When a family member from New York arrived at his parents' home on South Darien Street in South Philadelphia on Wednesday, he found his 76-year-old mother and brother unconscious and his 80-year-old father struggling to stay awake.

Authorities said the father, Lin Jing, phoned his son in New York for help as he slowly faded into semi-consciousness due to a potentially silent killer -- carbon monoxide poisoning.

Jing, his wife, Mingyu Wang, and their son, Zhonghu Chen, were rushed to the Hospital of the University of Pennsylvania Wednesday afternoon after carbon monoxide seeped throughout the home.

Wang and Chen were listed in critical condition. Jing was listed in stable condition, according to authorities.

"There's no carbon monoxide detector," fire commissioner Lloyd Ayers said as he stood outside the home on the 1100 block. "They didn't have an early warning at all."

The incident had neighbors shaken up.

"I have radon and the smoke detectors, but I don't really think of the carbon monoxide detectors," neighbor Arlene Amen said. She planned to put some in her home because of this incident, she added.

Detectives checked to make sure there was no foul play in the incident, but said it appeared a poorly maintained natural gas heater caused the poisonings.

Philadelphia Gas Works shut down the system.

Investigators said there were no carbon monoxide detectors in house.