PM must be inhaled and deposited in different parts of the human body to cause adverse
health effects. Figure 2-4 shows approximately how much of different particle sizes deposit in
different parts of the human respiratory system. The extra-thoracic region consists of the nasal,
oral, pharyngeal and laryngeal airways. The main mechanisms for deposition in this region are
impaction and diffusion. Most particles in the PM10-2.5 fraction typified by fugitive dust are
removed by impaction in this upper part of the respiratory system, sometimes resulting in
allergic reactions and sinus ailments. The intrathoracic region includes the trachea, bronchial
airways, and alveoli in the lung. Particles reaching this region can cause lung damage and
potentially transfer to the bloodstream for transport to other organs. Most of the UP pass through
the upper respiratory tract and deposit in the lower respiratory tract. Deposition efficiencies
increase at higher breathing rates, such as those experienced during exercise.