ABSTRACT
Everyone who has watched a sunbeam knows that the air is full of particles. They are not exactly constituents of the air like oxygen, and most are not exactly pollutants either. The fate of particles that reach the respiratory system depends on their size, shape, and density (see Fig.4.1). Large particles are collected by the nasal sinuses and throat, and are eventually swallowed. Small particles penetrate farther into the respiratory system and may reach the alveoli. All of this is normal and usually harmless. A few classes of particulate matter are of more concern, especially if they remain trapped in the bronchial tubes or alveoli. The health effects caused by particulates range from a sneeze to lung cancer.
