Hundreds of staff and students at Cardiff University could have been exposed to potentially lethal asbestos dust throughout the past five years, trade union Unison said this week.
The university last week closed almost all of its humanities building - where more than 200 staff are housed - following the unexpected discovery of asbestos during renovation work.
The Health and Safety Executive confirmed it was "monitoring the situation", and it is understood that the university may face a formal investigation.
Unison has demanded health checks and a monitoring programme for staff who believe they have been put at risk.
In a bulletin to staff, Unison's health and safety representative, Sue Hayward-Lewis, said that asbestos had been discovered in the same building five years ago, but staff were told at the time that it had been removed and the building had been made safe.
"Have staff been breathing circulating asbestos dust for five years?" she asked in the bulletin.
Unison said that a number of renovation works had been carried out in the time between the university declaring the building safe and rediscovering more asbestos recently.
The most recent work had been going on for more than three weeks before the asbestos was discovered. Asbestos is safe as long as it is not disturbed. If it is, then it can release particles into the air. If inhaled, these can cause fatal illness.
The university confirmed in a statement that it was aware that some asbestos was present before the latest work took place. "Some asbestos removal was identified and planned" as part of the building work, designed to provide a new fire-protection system in the building, off Colum Road in Cardiff. "But the extent of asbestos in the building has proven to be greater than anticipated," the university said.
Specialist contractors have been employed to remove all asbestos and decontaminate the area. The university has been advised that there are no relevant health tests or procedures to follow.