Choose an alarm that is listed with a testing laboratory, meaning it has met certain standards for protection. Whether you select a unit that requires yearly changing of batteries, or a 10-year unit that you change out at the end of the 10 years, either will provide protection.
CO alarms also have a battery backup. Choose one that is listed with a testing laboratory. For the best protection, use combination smoke and carbon monoxide alarms that are interconnected throughout the home. These can be installed by a qualified electrician, so that when one sounds, they all sound. This ensures you can hear the alarm no matter where in your home the alarm originates.