Roswell, GA – A 25-year-old Roswell man died of carbon monoxide poisoning Saturday while in his home, and authorities said this is a grim reminder of the dangers of the deadly odorless gas.

Roswell police found the man, who has not been identified, unconscious upstairs in his home on Lexington Drive, and he later died at the hospital, Channel 2 Action News reported.

Police told Channel 2 they believe someone left a car running in the garage, and carbon monoxide from the car filled his house, killing him.

Roswell Fire Capt. Pabel Troche told the news station this is a prime example why carbon monoxide detectors are as important as smoke detectors within homes.

“It’s an odorless gas that’s emitted by any burning product such as a generator, your vehicle, your furnace, your fireplace,” Troche said. “It’s incredibly dangerous.”

About 20,000 people end up in the hospital annually because of carbon monoxide poisoning — 400 of whom are killed by it, according to the Centers for Disease Control and Prevention.

The most common symptoms of carbon monoxide poisoning are headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion, the CDC said. The symptoms are often compared to the flu.

Breathing in the gas can make you pass out and potentially die, and people who are sleeping or drunk can die from carbon monoxide poisoning before they show symptoms, the CDC said.

The incident in Roswell is being investigated.