Particle deposition is an important phenomenon that prevents the majority of inhaled particles from reaching alveoli. Particle size and inspiratory flow rate are important determinants of particle deposition in upper airways. Deposition occurs in the nose and upper airways for particles of 10 μm or greater in diameter. Particles between 5 and 10 μm in diameter settle in the trachea, conducting airways, and alveoli. Particles 5 μm or less in diameter usually do not deposit in the lungs. Increase in the velocity of inspiratory flow also promotes deposition of particles by impaction in upper airways.