Authorities say the pool where about 40 kindergarten students became ill will remain closed indefinitely until it's determined what happened.
The Marathon County Health Department believes excessive chlorine levels are suspected in Friday's incident but that remains uncertain.
Gary Bezucha, CEO of North Central Health Care says water testing immediately after the students got sick found normal concentrations and the investigation into what were described as acrid fumes continues.
He says they are working the the Marathon County Health Department and their contacts at the State of Wisconsin Environmental Health Department.
The kindergartners from Lincoln Elementary School were at the center's therapeutic pool for water safety training when they began having difficulty breathing.
They were rushed to two area hospitals and most were treated and released.
One boy remains hospitalized today in good condition at St. Joseph's Hospital in Marshfield.
Bezucha says the pool has been open since the 1970's and they have never had a problem with it.