Yarmouth High School was back in session Tuesday, a day after students were dismissed after some complained that they were feeling dizzy and lightheaded.

Several students fainted on Monday, but a series of air quality tests found no issues.

The Yarmouth School Department initially evacuated the building late Monday morning, citing an “air quality complaint” in an announcement to families.

Since the early tests showed no worrying signs, administrators briefly reopened the school, before evacuating it again and closing it for the day after six and 10 additional students reported feeling light-headed or dizzy after returning to the building, Dolloff said. None of those students was transported to the hospital.

Tuesday began with additional testing by the fire department, which again found no contaminants, Superintendent Andrew Dolloff said. He added that facilities crews replaced all the building’s air filters overnight, and an outside contractor is conducting tests for other air contaminants.

“We had an uneventful day in terms of student illness and air quality issues,” Dolloff said after students were dismissed Tuesday. “Much better day than yesterday.”

Dolloff said three students fainted within 30 minutes of one another, “which is highly, highly unusual,” starting around 10:30 a.m. The students, who did not know each other and are in different grades, were each transported to a hospital, where they were tested for the presence of carbon monoxide, he said.

All three students returned home and were “doing well” by Monday evening, Dolloff said. He said he saw at least one back in school Tuesday morning.

Yarmouth fire Chief Michael Robitaille said his and other departments used four different multigas meters to test the building but found no indications of contamination. Crews also checked the rooftop vents and heating systems but found no blockages or leaks, he said.

“One medical (incident) turned into two; two turned into three,” Robitaille said. “Could it have just been a coincidence? Potentially. I don’t know. … It has me stumped also, let me just say that.”

One student tested positive for carbon monoxide, but less than would be in the bloodstream of a cigarette smoker, Robitaille said. Another student’s test showed trace amounts of the gas, but less than the threshold for a positive reading, he said. The third showed no presence of the gas.

Dolloff added that none of the students tested positive for drugs or alcohol.

It’s still not clear exactly what caused Monday’s incident, Dolloff said. One hypothesis, shared by the district Monday evening, is that an idling truck outside the facility may have wafted gas into the building, briefly contaminating the air.

In addition to increased testing, bus drivers were on standby in case students or staff experienced symptoms and needed to be relocated to other schools. The school department is also bringing in indoor air quality experts to further monitor the school’s air quality and is consulting with the district’s medical advisory panel to see if it has further guidance, Dolloff said.

“This is going to improve our air quality monitoring, if anything,” Dolloff said.

Carbon monoxide is an odorless, colorless gas that is produced by the incomplete combustion of fuels like gasoline, natural gas or wood. Gas- and oil-burning furnaces, portable generators, and gasoline engines produce carbon monoxide; inhaling it causes illness and death. The most common symptoms of CO poisoning are headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion, according to the U.S. Centers for Disease Control and Prevention.

Staff Writers Morgan Womack and Gillian Graham contributed reporting.