Los Angeles, CA – A man and a woman in Sylmar were taken to an area hospital for headaches and nausea after a carbon monoxide alarm woke them Sunday night and the gas was found inside their apartment.

The incident was reported at 10:45 p.m. at 14040 Foothill Boulevard, according to the Los Angeles Fire Department.

Authorities initially said the source of the carbon monoxide was possibly a natural gas powered appliance, and they were examining an oven/stove or water heater.

Amy Bastman, a spokeswoman with the Los Angeles Fire Department, said on Monday it was ultimately unclear what the source of the gas was, as the levels of carbon monoxide dropped with natural ventilation — such as opening the window — and were within normal and safe limits after fire officials investigated.

Fire officials were called Sunday night after the two residents complained of dizziness. The fire department later discovered a measurable presence of carbon monoxide inside the apartment.

The apartment is located in a 176,160 square-foot multi-story building with a total of 171 apartment units, officials added.

LAFD HAZMAT experts were initially called to the scene to investigate the source of the carbon monoxide, officials explained.