DEXTER, Kan. (AP) — Officials in south-central Kansas say an elementary school was quickly evacuated after students began having headaches from what turned out to be carbon monoxide.

The incident happened Wednesday morning at the elementary school in the Cowley County town of Dexter. The Winfield Daily Courier reports the fumes were traced to a faulty heat exchange on a heating and air conditioning vent.

The school’s 55 pupils were taken outside to be checked for possible carbon monoxide exposure. Winfield Fire Department Lt. Patrick Ramirez says four children who felt nauseated were taken to hospitals to be checked further and were expected to recover.

Dexter Superintendent Ron Ledford says the responders included a school librarian who is also an emergency medical technician.

The school was closed for the day and will reopen Thursday.

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DEXTER, Kan. (AP) — Officials in a small Kansas school district say at least 18 students are being checked for possible exposure to carbon monoxide.

KSNW-TV reports  the children are first- and second-graders at the elementary school in the Cowley County town of Dexter.

A hospital in Winfield was notified of the possible exposure around 11 a.m. Wednesday. At least one pupil was taken to the hospital in a private vehicle. That child’s condition was not released.

The school was closed Wednesday and will reopen Thursday.

Dexter superintendent Ron Ledford says a defective heat exchanger on the roof is the likely source of the carbon monoxide. He says the unit sits above the first and second grade classrooms. Those units have been shut down, and the school has been cleared of carbon monoxide.