Environmental Pathways of Toxic Chemicals

Two general types of processes govern the movement of chemicals through the environment  

Both forms of transport depend on environmental media, i.e., water, air, and soil or sediment (note soil and sediment are treated as a single environmental medium for the purposes of this discussion). Stated differently, diffusive and advective transport depend on the abiotic environment (with one notable exception, as discussed in Section 2.5). A third process that affects chemicals in the environment, chemical transformation, plays out in the abiotic environment as well as in the biotic (living) environment, e.g., in microorganisms such as bacteria. Chemical transformation results in changes in the number of atoms in a molecule and their arrangement in space. Changes in molecular architecture can have profound effects on the 

Essentials of Toxic Chemical Risk Science and Society 

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Envlroiunental testing Is a critical element In this process since It enables the qualitative and quantitative determination of toxic chemicals In the environment and the definition of environmental pathways which may lead to human exposure This paper briefly reviews the overall process of health risk assessments and the particular role which environmental testing plays Recent efforts to assess environmental health risks In relation to Love Canal Illustrate both the usefulness and the limitations of environmental testing In risk assessment ... 

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. 

As discussed in the introduction, there are an innumerable numbCT of environmental chemicals for which little or no toxicity information is available and for many others for which it is inadequate. HTS studies could be invaluable at three related areas. First, they could identify those chemicals with the greatest potential toxicity and hazard which would then be prioritized for further testing. Second, they may be able to identify pathways of toxicity. Third, they may be able to identify and quantify metabolic pathways. 

The four steps of the risk-assessment process are hazard identification, analysis of exposure, analysis of effect, and risk characterization. In the hazard identification step, the risk assessor identifies chemicals of concern, environmental pathways of exposure, and populations and subpopulations at risk. The exposure analysis develops exposure scenarios and estimates the chronic daily intake of each chemical of concern. In the analysis of effect, the risk assessor combines the chronic daily intake calculated in the exposure analysis with toxicity data from animal studies (and/or human epidemiological studies, if available) to estimate the risk of toxic effects in exposed populations, whereby risks to public health are divided into two broad categories noncancer health effects and cancer. The final step of the risk-assessment process, risk characterization, is a narrative that marshals all the evidence of risk to public health, including quantitative risk assessments and qualitative evidence of risk. The risk assessor weighs all the evidence and uses professional judgment to draw conclusions about risks. 

Tlie knowledge of the chemical pathways of degradation is also of interest for forecasting die intermediate products, their time evolution, the treatment times, and the eventual toxicity of the effluent, because changing the process conditions could form different species at dissimilar concentrations. This problem would address die environmental compatibility of the process. 

Toxicants are released into the environment in many ways, and they can travel along many pathways during their lifetime. A toxicant present in the environment at a given point in time and space can experience three possible outcomes it can be stationary and add to the toxicant inventory and exposure at that location, it can be transported to another location, or it can be transformed into another chemical species. Environmental contamination and exposure resulting from the use of a chemical is modified by the transport and transformation of the chemical in the environment. Dilution and degradation can attenuate the source emission, while processes that focus and accumulate the chemical can magnify the source emission. The actual fate of a chemical depends on the chemical s use pattern and physical-chemical properties, combined with the characteristics of the environment to which it is released. 

Catal5i ic reactions are commonly used in environmental technologies with applications ranging from pollution abatement to pollution prevention. Examples include complex hiocatalytic pathways embedded into natural microbial communities for the bioremediation of toxic compounds into harmless end products, and new and cleaner catalytic synthetic routes for the production of bulk and fine chemicals [1]. The Organization for Economic Cooperation and Development (OECD) has recognized enzyme technology as an important component of sustainable industrial development [2,3]. 

Creating synthetic versions of useful natural products certainly benefits society by producing cheaper compounds. However, synthetic chemistry can also be environmentally friendly. Improved synthetic pathways can reduce the amount of toxic by-products formed during some chemical reactions. In addition, the availability of synthetic compounds eliminates the need to continually harvest large quantities of rare plants or other organisms in order to isolate the natural product. 

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