Herndon, VA – A Herndon apartment complex was evacuated after high levels of carbon monoxide were found in the building, according to officials.

Crews were dispatched to the 1200 block of Elden Street around 8:30 Sunday night for reports of several residents not feeling well. After investigating, crews found elevated levels of carbon monoxide in the building. Seven residents were taken to an area hospital for feeling ill, and treated for non-life threatening injuries.

Crews cleared the scene around 10 p.m. Crews were then dispatched again shortly after 11 p.m. to the same complex, and found several more patients reporting symptoms similar to the effects of carbon monoxide poisoning. After more investigating, firefighters found that the furnaces on the terrace level were buried under drifts of snow, causing backup of carbon monoxide and poor ventilation.

Crews cleared the vents, and noticed an immediate drop off in the carbon monoxide levels.

Firefighters evacuated dozens of residents from the building, but they have been allowed back into their apartments.