TUPPER LAKE — Elevated carbon monoxide levels that triggered an evacuation of Adirondack Health’s Tupper Lake Health Center Wednesday morning have returned to zero, but the building will stay closed until Friday as employees and the clinic’s fuel provider investigate the cause.

Adirondack Health Communications Director Matt Scollin said they have narrowed the cause down to something involving the building’s boiler. A technician from the center’s fuel provider was working on the boiler at the time of the leak.

Though carbon monoxide is odorless, employees across the building reported smelling something. Scollin said it might have been exhaust mixed with the carbon monoxide or an added odor that he knows can be used to warn people the dangerous gas is present.

Employees also reported experiencing headaches and nausea. At around 11 a.m., carbon monoxide alarms started signaling, too.

Scollin said one of the clinical providers, seeing all these warning signs, pulled the fire alarm, signaling the whole building to evacuate and bringing the fire department.

“(It) was really the right thing to do, and we’re glad that that happened,” Scollin said.

Employees were sent home and compensated, and patients had their appointments rescheduled.

“We will be moving around both staff and patients (Thursday) trying to reschedule and see them in other health centers,” Scollin said. “We really want to limit the impact to our patients, and we want to try to keep as many of those appointments as we can, although it will be in another location, if that works for them.”

The leak, though widespread through the building, was not severe and was reduced through ventilation.

Fifteen people who experienced symptoms of carbon monoxide exposure were transported to Adirondack Medical Center in Saranac Lake for precautionary evaluation and treatment, Scollin said. They have all since been released.

“However, we are working on tracking down anybody that may have been in that building this morning prior to the evacuation,” Scollin said. “If anybody was in there today and has any feelings that they’re not well, we would advise them to please give us a call at … 518-359-7000.”

Dr. Darci Beiras, Adirondack Health’s chief medical officer, said symptoms of carbon monoxide exposure include headaches, dizziness, nausea and vomiting.

“Most of the effects of exposure are related to short-term,” Beiras said. “Usually it’s treatable with oxygen.”

She said if people have underlying issues as well, they can experience more severe symptoms including shortness of breath, confusion and vision issues.

Scollin said he is not sure exactly what happened in the boiler to cause the problem but said an official report will be finished soon. He said he would not name the fuel company because he did not want to point fingers, especially when the investigation has not been finished.