Lead toxicity indirect exposure

The bioaccumulation of a substance into an organism is not an adverse effect hazard in itself. Bioconcentration and bioaccumulation may lead to an increase in body burden which may cause toxic effects due to direct and/or indirect exposure. Bioaccumulative substances characterized by high persistence and toxicity, negligible metabolism and a log ATow between 5 and 8 may represent a concern when widely dispersed in the environment. The potential of a substance to bioaccumulate is primarily related to its lipophilicity. A surrogate measure of this quality is the n-octanol - water partition coefficient (/fow), which is correlated with bioconcentration potential. Therefore, /fow values are normally used as predictors in quantitative structure - activity relationships (QSARs) for bioconcentration factors (BCFs) of organic non-polar substances. 

Toxic substances can interfere with normal neurotransmission in a variety of ways, either directly or indirectly, and cause various central effects. For example, cholinesterase inhibitors such as the organo phosphate insecticides cause accumulation of excess acetylcholine. The accumulation of this neurotransmitter in the CNS in humans after exposure to toxic insecticides leads to anxiety, restlessness, insomnia, convulsions, slurred speech, and central depression of the respiratory and circulatory centers. 

These comments deal with environmental levels likely to produce toxic responses at the indicated species level. The focus has been on the probability of risk groups undergoing ecotoxic responses, mainly for comparative purposes. We do not deal with complex effects that may be expressed at low lead exposures in systems and populations, or interplays between relatively larger groupings that occur or may occur because certain populations are reduced, directly or indirectly, by changes in environmental compartment levels of lead. Lead readily accumulates in sediments so that there may be different sedimentary populations that relate to that lead accumulation more detrimentally than others. This topic of subtle, large-scale ecotoxic effects as an aspect of concern largely unknown in the lead literature is presented in WHO s 1989 report on the ecotoxicity of lead (WHO, 1989) and sections of the current EPA Pb criteria document (U.S. EPA, 2006). 

Consequently, this review is designed to briefly summarize many of the available techniques for accurate measurements of environmental and human lead contamination. This includes the importance of ultraclean techniques for lead analysis as well as brief descriptions of some current and emerging analytical techniques for measuring lead exposures in humans. The descriptions are preceded by abbreviated discussions of the chemical properties of lead, natural and anthropogenic variations in its stable isotopic composition, and historical records of lead contamination in the environment. The report concludes with a summary of some indirect methods of measuring lead exposure and toxicity in humans. Much of the material in this report is based on reviews written for several recent reports Measuring Lead Exposure in Infants, Children, and Other Sensitive Populations (NRC 1993), Lead in the Biosphere Recent Trends (Smith and Flegal 1995), and In Vivo Measurement and Speciation of Nephrotoxic Metals (Smith and McNeill 1995). 

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