To set the sensor breakboard we look at the datasheet to search for the resistance:
"Resistance value of MQ-135 is difference to various kinds and various concentration gases. So,When using this components, sensitivity adjustment is very necessary. we recommend that you calibrate the detector for 100ppm NH3 or 50ppm Alcohol concentration in air and use value of Load resistancethat( RL) about 20 Kohm;(10K ohm; to 47K ohm;)."
We used a 20K ohm resistance. (In case of specific used for more details is better to contact the manufacturer)
Sensor set Arduino: Gas Sensor MQ-135 + Humidity Sensor HIH-4030 + Temperature Sensor TMP36: