The Western Education and Library Board said they have dealt with radon gas, which is associated with lung cancer, in 18 local schools.
The Board said that is has made the health and safety of pupils is a priority and that it is dealing with with the issue "immediately and effectively".
There are areas within two schools, Gortin Primary School and Strabane Academy, which are still above the action level and need more work.
Dr Ann Wilson, consultant in health protection with the Public Health Agency, has urged parents not to panic.
"Radon is a naturally occurring gas. It's in the atmosphere, all around us all of the time but usually in very low levels.
"Radon when it disintegrates gives off radio active dust. That is not a problem unless you breathe in a lot over a long period of time.
"Work has been done by the education board and we have been involved in that.
"Parents should not panic here.
"The areas that still need work in the two schools, are not areas in which the children are."
In a statement the WELB said they survey of Radon monitoring was carried out in all relevant schools in the 2010/11 school term.
They said they are working with the schools who need more work to ensure that staff or pupils are only allowed back into the areas when the levels have been brought up to the required standard.