The Human Resource Building was evacuated Thursday morning after poisonous gas leaked into the air conditioning system.

After the Environmental Health and Safety Service detected carbon monoxide, it evacuated the building, which houses the School of Architecture.

A mechanical malfunction inside the building led to the leak, according to University Police. Students, faculty and staff were allowed to return to the building after 15 minutes. There were no symptoms of carbon monoxide poisoning or complaints.

Mary Hayward, an executive administrative specialist in the School of Architecture Dean’s Office, reported an odor to the Environmental Health and Safety Service at 10:22 a.m.

“We smelled something in the stairwell here,” Hayward said.

The service had been chasing an odor in the building for the past few days, said Charles Brown, the safety and compliance manager.

When he went to the stairwell, he said it smelled like burnt sulfur. The gas detector used by the service found carbon monoxide in the air. The higher the level of the building, the more the carbon monoxide levels rose.

“It wasn’t dangerous but it was higher than I was comfortable with as far as leaving the building occupied,” Brown said.

It is unclear how the sulfurous odor related to the presence of carbon monoxide. As an invisible, odorless gas, carbon monoxide is impossible to see, taste or feel, making it hard to detect and dangerous.

Brown ordered an evacuation at 10:45 a.m. and called University Police to help with the evacuation.

The building is under renovation and a generator was being tested in the basement near the air conditioner. The exhaust system from the generator was leaking into the building. It circulated the gas throughout the building, Brown said. The generator is a back up for the building’s electrical system, he said.

Lt. Meg Ross, University Police spokeswoman, said the test lasted for two hours, which seemed strange to her.

“We discovered that the facilities department and physical plant were testing a small generator in the basement next to the air conditioner,” Brown said. “The exhaust system from the generator was leaking into the building and since you had the air conditioner there with it, it was pulling it and circulating it throughout the building.”

Brown said carbon monoxide levels weren’t high enough to pose a threat.

Carbon monoxide can be deadly: about 200 people die each year from carbon monoxide inhalation in the U.S. according to the Consumer Product Safety Commission.

“If it got high enough, their skin would turn red because carbon monoxide combines in the blood with the hemoglobin and doesn’t allow the blood to absorb oxygen. It doesn’t allow the body to function. That’s the danger of carbon monoxide – people can suffocate from oxygen deficiency,” said Brown.

“That was the reason why, even though it was a low level, I didn’t want there to be a pocket of it somewhere in the building and (have) someone die. We are always on the side of caution and safety.”