Three people were taken to a hospital by ambulance Friday and firefighters evacuated two floors of a 10-story office building on Marginal Way in Portland because of a carbon monoxide problem. Despite air sampling tests, no source for the problem was found.

Staff members at InterMed, which occupies the top four floors at 84 Marginal Way, contacted the Portland Fire Department twice Friday, first around 9 a.m. and again at 11:30 a.m., said Assistant Fire Chief Keith Gautreau.

Workers on the seventh and eighth floors of the health care company had symptoms of dizziness, nausea and headaches during both calls, Gautreau said.

As a result of the first call, the fire department took one person to Maine Medical Center and evaluated many more.

“We went through the building with our monitors,” Gautreau said. “We evaluated close to 20 people. Some of them had elevated levels of carbon monoxide, but nothing that was life-threatening or even critical.”

Firefighters could not detect any problem on their meters inside the building or a source of the problem, and let people go back to work as a heating, ventilation and air conditioning crew continued to work inside.

During the second call, firefighters took two more people to the hospital and evaluated another 50 workers, many with the same symptoms. Firefighters who walked through the building even evaluated one another and found that some of them had elevated levels of carbon monoxide in their systems, Gautreau said.

“It’s really kind of a mystery,” he said. “We’ll do some follow-up for sure.”

The Drummond Woodsum law firm occupies two floors of the building, but no one there had any symptoms. A pharmacy on the ground floor also had no problems. The other floors are occupied by a parking garage.

Dan McCormack, InterMed’s chief executive officer, said that a fourth InterMed employee went to a walk-in clinic to be treated for symptoms and had blood tests done. Everyone else was allowed to go home for the rest of the day.

“We just found out that all four had carbon monoxide levels that were within normal levels,” McCormack said. “We decided to close for the afternoon out of an abundance of caution.”

InterMed staff members contacted as many patients as possible to tell them that appointments had been canceled for the day, but some who arrived were turned away, he said.

McCormack said the evacuation and cancellations were a disruption to patients but he would rather err on the side of caution than put anyone at risk.

InterMed’s Portland office does not see patients on weekends, so McCormack said he hopes that the workers assessing the ventilation system will be able to solve the mystery before Monday.

Jim Hanley, the chief executive officer of Atlantic Bayside Trust, which owns 84 Marginal Way, said the building’s heating, ventilation and air conditioning contractor inspected the rooftop systems and found no problems.

“Out of an abundance of caution, we hired Northeast Air Quality Services to perform comprehensive air sampling tests with specialized equipment on every floor of the building. They found normal carbon monoxide levels and no other issues,” Hanley added in an email. “We will continue to test the air quality over the weekend, but it appears that there is no problem with the building.”