I don't have any field expierience for that sensor, but I would put them where it matters.
You want to monitor the air quality, so you should monitor it in "breathing" locations and height. I wouldn't put it directly next to an exhaust of the CNC machine, but more to the input device of the CNC machines.
Same goes for the ventilation of the room. Don't put it next to the supply air vent. Your readings would always be good there and in reality the people are breathing "poison".
The same goes for the height. Heavy particles would stay in the lower area, so the sensor would never catch them in a height of 5 meters(or whatever the height of the facility is).