PRINCIPLES OF TOXICOLOGY

suspended in air over time. Some terms that are also used are dusts, fumes, smokes, mists, and smog.
Dusts, which result from industrial processes such as sandblasting and grinding, are considered to be
identical to the compounds from which they originated. In contrast, fumes usually result from a
chemical change in compounds during processes such as welding, in which combustion or sublimation
occurs. Smokes result when organic materials are burned; mists are aerosols composed of water
condensing on other particles; and smog is a conglomerate mixture of particles and gases that is
prevalent in certain environments such as areas with mountains, plenty of sunlight, and periodic
temperature inversions. The toxicity of inhaled particulates has been known for a long time, especially
in relation to occupational exposure. The early (1493–1541) famous toxicologist Paracelsus described
the relationship between mining occupations and pulmonary toxicity in the sixteenth century.
Particle Size
9.1 LUNG ANATOMY AND PHYSIOLOGY 175
Figure 9.6 Scanning electron micrograph showing interior of an alveolus and its pores of Kohn. (Reproduced
with permission from D. V. Bates et al., Respiratory Function in Disease. An Introduction to the Integrated Study
of the Lung. Saunders, Philadelphia, 1971.)
In the case of particulates, size is the primary critical determinant of how much of and where the agents
will be deposited. The range in particle size for various aerosols is generally as follows: dusts, up to
100 µm; fumes, from 10 Å to 0.1 µm; smokes, less than 0.5 µm. The pattern of airflow in the respiratory
system and anatomic features of the exposed individual are also important.
Most inhaled particles are not spherical, but highly irregular in shape. In order to categorize the
highly heterogeneous nature of inhaled particles, the aerodynamic diameter is calculated for the
population of particulates of interest. This value is based on the settling velocity of the population of
particles and roughly approximates what the particles’ diameter would be if it were compared to a