As convenient as it sounds, smoke and carbon dioxide detectors do not last forever. It is best to replace smoke detectors every 10 years. Like most devices, the store or company should allow a return policy and issue warranty when buying smoke detectors. Limited warranties range from 2 10 years and return policies are permissible up to 90 days. Smoke detectors must indicate that it has met Underwriters Laboratories UL Standard 217 and carbon dioxide detectors must meet UL Standard 2034. UL conducts certification training on every electrical appliance. Smoke detectors can only alert you if there’s fire. The rest depends on the homeowner’s preparation and other fire prevention safety measures. Equip yourself with fire extinguishers and sprinklers, too, if necessary. Photo Electric or Optical Sensors They are triggered by smoke disturbing an existing beam of light inside the detector. When smoke is not present, the beam from a light source inside the device travels in a straight line to a detector, but when smoke particles enter, some of the light is scattered and triggers the electrical circuit that sounds the alarm.