About a dozen residents were treated on Saturday night after being exposed to carbon monoxide.

The exposure occurred at an apartment complex in the 3400 block of Pin Oak Drive and Oakdale.

Fire officials said they got the call about 5 p.m. from hospital staff after a family of three checked themselves into the hospital with flu-like symptoms.

After examining the apartment building, crews evacuated 15 people from that complex and also had to force their way into two units to get to three of the residents who were already unconscious.

"If we use the toxicological data that is available to us, it tells us that they can be in that area for up to ten minutes without having significant life threatening health effects," said  Jonathan Jones, San Antonio Fire Department  battalion chief. "So we know that they have been exposed to those levels for some time."

Jones said they believe the carbon monoxide was coming from the boiler room where the  insulation around the boiler had caught fire.

After ventilating the building, crews let everyone return to their homes Saturday night.

Jones said one of the three people found unconscious, a woman in her 20s, is in critical condition. The other two are expected to be okay.

Jones said there were no carbon monoxide detectors at the complex.