's lungs for anywhere between 15 and 50 years after initial exposure.
It is only hazardous when it is in a loose form, damaged, disturbed or worked on, as this releases the asbestos fibres into the air.