Particle counter project - Some questions

I would like to make a project for a particle counter. To do this, I’ll connect in a box one or two dust sensors to an Arduino (or clone).

I wish to know which part of the dust sensor need to be exposed to the outside, and if necessary create an air flow circuit, and why it is necessary that the sensor be in a vertical position. What would happen if it were in a landscape position?

Is it better to use two sensors and get an average of its measurements?

Thank you!