An invisible killer has ended the lives of two people and sickened four others.

Police said that in separate incidents - one in Cheltenham Township, Montgomery County, and the other in Philadelphia - carbon monoxide, a colorless and odorless gas, snaked its murderous way through two homes.

Police, contacted by school officials when a 15-year-old boy didn't show up for school Tuesday, entered a home on Central Avenue in the Cheltenham Village section of Cheltenham just after 5:15 p.m. and found the teen and a 50-year-old man dead in their bedrooms, apparently from carbon monoxide poisoning.

A 44-year-old woman was found unconscious and rushed to Elkins Park Hospital, where her condition wasn't immediately available.

Cheltenham Township police said Peco Energy and the county fire marshal found a natural-gas heating system to be malfunctioning, the stack clogged with soot.

A final determination of the cause of death was awaited from the county coroner's office.

The second incident happened early yesterday morning in West Kensington. According to Executive Chief Daniel Williams, fire officials were called to a home on Ontario Street near Allegheny Avenue shortly after midnight.

"Firefighters found three people in the house, all suffering from carbon monoxide poisoning," Williams said.

An unidentified man and woman were taken to Temple University, where their conditions were unavailable. Another victim, an unidentified teenage girl, was taken to the Children's Hospital of Philadelphia, where her condition was also unavailable.

Early symptoms of carbon monoxide poisoning are nausea, sleepiness, headaches and slurred speech, Williams said.

If you think you may be suffering from carbon monoxide poisoning, immediately get fresh air and, depending on the seriousness of the situation, seek medical attention.

Fire Commisioner Lloyd Ayers yesterday stressed the importance of trying to prevent a situation such as this from occurring.

Ayers said that every home should be equipped with a carbon monoxide detector, which costs approximately $40.

"You need to make sure that it is properly installed and you should install it in the sleeping area of the home," he said.

"If the levels are dangerous it will go off and emit a sound that is similar but not the same as a smoke detector. That's when you'll know that the levels are too high."

Ayers also suggested inspecting all appliances and keeping chimneys clean because residue can build up and insufficient ventilation can be deadly. *