Carbon monoxide is an odorless, colorless gas that is toxic to humans and pets at varying levels. Did you know that UL listed (Underwriters Laboratories) carbon monoxide detectors are not required to sound off until CO levels exceed 30 ppm (parts per million). While exposure to 30 ppm of carbon monoxide might not be enough to kill you, it might be enough for the young or elderly or for small pets to experience symptoms of exposure. This is why you should consider replacing your standard carbon monoxide detector with a low-level carbon monoxide detector which will alert you when levels exceed 5 ppm. Don't risk being exposed to low levels of carbon monoxide without knowing it.