Biological monitoring of exposure to chemicals in the environment and workplace The relationship between external and internal doses of organic solvent vapours

INTRODUCTION According to an evaluation made by the International Agency for Research on Cancer (IARC), 66 agents or exposures have been recognised as human carcinogens. About 66% of all cancer cases occur in people over 65 years of age. The implementation of effective measures for reducing the human disease burden depends primarily, although not entirely, on our knowledge of what causes the disease. For instance there is a general consensus that about 50% of cancers would be avoided if the existing aetiological knowledge was applied. In most western countries the objectives of biological monitoring is to secure the health of the worker. Generally monitoring toxic chemicals in the air of the workplace is commonly used as a measure of the degree of exposure. However, the information obtained from air monitoring is insufficient if the chemical penetrates the skin or if respirators are in use. The more we learn about the way chemicals enter the body, their physical activities, and the physiological and health status of the worker to health risks of the workplace, the more we recognise the inadequacy of air monitoring and the more we appreciate biological monitoring as a powerful tool in the evaluation of chemical safety in the workplace. The dose-effect relationship is an important concept in toxicology. In inhalation, dose is usually expressed as a product of exposed concentration and duration (Table 1) referred to as ‘external’ dose. On the other hand the amount of chemical that has entered the body is referred to as the ‘internal’ dose. Toxic response is thus more closely related to the internal dose than the external dose. Biological monitoring of exposure is of practical use only when the relationship between external and internal doses is known. This relationship will be discussed using a commonly commercial solvent, Trichloroethylene (TRI), as an example. It is common knowledge that TRI is a CNS depressant and is carcinogenic in experimental animals. Neurological effects have been ascribed to exposures of TRI at concentrations as low as100–200 ppm. A study done by a scientific group on TRI compared the external and internal doses in a controlled environment of a worker on two separate occasions. The internal dose resulting from intermittent exposure was compared with an internal dose resulting from continuous exposure. Continuous exposure was measured in a standard man inhaling 50 ppm of TRI for 8 hours. Intermittent exposure was measured for the same man two weeks later working in an environment where the exposure to TRI alternated between 0–100 ppm over 1 hour. The total duration for exposure in both cases was 8 hours. During continuous exposure, the TRI concentration rose rapidly in the vessel rich tissue, slowly in muscles and very slowly in fatty tissue (Figure 1). At the end of exposure, concentration in both vessel-rich tissue and muscle approached steady state, whereas the fatty tissue still had a vast capacity to retain TRI. After exposure, the TRI concentration declined rapidly in the vessel-rich tissue, slowly in muscle and very slowly in fatty tissue. Sixteen hours after exposure the concentration in the fatty tissue was still 140 times higher than in the vessel-rich tissue. Dur ing four hours in te rmi t ten t exposure to 100 ppm, vessel-rich tissue rapidly responded to exposure changes in inhaled TRI. Muscle tissue also reflected the changes, although more slowly than in the vessel-rich tissue. However, the concentration in