DALLAS (CBSDFW.COM) – The school day was disrupted for students and staff at Rangel Women’s Leadership High School near Fair Park in Dallas.

Just before 10 a.m. Dallas Fire Rescue was called to the school in the 1700 block of Robert B. Cullum Boulevard.

Nearly a dozen people were complaining of headaches and nausea, possibly from a carbon monoxide leak.

“Whoever called said that they were getting a strong gas odor in the hallways,” explained Dallas Fire-Rescue spokesman Jason Evans.

Before fire crews arrived officials at the school took precautionary measures.

“When we got out there luckily they had about 500 students and faculty that had already been self-evacuated to a gymnasium across the street,” said Evans. “Our HAZMAT team came in with their handheld detectors.”

With the passage of time both DFR crews and Atmos Energy said the CO levels became undetectable.

“While we were trying to locate the leak the building had naturally ventilated so weren’t able to pick anything up,” Evans said.

Students and staff remained at the gym until HazMat crews declared the school safe. Of those complaining of illness no one had to be transported to the hospital.

Around 1 p.m. firefighters again tested CO levels at the school and found no trace.

A DFR investigation later determined the source of the leak had been an improperly ventilated boiler.  The problem was corrected and everyone was allowed back in the school.