Single chemicals effects

Single chemical effects are not specifically addressed here. Rather, the focus here is on illnesses that ensue following exposures to mixtures of chemicals that cannot be attributed to any one component of an exposure mixture. 

The chemicals listed in Tables 7.1 and 7.2 contain numerous hydro-philes and lipophiles. The number of mixtures possible is impossible to calculate. When people are stricken following exposure to polluted air, health effects are often attributed to an inordinately high concentration of a particular toxicant. An example of such a situation is what occurred in Bhopal, India, in 1984 when a huge quantity of methyl isocyanate was released from a chemical plant. Such single chemical effects, however, are the exception, rather than the rule. Most polluted air contains complex mixtures of chemicals that often produce effects that cannot be attributed to the known toxicology of the individual species. 

Flavoringingredient. Any single chemical entity or natural mixture added to food, dmgs, or other products taken in the mouth, the clearly predominant purpose and effect of which is to provide all or part of the particular flavor of the final product. 

The ECOTOXicology database is a source for locating single chemical toxicity data for aquatic life, terrestrial plants and wildlife. ECOTOX integrates three toxicology effects databases AQUIRE (aquatic life), PHYTOTOX (terrestrial plants), and TERRETOX (terrestrial wildlife). These databases were created by the U.S. EPA, Office of Research and Development (ORD), and the National Health and Environmental Effects Research Laborator) (NHEERL), Mid-Continent Ecology Division... 

When die hazard index exceeds miity, diere may be concern for potential health effects. While any single chemical with an exposure level greater than the toxicity value will cause die hazard index to e.xceed unity, die reader should note diat for multiple chemical exposures, die hazard index can also exceed unity even if no single chemical exposure exceeds its RfD. 

Catalytic processes frequently require more than a single chemical function, and these bifunctional or polyfunctional materials innst be prepared in away to assure effective communication among the various constitnents. For example, naphtha reforming requires both an acidic function for isomerization and alkylation and a hydrogenation function for aromati-zation and saturation. The acidic function is often a promoted porous metal oxide (e.g., alumina) with a noble metal (e.g., platinum) deposited on its surface to provide the hydrogenation sites. To avoid separation problems, it is not unusual to attach homogeneous catalysts and even enzymes to solid surfaces for use in flow reactors. Although this technique works well in some environmental catalytic systems, such attachment sometimes modifies the catalytic specifici-... 

For some toxins it is possible to demonstrate an apparent improvement in functional response at levels of exposure which are below a threshold. This effect, which has been termed hormesis , is most effectively demonstrated in the consistently improved longevity of animals whose caloric intake is restricted rather than allowing them to feed ad lib (Tannenbaum, 1942). Clearly in this instance, the observed effects are the result of exposure to a complex mixture of chemicals whose metabolism determines the total amount of energy available to the organism. But it is also possible to show similar effects when single chemicals such as alcohol (Maclure, 1993), or caffeic acid (Lutz et al., 1997) are administered, as well as for more toxic chemicals such as arsenic (Pisciotto and Graziano, 1980) or even tetrachloro-p-dibenzodioxin (TCDD) ( Huff et al., 1994) when administered at very low doses. It is possible that there are toxins that effect a modest, reversible disruption in homeostasis which results in an over-compensation, and that this is the mechanism of the beneficial effect observed. These effects would not be observed in the animal bioassays since to show them it would be necessary to have at least three dose groups below the NOAEL. In addition, the strain of animal used would have to have a very low incidence of disease to show any effect. 

Fig. 5.5.5 1 D CSI datasets showing the extent of conversion during a batch reaction. The form of the feature identified as peak B is associated with a single chemical shift i.e., it is of constant form at all positions across the bed, and therefore shows that the extent of conversion is uniform throughout the bed. The low intensity horizontal streaking" effect observed in these datasets and that shown in Figure 5.5.6 are artifacts arising from the automatic phase correction applied to the data ...

Chemical flowshops are widely used in industry for the medium-scale synthesis of chemicals. In a flowshop, a number of different physical units, such as reactor vessels, ovens, dryers, and distillation columns are placed in a serial, or largely serial, arrangement. Rather than being devoted to the preparation of a single chemical, many different chemicals are synthesized one after another in a flowshop by feeding in the appropriate precursors and reagents as the intermediates pass through most or all of the units. This mode of operation is an effective use of the physical plant because no part... 

Therefore, chemical reactivity is not necessarily an intrinsic property of a single chemical substance. The severity of reactive hazards is influenced by process-specific factors, such as operating temperatures, pressures, quantities handled, chemical concentrations, impurities with catalytic effects, and compatibility with other chemicals onsite. 

In short, liquid-liquid extraction has been employed predominantly and effectively not only for the pre-concentration and isolation of a single chemical entity just before its actual estimation, but also for the extraction of classes of organic compounds or groups of metals, just prior to their usual estimation either by chromatographic techniques or by atomic-absorption methods. 

For many, familiarity with the TSCA generally stems from its specific reference to polychlorinated biphenyls, which raise a vivid, deadly characterization of the harm caused by them. But the TSCA is not a statute that deals with a single chemical or chemical mixture or product. In fact, under the TSCA, the EPA is authorized to institute testing programs for various chemical substances that may enter the enviromnent. Under the TSCA s broad authorization, data on the production and use of various chemical substances and mixtures may be obtained to protect public health and the environment from the effects of harmful chemicals. In actuality, the TSCA supplements the appropriate sections dealing with toxic substances in other federal stamtes, such as the Clean Water Act (Section 307) and the Occupational Safety and Health Act (Section 6). 

The extracellular matrix of mammalian tissue is composed of a complex mix of constitutive proteins. This matrix must be broken down to recover single cells effectively for culture and/or staining (1). Tissue dissociation and its affiliated problems were described and defined over 80 yr ago by Rous and Jones (2). More recent reviews (3,4) have revealed newer methods for creating single-cell suspensions. Numerous procedures exist for dissociating solid tumors. They are usually multistep procedures involving one or a combination of mechanical, enzymatic, or chemical manipulations. Ideally, the dissociation protocol is individualized for the tissue of interest and evaluated relative to both optimal and representative cell yield. 

Advantages of animal studies include standardized test conditions, the possibility to detect far more effects through invasive measurements and postmortem examinations, the possibihty to investigate modes and mechanisms of action, and to investigate dose-effect relationships for single chemical substances. 

When epidemiological studies form the basis for the risk assessment of a single chemical or even complex mixtures, such as various combustion emissions, it may be stated that in those cases the effects of combined action of chemicals have been incorporated. Examples can, for instance, be found in the updated WHO Air Quality guidelines (WHO 2000). Thus, the guideline value for, e.g., ozone was derived from epidemiological studies of persons exposed to ozone as part of the total mixture of chemicals in polluted ambient air. In addition, the risk estimate for exposure to polycyclic aromatic hydrocarbons was derived from studies on coke-oven workers heavily exposed to benzo[fl]pyrene as a component of a mixture of PAH and possibly many other chemicals at the workplace. Therefore, in some instances the derivation of a tolerable intake for a single compound can be based on studies where the compound was part of a complex chemical mixture. 

The in vitro and in vivo test methods available to study combined actions and toxicological and biochemical interactions of chemicals in mixtures are essentially the same as those used for the study of single chemicals in order to examine their potential general toxicity and special effects such as mutagenicity, carcinogenicity, and reproductive toxicity. 

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