Persistent environmental chemicals

Polychlorinated biphenyls (PCBs section 1.2) and dioxins have been most widely studied in this category. PCBs were used in a wide variety of industrial applications, for example as dielectrics in transformers. But they are very persistent contaminants, both in the general environment and in human fat. In theory the routes of entry into food are  

In practice there is little evidence for the migration of PCBs from packaging (JFSSG, 1999a). There is considerably more evidence for the other two routes leading to PCBs in food. Indeed, historical trends can be drawn up for residues of PCBs in human fat, breast milk and fish. There has been a very gradual decline in levels of these organochlorine compounds in the environment, food and human tissues. This is entirely consistent with the persistence of these compounds. 

There has been considerable work on the individual congeners of PCBs, so much so that analysis for these is now commonplace. Toxicological review of these substances has also been extensive. This has identified dioxin-like PCBs, probably the first time that such close parallels have been drawn between the toxicological and other properties of two major groups of chemical contaminants in food. 

Dioxins in food and the environment have been intensively studied over the past twenty years (see for example Steering Group on Chemical Aspects of Food Surveillance, 1992a). The term dioxins has come to be used for polychlorinated dibenzo-p-dioxins and in some cases also polychlorinated dibenzofurans. Both of these are ubiquitous environmental contaminants. They are highly resistant to breakdown in the environment. They are particularly difficult to study because of the large number of substances involved and the very low levels of detection needed. Nevertheless, much surveillance work has been done on polychlorinated dibenzo-p-dioxins, particularly 2,3,7,8-tetrachlorodibenzo-p- 

There has been a huge amount of analytical work on metals such as lead in food. Indeed a large part of the periodic table has been covered. Early work on metals identified that analytical quality assurance is a key tool in the surveillance of food for chemical contaminants. It also led to the development of toxicological standards which can be used to define whether or not surveillance results show there is a hazard to consumer health. Both of these types of approaches are now standard in the best surveillance programmes, whether they are on contaminants or additives in food. 

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The exposure pathway and physicochemical characteristics of a particular environmental chemical can provide useful information on the likelihood of childhood exposure (chapter 5). Age-specific exposure pathways of particular relevance include those during the prenatal and early developmental years postnatally (e.g. placental transfer, breast milk, toys, soil, indoor air/dust, child-care centres). Exposures to persistent environmental chemicals are of special concern, since the internal exposure may continue and result in a level sufficient to cause effects during critical developmental stages, even after the external exposure has ceased or been removed. 

As long as chemical regulation is based on this risk-based philosophy, human and environmental exposure to dangerous chemicals - substances of very high concern - will continue. The disperse and dilute model does not work for persistent bioaccumulative chemicals because Nature quite simply collects and concentrates these materials overtime. 

Scheringer, M. Persistence and Spatial Range of Environmental Chemicals New Ethical and Scientific Concepts for Risk Assessment (Wiley-VCH) Weinheim i.a. 2002... 

Insect resistance and environmental pollution due to the repeated application of persistent synthetic chemical insecticides have led to an Increased interest in the discovery of new chemicals with which to control Insect pests. Synthetic insecticides, including chlorinated hydrocarbons, organophosphorus esters, carbamates, and synthetic pyrethroids, will continue to contribute greatly to the increases in the world food production realized over the past few decades. The dollar benefit of these chemicals has been estimated at about 4 per 1 cost (JJ. Nevertheless, the repeated and continuous annual use in the United States of almost 400 million pounds of these chemicals, predominantly in the mass agricultural insecticide market (2), has become problematic. Many key species of insect pests have become resistant to these chemicals, while a number of secondary species now thrive due to the decimation of their natural enemies by these nonspecific neurotoxic insecticides. Additionally, these compounds sometimes persist in the environment as toxic residues, well beyond the time of their Intended use. New chemicals are therefore needed which are not only effective pest... 

The effects of a chemical in a tissue frequently depend on the chemical s interaction with cell surface or cytoplasmic receptors. In some cases, a chemical interacts directly with the cell membrane and alters its permeability. The pharmacodynamic actions of drugs are usually mediated by interactions with a receptor, and a drug often competes with endogenous ligands of a receptor. The toxicity of environmental chemicals can also depend on and be mediated by interactions with receptors. In some cases, the responses are different for chemical exposures at different fetal stages of development, and it is possible to explain the different responses by the chronology of the development of fetal receptor systems. The fetus may develop receptor systems for a compound before it develops the ability to metabolize that compound thus, a low level of an active chemical can have greater and more persistent effects in the fetus than in the mother, whose metabolism limits the duration and extent of the effect. This is one mechanism for selective developmental toxicity of chemicals. 

Much of the effort on environmental chemicals that contaminate food has concentrated on a small range of chlorinated chemicals. In addition to the chlorinated PCDDs, PCDFs and PCBs already mentioned, other chlorinated compounds can be separated into two groups chlorinated aromatic compounds and chlorinated aliphatic compounds. Although there is a number of organochlorine pesticides that are persistent in the environment, these will not be considered here, as they comprise an extensive field of study in their own right. 

Environmental fate models make use of chemical properties to describe transfer, partitioning, and degradation (Mackay et al. 1992a Cahill et al. 2003). For organic chemicals, quantitative structure-property relationships (QSPRs) may be used to predict partitioning from physical-chemical properties, such as Kow and Kov Such properties may also allow for a prediction of the transfer of chemicals between compartments. Recently, some successful attempts have also been made to predict persistency of chemicals (Raymond et al. 2001), although this mainly concerns... 

Green N., Bergman A. (2005) Chemical reactivity as a tool for estimating persistance. Environmental Science and Technology 39 480A-486A. 

The extent to which a POP is reconcentrated in terrestrial and aquatic food webs is a function of its persistence, physical-chemical properties and properties of the receiving system. According to Kelley et al (2007), persistent compounds with values of log ATo between 4—8,5-7 and 6-8 and log Koa above 8 that do not metabolize will biomagnify up to 400,8000 and 4000 times in terrestrial mammalian, marine mammahan and human food chains, respectively. Compounds with values of log Kq from 6 to 8 will biomagnify up to 75 times in aquatic food webs. The process of environmental transport and incorporation into food supplies will be accelerated if the compound is within biosolids applied to agricultural lands, in wastewater effluents discharged to surface waters and in dust deposited to landfills adjacent to agricultural lands, and if industrial facilities that use the compound are located nearby our sources of food. 

The acronym, POP, is gaining world-wide acceptance, although some national agencies still use other terms, e. g. persistent environmental pollutants (PEPs), for these chemicals. The chemical industry, for instance, terms them persistent, bioaccumidative, toxic substances (PBTs). The US Environmental Protection Agency (EPA) prefers bioaccumulative chemicals of concern (BCCs). Much of... 

The rate of chemical processes affects many facets of our lives. Aspirin is an effective antiinflammatory agent because it rapidly inhibits the synthesis of prostaglandins (Section 19.6). Butter turns rancid with time because its lipids are only slowly oxidized by oxygen in the air to undesirable by-products (Section 15.11). DDT (Section 7.4) is a persistent environmental pollutant because it does not react appreciably with water, oxygen, or any other chemical with which it comes into contact. All of these processes occur at different rates, resulting in beneficial or harmful effects. 

Drug metabolising enzymes were developed during evolution to enable the body to dispose of lipid-soluble substances such as hydrocarbons, steroids and alkaloids, that are ingested with food. Some environmental chemicals may persist indefinitely in our fat deposits, e.g. dicophane (DDT), with consequences that are as yet unknown. 

White, R., S. Jobling, S.A. Hoare, J.P. Sumpter, and M.G. Parker. 1994. Environmentally persistent alkylphenolic chemicals are estrogenic. Endocrinology 135 175-182. 

Wagner, Hazardous Waste. PCBs C. Q. Gustafson, PCBs—Prevalent and Persistent, Environmental Science and Technology. vol. 4, pp. S14—S19 (1970) U.S. Aides Seek to Calm Fears over Chemical PCB, New York Times, Sept. 30, 1971 Polychlorinated Biphenyls and the Environment, Interdepartmental Task Force on PCBs Report COM-72-10419, May 1972. 

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