Data set of MQ135 clearly suggests it need to be caliberated inorder to work it correctly. code "MQ135_Ro_Calculation" calculates the values for the sensor caliberated at home. code "MQ135_Smoke_Detection" calculates the value of the level "Defined by the dataset". Compare the result with the threshold given by the dataset of MQ135.
You can follow the result which is published "http://ijirt.org/Article?manuscript=146594" on detection of Carbondioxide level.