Norwalk, CT – Six people were sent to the hospital Saturday night in Norwalk after a carbon monoxide leak.

Firefighters arrived on scene to find two adults unconscious and extreme levels of carbon monoxide in the air. It happened in a multi family house on Woodward Avenue in South Norwalk Saturday night.

Six people in all were taken to Norwalk Hospital suffering from carbon monoxide poisoning from a malfunctioning furnace.

Related: Dangers of carbon monoxide poison in homes

Carbon monoxide is dubbed the “silent killer” because it is both colorless and odorless, meaning it is virtually undetectable. It is especially dangerous this time of year when windows and doors stay closed and people run fuel burning appliances like fireplaces and gas heaters.

“What they thought was gas was furnace malfunctioning. They weren’t smelling CO. It’s colorless, odorless? Silent killer. You don’t know until it’s too late. And it affects very young and very old first,” said Captain Mark Franze, Norwlak Fire Department.

So experts urge safety and caution to avoid CO poisoning. They say the best method of protect is to install a carbon monoxide detector.