Fire Detection sensors are used to detect fire.
Toxic Sensors are used to detect oxygen, Carbon Dioxide, Carbon Monoxide, etc.
Combustible Sensors are used to detect gases that can be burned such as Methane, Ethane, Propane and hydrocarbons as well as hydrogen, acetone, ammonia, alcohols, gasoline and 80 other gases.
Environmental Sensors are used to detect humidity, tempature, airflow, vibration, pressure, etc.