MQ135 gas sensor is specialized in AIR QUALITY measurement and is sensitive for Benzene, Alcohol, and smoke.
An example how to use it: http://wiring.org.co/learning/basics/airqualitymq135.html
An example with calculation of the CO2 value: http://davidegironi.blogspot.it/2014/01/cheap-co2-meter-using-mq135-sensor-with.html
VCC – 5v
GND – Ground
DO – Digital Output = HIGH /LOW (Sensitive can be adjustable by potentiometer)
AO – Analog Output = 0-1023 value (Arduino)