CHARLOTTE, N.C. -- Nearly 50 employees inside a southwest Charlotte office building felt sick and had to evacuate Thursday. People thought it was a possible carbon monoxide exposure.

• Five patients were transported with non-life threatening injuries
• Nearly 50 people experienced nausea and headache symptoms
• The odor came from tar being laid on the roof

Just before they were about to take lunch hour, employees inside the Regency Executive Park building were startled from their work stations.

"What we did hear was the alarm beeping," Yolando Cato said. She told us it took a moment before panic set in.

However, the situation warranted hazmat and mass casualty buses as workers evacuated to the parking lot.

"I did see a woman that seemed quite lethargic in her office chair that was being wheeled out," Cato said.

Medic says they treated 46 employees who were experiencing nausea and headache symptoms.

Officials say five patients were transported with non-life threatening injuries to local hospitals.

At first thought, it was a gas leak. However, Charlotte fire captain Dennis Gist determined the odor was coming from tar being laid on the roof.

“That odor from the tar is being sucked in thru the HVAC system. So, it’s being dispersed throughout the building," he said.

We're told that's not uncommon.