Page 304 - Industrial Ventilation Design Guidebook

260 CHAPTER 5 PHYSIOLOGICAL AND TOXICOLOGICAL CONSIDERATIONS

6/ood Solubility
Absorption of a gaseous compound from the lungs depends on its
blood solubility. For most compounds, blood solubility is similar to water
solubility. However, the blood solubility coefficient may become much
higher than the water solubility coefficient if the blood proteins have a high
affinity for the compound. Carbon monoxide, for which hemoglobin has a
high affinity, is a good example (see Section 4.3.3). Blood solubility is deci-
sive for the rapidity of the action of the compound, especially on the central
nervous system, but also on other organs. Often lipid-soluble vapors such
as diethyl ether or organic solvents such as xylenes also have a high blood
62 63
solubility. '
The toxic effect depends both on lipid and blood solubility. This will be il-
lustrated with an example of anesthetic gases. The solubility of dinitrous ox-
ide (N-jO) in blood is very small; therefore, it very quickly saturates in the
blood, and its effect on the central nervous system is quick, but because N 2O
is not highly lipid soluble, it does not cause deep anesthesia. Halothane and
diethyl ether, in contrast, are very lipid soluble, and their solubility in the
blood is also high. Thus, their saturation in the blood takes place slowly. For
the same reason, the increase of tissue concentration is a slow process. On the
other hand, the depression of the central nervous system may become deep,
and may even cause death. During the elimination phase, the same processes
occur in reverse order. N 2O is rapidly eliminated whereas the elimination of
halothane and diethyl ether is slow. In addition, only a small part of halothane
and diethyl ether are eliminated via the lungs. They require first biotransfor-
mation and then elimination of the metabolites through the kidneys into the
62 63
urine. '
Partition Coefficients
Other important determinants of the effects of compounds, especially sol-
vents, are their partition coefficients, e.g., blood-tissue partition coefficients,
which determine the distribution of the compound in the body. The air-blood
partition coefficient is also important for the absorption of a compound be-
cause it determines how quickly the compound can be absorbed from the air-
space of the lungs into the circulation. An example of a compound that has a
high air-blood partition coefficient is trichloroethane (low blood solubility)
whereas most organic solvents (e.g., benzene analogues) have low air-blood
62 63
partition coefficients (high blood solubility). '
Vapor Pressure
Vapor pressure is important simply because a compound that is easily va-
porized can also readily cause a marked exposure through the lungs. Organic
solvents are good examples of volatile compounds, and known to cause
marked exposure via the lungs, in addition to exposure via the skin, 64

Porticle Siie
The size of inhaled particles varies markedly. The size distribution ap-
proximates a log-normal distribution that can be described by the median or
the geometric mean, and by the geometric standard deviation. For fibers, both