Oklahoma City, OK – A carbon monoxide leak at Lee Elementary School in southwest Oklahoma City sent three students and three staff members to local hospitals Monday afternoon, authorities said.

People began complaining of dizziness, nausea, and stinging eyes in the late afternoon. Oklahoma Natural Gas and the Oklahoma City Fire Department later confirmed that carbon monoxide was detected.

Those taken to the hospital were in good condition, as were four others treated at the scene.

Tierney Tinnin, a spokeswoman for Oklahoma City Public Schools, said the source of the leak is believed to be a recently installed heating unit on the school’s roof.

Shortly after school let out, Tommy Iago, Oklahoma City Fire Department battalion chief, said it was safe to re-enter the school, and several students, parents, and staff did go back inside.

However, Oklahoma City Public Schools decided to cancel school for Tuesday while the heating unit in question is replaced.

The school district’s operations team also will conduct additional scans for carbon monoxide Tuesday to confirm the building is safe.