PRINCIPLES OF TOXICOLOGY - Biology East Borneo

22.2 EXPOSURE LIMITS 529observation process, followed by recommendations and control measures. Even if the recommendedcontrol measures are taken, it is not clear that the workers are adequately protected. To overcome thisdeficiency, as well as for other reasons, biological exposure indices (BEIs) have been proposed, and,for some chemicals, have been adopted by ACGIH. In addition, OSHA requires biological monitoringin the form of measurement of blood lead levels as part of its lead standard; cadmium and2-microglobulin in urine, as well as cadmium in the blood as part of the cadmium standard, and, inemergency situations, urinary levels of phenol after exposure to benzene.BEIs are reference values for biological samples from exhaled air, urine, or blood. Theoretically,hair, nails, teeth, fat deposits, and other tissues could be used, but they are either more difficult to obtainor the chance of uncontrollable sample contamination may be too great. Each BEI has a specificdeterminant and biological specimen to be monitored, as well as a time to monitor relative to theexposure period. The determinant can be the chemical itself, a metabolite, or a characteristic reversiblebiochemical change induced by the chemical. The time of monitoring is determined by the eliminationhalf-life of the determinant. Timing becomes less critical as the elimination half-life increases.BEIs are similar to TLVs ® in that they are intended to indicate exposure level, and do not providesharp distinctions between “safe” and “unsafe” exposures. The major advantage of BEIs is that, atleast theoretically, they are capable of integrating all exposure routes to yield a better estimate of dose.However, BEIs cannot distinguish between nonoccupational and occupational exposure sources. SomeBEIs are non-specific and may be applicable to several chemicals so that in the event of multipleexposures, interpretation of biological determinants becomes more difficult.Biological exposure indices are subject to a variety of sources of variability. The same airborne,ingested, and skin exposures do not necessarily produce the same levels of the determinant in differentindividuals. Absorption, distribution, and metabolic processes (pharmacokinetics) vary from oneindividual to another. The choice of a determinant is based on the highest correlation with the level ofexposure, but the determinant may not best represent the dose to the target organ, which is the mostuseful indicator for evaluating the health risk to the individual worker.The use of BEIs is likely to increase with time, particularly as the techniques become more routine.Biological monitoring is not likely to completely replace the more traditional exposure assessmenttechniques. Rather, the two approaches will be used in tandem to complement each other and tominimize the respective weaknesses associated with each.Integrated ExposuresBecause exposures occur over a lifetime from many different sources, and sensitivities to a chemicalmay vary at different stages in life, the eventual public health goal should be to integrate occupationalexposures into an overall strategy for limiting chemical exposures throughout the lifetime of anindividual. Such a scheme would likely involve limits at varying ages and from varying sources: diet,air, and water as well as from occupational exposures. Computer models, which incorporate many ofthese considerations, are already well underway for specific chemicals such as lead. However, progressis likely to be slow and tedious because of the great amount of biological variation, large uncertainties indose–response data (particularly at low doses) for different substances, and disparate political/economicforces.Another factor to be considered is chemical interaction. Although synergism, potentiation, andantagonism are known to occur for specific chemical combinations, in current practice, exposures tochemicals which have similar effects are considered to be additive relative to their respective exposurelimits, because little information is available to the contrary. This is especially troubling becauseworkers and even the general population are routinely exposed to many chemicals simultaneously, andmost commercial products are mixtures of chemicals, not pure substances. Without more specificinformation, additivity is strictly an estimate of the combined effects and should be viewed withconsiderable suspicion. In these cases increased observation and monitoring of the physical well-beingof the workers and surveillance for the presence of symptoms of exposure should be done in additionto routine exposure monitoring.