Particulate pollutants bigger than  microns are likely to lodge in the nasal passage whereas particles of about microns enter to the lungs easily  and they size are respectively in microns,

Particulate pollutants > 5 microns are likely to lodge in the nasal passage. Particulate pollutants < 1 micron, enter into lungs easily.  If x > 5 microns – nasal passage is blocked, 1 micron – enter into lungs .