Stoke's Law is used in several ways. We can predict settling rate for a given particle if its diameter and density are known. When air is sampled for particulate matter, particles are collected according to the effective or Stoke's diameter. Since particulate matter varies widely in its composition and size, an average value is used to describe experimentally collected aerosols. The calibration plots from the standard particulates are then used to determine the percentage of mass less than a specified diameter of actual air samples.
Particulates affect health by transporting toxic substances into the respiratory tract.
If the particles Figure 34.2 shows the efficiency with which particulate matter is retained in the respiratory tract for mouth breathing. The particles that are not retained are small enough to navigate the air passages of the mouth and throat, and move slowly enough that they are expelled when a person exhales. Unfortunately, the tiny particles that are retained deep in the lungs contain the highest concentrations of toxic substances. Figure 34.2 Respiratory tract deposition efficiency for aerosols resulting from mouth breathing.