A smoke detector is a device that senses smoke, typically as an indicator of fire. Commercial security devices issue a signal to a fire alarm control panel as part of a fire alarm system, while household smoke detectors, also known as smoke alarms, generally issue a local audible or visual alarm from the detector itself.

Carbon monoxide sensors detect potentially fatal concentrations of carbon monoxide gas, which may build up due to faulty ventilation where there are combustion appliances such as gas heaters and cookers, although there is no uncontrolled fire outside the appliance.
High levels of carbon dioxide (CO2) may indicate a fire, and can be detected by a carbon dioxide sensor. Such sensors are often used to measure levels of CO2 which may be undesirable but not indicative of a fire; this type of sensor can also be used to detect and warn of the much higher levels generated by a fire. One manufacturer says that detectors based on CO2 levels are the fastest fire indicators, and also, unlike ionization and optical detectors, detect fires that do not generate smoke, such as those fuelled by alcohol or gasoline. CO2 fire detectors are not susceptible to false alarms due to particles, making them particularly suitable for use in dusty and dirty environments.