Toxic chemicals, persistence

Measurement of exposure can be made by determining levels of toxic chemicals in human serum or tissue if the chemicals of concern persist in tissue or if the exposure is recent. For most situations, neither of these conditions is met. As a result, most assessments of exposure depend primarily on chemical measurements in environmental media coupled with semi-quantitative assessments of environmental pathways. However, when measurements in human tissue are possible, valuable exposure information can be obtained, subject to the same limitations cited above for environmental measurement methodology. Interpretation of tissue concentration data is dependent on knowledge of the absorption, excretion, metabolism, and tissue specificity characteristics for the chemical under study. The toxic hazard posed by a particular chemical will depend critically upon the concentration achieved at particular target organ sites. This, in turn, depends upon rates of absorption, transport, and metabolic alteration. Metabolic alterations can involve either partial inactivation of toxic material or conversion to chemicals with increased or differing toxic properties. 

The Danish List of Undesirable Substances is a list of chemicals of concern that the government believes should be avoided to the extent feasible in commerce. Using a systematic analysis, substances are selected automatically if they meet some clear and defined criteria, for example, problematic classifications, because they are imder suspicion for being PBT/vPvB (Persistent, Bioaccumulative, Toxic/very Persistent, very Bio accumulative) or endocrine-disrupting. 

Van Voris P, Cataldo DA, Ligotke MW, et al. 1987. Acute environmental toxicity and persistence of selected chemical agent simulants diisopropyl flourophosphate (DFP) and diisopropyl methylphosphonate (DIMP). NTIS No. AD-A181-309. 

Kellogg, S.T., Chatterjee, D.K., and Chakrabarty, A.M., Plasmid-assisted molecular breeding New techniques for enhanced biodegradation of persistent toxic chemicals, Science, 214, 1133-1135, 1981. 

Monitoring of background concentrations of oxychlordane in wildlife, since this metabolite is more toxic and persistent than the parent chemical (Kawano et al. 1988)... 

Chlordane is readily absorbed by warm-blooded animals via skin, diet, and inhalation, and distributed throughout the body. In general, residues of chlordane and its metabolites are not measurable in tissues 4 to 8 weeks after exposure, although metabolism rates varied significantly between species. Food chain biomagnification is usually low, except in some marine mammals. In most mammals, the metabolite oxychlordane has proven much more toxic and persistent than the parent chemical. 

The chlorinated chemicals assessed do not have the same risk profile. For the more volatile chemicals the safety margins between the actual exposure and the level at which no effect on the environment would be expected is quite high. For more persistent chemicals there is a need to look to the environmental compartment where they can be accumulated (mainly in sediments and biota). For some of these chemicals the safety margin is quite low and in worst-case situations serious effects may occur. For the very persistent, bioaccumulative and toxic chemicals (like dioxins, PCBs and DDT), acceptable environmental concentrations are so low and difficult to control that the industry is committed to reducing as far as possible releases to the environment through application of Best Available Techniques (BAT), mainly with respect to dioxins. For other chemicals (PCBs, DDT), production has already been halted for some years. 

The so-called persistent organic pollutants (POPs) are the subject of the Rio Declaration (1994) and the Stockholm Convention (2001) these international agreements (yet to be ratified in all signatory countries, including the United States) call for the elimination from production of 12 persistent chemicals, including the chlorinated pesticides and PCBs mentioned above (all of which have already been eliminated from production in the United States). Current regulatory efforts in the European Union and the United States place emphasis on elimination or restriction of all PBTs (persistent, bioaccumulative, and toxic chemicals). 

There are occasional outbursts of skepticism about the value of the risk assessment approach. Some argue its full use is overly burdensome, that the only real need is to identify toxic chemicals and then eliminate them or reduce their presence. Perhaps this approach is appropriate for a few chemicals that exhibit extremely high toxicity or that are excessively persistent and bioaccumulate in an environment, and for which there are reasonably good substitutes available. But as a general approach, it ignores the elementary principle that all chemicals will... 

Article 54 a-f. Criteria for chemicals to be added to Annex XI11 and thus require authorisation category 1 or 2 carcinogens, category 1 or 2 mutagens, category 1 or 2 reproductive toxins, substances which are persistent, bloaccumulative and toxic, very persistent and very bloaccumulative, endocrine disrupting or of equal concern. 

Williams, B. L., Brodie, E. D., Jr., and Brodie E. D., Ill (2004). A resistant predator and its toxic prey persistence of newt toxin leads to poisonous (not venomous) snakes. Journal of Chemical Ecology 30,1901-1919. 

Governments from around the world agreed to eliminate or minimize use of toxic chemicals that persist in the environment, now called persistent organic pollutants (POPs), which include many chlorinated hydrocarbons such as DDT and PCBs. The accord allows some developing countries to continue using DDT for controlling malaria. It also allows use of PCBs in electrical equipment until 2025, provided the equipment is maintained to prevent leaks. 

The US EPA lists chlordane as a persistent bioaccumulative toxic chemical. In 1978, the EPA cancelled use of chlordane on food crops and by 1988 all use was banned. 

There is further analogy with yet "extra concern for toxic chemicals that may also persist in the environment and be transported great distances from their point of entry into the environment. Here the unifying general notion is that unsuspecting individuals are placed at risk, and are thus less able to defend than are the perpetrators. Mercury is a classic example of such a chemical. Mercury is extremely toxic to the CN S. Fetuses, infants, and toddlers are especially sensitive and susceptible to the neurotoxic properties of mercury. Mercury also persists in the environment, and is known to bioaccumulate in the food web and biomagnify up the food chain. 

Industrial research and development increasingly incorporates the principles of green chemistry and development of safer substitutes, but safer has usually meant less toxic. However, persistence should also be considered in product design. Chemicals that persist remain potentially available to biota to exert toxic effects, not all of which may be known or predictable at the outset Persistent chemicals that are bioaccumu-lative are of even greater concern because levels may be achieved in organisms that... 

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