COVENTRY, Vt. (AP) — Classes were cancelled at a small school in northern Vermont on Wednesday after several students were sent to the hospital for carbon monoxide exposure, officials said.

Several students and one adult at the Coventry Village School were showing symptoms of exposure, including nausea, fatigue and headache on Tuesday, North Country Supervisory Union superintendent Elaine Collins wrote in a message to the school community.

Emergency crews arrived quickly to the kindergarten-through-eighth grade school and some students were sent to the hospital for further evaluation, she said. Several students tested positive for slightly higher levels of carbon monoxide even though initially there were no detectable levels in the building, Collins said. The students were treated and released.

The affected students and adult had been in a classroom closest to an outside construction site on the school property. While heavy machinery was running outside, air conditioning was running inside the building, pulling outside air in, Collins said.

It’s likely that the carbon monoxide came from the operation of the heavy equipment. The school decided to cancel classes on Wednesday out of an abundance of caution to test levels and come up with a mitigation plan, Collins said. The Newport Fire Department was at the school on Wednesday morning and found no detectable levels of carbon monoxide, she said.