Houston, TX – By Laura Whitley, KTRK TV – A northwest Harris County apartment complex got a carbon monoxide scare Friday morning. Several residents had to be treated with oxygen.

That’s after one of their neighbors left a car running in a closed garage, sending carbon monoxide into the complex. That mistake could have been deadly.

A woman on the first floor left her car running inside of a garage for about 12 hours. That sent carbon monoxide seeping up to the three-story apartment building on Cypresswood Drive.

Around 1am, a neighbor on the second floor heard her carbon monoxide alarm sounding. At first, she thought it was a false alarm, but then her dog got sick. That’s when firefighters were called.

When fire crews arrived, the carbon monoxide levels on the first floor had reached deadly levels. Paramedics treated the woman who left her car on with oxygen.

“I kind of second guessed whether I need to get out or whether the alarm was just acting funny,” said resident Elizabeth Braden. “But when I saw my dog sick, I knew there was something that was serious going on.”

“We found a high concentration of carbon monoxide on the bottom floor where there was a car that had been left in the garage, which caused the building (to fill) with the carbon monoxide,” said Lt. Curt Crandall with the Cypresswood Volunteer Fire Department.

In all, seven units were evacuated. All of them were aired out and the residents were allowed to go back in.