More than 1,000 school buildings in Northern Ireland currently contain asbestos, it has emerged.
The Education Minister confirmed there are 1,036 Education Authority (EA) properties which contain some type of the substance.
White, blue and brown asbestos are all potentially dangerous and have been banned in the UK.
The Department of Health has previously said asbestos that was not disturbed or damaged was not a significant health risk.
Asbestos was extensively used in building during the 1970s because of its strength and heat resistance.
If asbestos is damaged, however, the fine fibres become airborne and can be inhaled, causing serious diseases including lung cancer and asbestosis.
In an Assembly question, SDLP MLA Justin McNulty asked DUP Minister of Education Peter Weir to detail "the number of school buildings which have asbestos as part of the fabric of the school structure".
The Newry and Armagh MLA also asked Mr Weir to confirm "any planned works to remove asbestos from schools".
Mr Weir confirmed that 1,036 properties within the Education Authority (EA) estate "have some type of asbestos containing materials (ACM)", adding that the number "does not include schools within the voluntary grammar or grant maintained integrated estates".
The minister said the EA "is continually removing asbestos from its estate as part of its refurbishment and demolition programmes".
Mr Weir also said that "ACM is removed throughout the EA estate when the risk assessment dictates that removal is required", adding that "the approximate cost of asbestos removal is in the region of £3-5m annually".
He added: "Unfortunately, it is not possible to be absolutely certain all ACM have been removed from a building until that building is demolished, as ACM was used extensively in the structure of buildings constructed before the year 2000."
Jacquie White from the Ulster Teachers' Union said "the total removal of asbestos is the safest option".
"Nothing can be more important than the safety of children and teachers, so this is a worrying finding," she said.
"It is compounded by the fact that, according to the Health and Safety Executive (HSENI), 77 people here died from asbestos-related conditions in 2017, the last figures available.
"In the UK as a whole over 200 teachers have died in the last decade from the effects of being exposed to asbestos, according to government statistics.
"Add to this that for each teacher fatality, nine ex-pupils can also be expected to fall victim to the silent killer, according to the American Environmental Protection Agency, and you can appreciate just how insidious the problem is." Ms White said "the deadliest form - 500 times more lethal than the commonest - was still present in 21 schools here, according to an audit in 2012" following a health scare in a Belfast secondary school.
"Asbestos truly is a hidden killer and as such it's easy for teachers and pupils not to realise the potential for harm," she said.
"Something as simple as stapling posters on to walls containing asbestos could release the harmful fibres, while some items, such as old gas masks used in history lessons, can also contain the material.
"Schools are required to hold an asbestos register and management plan and advice from the HSENI is that unless it is damaged, asbestos can be left in place and monitored.
"However, we believe the total removal of asbestos is the safest option, unless there is good reason not to do so."
Mr McNulty said the ongoing asbestos removal works is a step in the right direction.
"Only when asbestos is disturbed is it a danger, however, it shows that our school estate is aging and I think we need to seek further clarity from the minister," he said.
"I want to see a dedicated programme to have all asbestos removed from schools. We have to endeavour to do this."
The Health and Safety Executive Northern Ireland website states: "Asbestos can be found in any building built before the year 2000, including houses, factories, offices, schools or hospitals, and causes around 5,000 deaths every year in the UK."
The Department of Education was contacted for comment.