Exposure tests in industrial toxicology
The purpose of exposure tests for industrial poisons or other hazards is to determine if sufficient quantities of the harmful agent are being (or have been) absorbed to cause intoxication. In interpreting such tests, the three major factors which determine whether or not intoxication occurs should be considered. The first of these is the time factor. Over a short period the probability of intoxication may be proportional to the length of exposure. Over longer periods this is not true but as a general rule, the longer the exposure the greater the likelihood of injury. The second factor is the susceptibility of the individual. This cannot be measured quantitatively and in many instances, cannot even be qualitatively estimated. This factor not only varies among different individuals but will, undoubtedly, change considerably with the same individual from time to time, reflecting stresses from infections, fatigue, and other causes. The third factor, and the one which concerns us primarily, is the concentration of the toxic agent; not the concentration in the environment nor even, ordinarily, in the body as a whole but the concentration in that Organ or tissue of the body which is most sensitive to attack by the toxic agent in question. For purposes of simplicity we will call this the critical organ. In Figure 1 a highly simplified version of the flow of a toxic substance through the body is depicted. It is seen that there are four primary media which we can analyse, indicated by the asterisks. These are the air, blood, urine, and breath. None of these analyses will give us directly the information we want most, namely, the concentration in the critical organ (Ce) which may be the brain or liver for example. Ordinarily it is impossible to measure this value directly (except, perhaps, at autopsy) and we can only infer that it rises or falls with increases or decreases in the concentrations which we can determine. There are, of course, analyses which can be made—other than those mentioned. Environmental tests may include analysis of water or food, wipe tests of bench tops, etc. or the contamination of the skin can be determined. Biological specimens sometimes analysed include lung tissue, fat and skin, as well as blood, while other excretion products, in addition to urine and breath, are sweat, fces, hair, and nails. For purposes of classification we can divide industrial poisons into four categories: (1). Volatile vapours or gases relatively insoluble in water; examples— methyl chloroform, ether, and hexane. The solubilities of these substances in blood and urine are not high, and the analysis of breath is the most