Paicle size which is inhaled by lung and causes damage is:

Paicle size and lung damage: Paicle size Remarks <0.5 micron Brownian motion 0.5 - 3 micron Most dangerous paicle size 3-5 micron Trapped by mid - respiratory tract 5-10 micron Trapped by upper - respiratory tract > 10 micron Settle by gravity