A kindergartner hospitalized in critical condition after being exposed to a high level of chlorine at a swimming pool was upgraded to fair condition Saturday, a hospital official said.

A group of more than 40 kindergartners from Lincoln Elementary had difficulty breathing Friday while they were in a swimming pool learning about water safety, said Mary Ellen Marnholtz, spokeswoman for the Wausau School District in northern Wisconsin. The pool was at a nursing home run by North Central Health Care in Wausau.

Thirty-eight students and two adults were taken to hospitals but most were released within about two hours, officials said. However, one boy was taken to a pediatric intensive care unit at St. Joseph's Hospital in Marshfield.

A nursing supervisor at St. Joseph's Hospital, who wouldn't give her name, said the boy's condition had improved Saturday.

The chlorine problem was caused by a pump malfunction, said Gary Bezucha, chief executive of North Central Health Care.