Remedial works to lower radon levels in a Kerry mental health institution will commence on Monday.
The HSE confirmed that monitoring found levels of Radon over the EPA guidelines in Deer Lodge, Killarney.
The Psychiatric Nursing Association have questioned the failure of a Radon barrier installed in the newly-constructed building and when the HSE knew about the issue.
Radon levels at Deer Lodge were recorded at over 170 points, with the majority being below the EPA guidance of 200 becquerels per metre-cubed.
Levels were found to be above 200 becquerels in 21 points, and twelve of these were above 400 becquerels.
Two points were found to be in excess of 500 becquerels per metre-cubed, two-and-a-half times the recommended safe level.
The monitoring took place between March and late-June of this year by an external company, before the facility opened in July.
The HSE say the results were not received until after the opening of Deer Lodge.
They have confirmed remedial works will begin on Monday, which will include the fitting of fans to reduce the levels of the cacogenic gas.