The MQ series of gas sensors use a small heater inside with an electro-chemical sensor. They are sensitive for a range of gasses and are used indoors at room temperature.

They can be calibrated more or less (see the section about "Load-resistor" and "Burn-in") but a know concentration of the measured gas or gasses is needed for that.

The output is an analog signal and can be read with an analog input of the Arduino.

They are used in gas leakage detecting equipments in family and industry, are suitable for detecting of CH4,Natural gas.LNG, avoid the noise of alcohol and cooking fumes and cigarette smoke.