Exposure environmental media

The product of exposure analysis is an exposure profile. For chemicals, the profile should include the nature of the source pathways of exposure environmental media of concern (e.g., soils, water, sediments, contaminated biota) exposure concentrations (magnitude, timing, duration, recurrence) and uncertainties associated with these exposures. Analogous exposure profiles would be developed for nonchemical stressors included in an ERA. 

Exposure of the general population to hexachloroethane is expected to be low. Analysis of blood from 13 individuals, using a method with a detection limit of 0.028 ppb, did not identify hexachloroethane in any sample (Ashley et al. 1992). The chemical has not been frequently detected in any environmental medium. Ambient air is the most likely source of hexachloroethane for exposed individuals in the general population (Howard 1989). Due to the stability of hexachloroethane (see Section 5.3.2.1), it may remain in the atmosphere for extended periods. 

Cyanide metabolizes in the human body to thiocyanate, and its biodegradation products include ammonia, carbon dioxide, nitrate, or nitrogen (Richards and Shieh 1989). The detection of thiocyanate in body fluids may indicate cyanide exposure. Similarly, the amounts of cyanide degradation products formed in an environmental medium could be used to measure cyanide s biodegradation rate. A summary of methods for determining environmental degradation products is shown in Table 6-4. Suitable analytical methods are available to detect all of these compounds (Pettigrew and Fell 1973 Richards and Shieh 1989). 

Exposure, as we have seen, refers to an individual s contact with an environmental medium containing a chemical or, in some cases, with the chemical itself. The amount of chemical that enters the body as a result of the exposure is called the dose. 

An example of a concentration limit is a cleanup level, which is a type of an action level used in remediation projects. Pollutant concentrations in an environmental medium that are greater than the cleanup levels warrant a remedial action. Cleanup levels are often developed based on site conditions, such as future land use exposure pathways the presence of groundwater and the likelihood of its beneficial use the presence of specific ecological receptors background concentrations. 

Lethal concentration-50 (LC50) concentration of a potentially toxic substance in an environmental medium that causes death of 50% of the experimental organisms over a certain period of exposure. 

The mere presence of any single chemical or chemical mixture in the environment does not indicate that a health threat exists. An important step of mixture risk assessment is the evaluation of completed exposure pathways. Completed exposure pathways link together the source of contamination, environmental medium, point of exposure, route of exposure, and a receptor population. It means that without the potential for chemicals actually entering (or contacting) the human body, no threat is present. 

The residues and metabolites of a substance can be measured in an organism or an environmental medium. Alternatively, biological effects known as biomarkers that are known to be the result of exposure to a hazard can be used to determine exposure levels [131]. In some cases, monitoring biomarkers in employees (e.g., metabolites in urine) can prove cheaper than measuring airborne concentrations of a substance in the workplace [132],... 

Total human exposure Accounts for all exposures a person has to a specific contaminant, regardless of environmental medium or route of entry (inhalation, ingestion and dermal absorption). Sometimes, total exposure is used incorrectly to refer to exposure to all pollutants in an environment. Total exposure to more than one pollutant should be stated explicitly as such (NRC, 1991). 

Methods for Determining Parent Compounds and Degradation Products in Environmental Media. The levels of this conpound in environmental media can be used to indicate exposure of humans to this compound through the inhalation of air and ingestion of drinking water and foods containing N-nitrosodimethylamine. If a correlation with human tissue or body fluid levels were available, the intake levels from different environmental sources could be used to estimate the body burden of the chemical in humans. Such studies correlating the levels of this conpound in any environmental medium with the levels in any human tissue or body fluid are not available. 

Exposure Pathways. An exposure pathway for a toxic chemical consists of a means by which the chemical is released to the environment, its transport over a short or long distance to an area in which people may come in contact with it, which may or may not involve its movement from one environmental medium to another, and its coming into contact with the skin, digestive tract, or respiratory system of a person. 

In a review of previous work, DeLollis concluded that the nitrile-phenolic is the most durable adhesive, surviving all environmental exposures. The nylon-epoxy adhesive performed well on exposure to medium-humidity environments, but was susceptible to high humidities. The nitrile-epoxies showed good resistance to most outdoor exposures except the seacoast environment, where bonds failed after four years. Epoxy/polyamide bonds gave good results after four years exposure to an industrial environment and joints exposed to a tropical climate had retained about 33% of their initial strength after three years. This performance should be compared with that of some heat-cured adhesives, for which the bonds are completely degraded by tropical exposure. 

Ad hoc, site-specific statistical exposure modeling approaches have often been applied in the form of slope-factor relationships, whereby Pb in some environmental medium is linked to PbB via a numerical slope factor. In the case of air Pb, the slope factor is the ratio of community or larger area blood Pb level to an air Pb concentration. In their application, the product of a selected slope factor and air Pb value provides an estimated blood Pb figure for a cohort of Pb-impacted individuals. The currently reported range of slope factors using an air Pb is 5—10 and in some cases higher (Bmnekreef, 1984 Brunekreef et al., 1983 Hayes et al., 1994 Hilts, 2003 U.S. EPA, 1986, Ch. 11, Vol. 3 WHO, 2000). 

The Pb uptake module, where Pb uptake is indexed as pg Pb absorbed per day, is the interface between the exposure module and the biokinetic processing of absorbed Pb quantities to provide an output as PbB, in the form of a probabilistic expression. The daily amounts of absorbed Pb are calculated for each environmental medium through computations of Pb levels in the medium, amount of the medium ingested, and the bioavailability or fraction of daily Pb intake that is absorbed. Bioavailability can be the default selection diet and water Pb uptake of 50% soils and dusts, 30%. Alternatively, users can select bioavailabilities other than default if additional data exist for other percentage uptake rates. The specific topic of bioavailability of lead as a function of environmental medium containing Pb was described in the previous chapter. 

Lead is a multimedia pollutant, i.e., it provides exposures through diverse environmental media. The specific characteristics of each environmental contributor, such as Pb concentrations and intake amounts of some environmental medium, help determine the extent to which different individuals and populations sustain actual exposures. This also means that while some Pb sources can be characterized on a national, international, or other macroscale as being significant, the actual sources and pathways in specific cases for substances such as lead or other elements require specific evaluation. Some segments of human populations may sustain exposures from Pb in more than one environmental medium, in which case one employs methods to sort out potential contributions or the relative total contributions to total exposures. 

A second scientific factor that complicates effective Pb regulation and population exposure reduction efforts is the environmental medium-specific temporality and persistence of Pb in environmental media. Pb emitted from a stationary source to the atmosphere results in relatively transitory elevations... 

The interactive, feedback-loop relationships for lead among scientific research, risk assessments, and regulatory initiatives graphically depicted in previous chapters would predict that lead regulation in whatever environmental medium in the human environment arises from demonstrated threats to health shown by both scientific research and health-risk assessment. Consequently, discussions of lead in this and subsequent chapters require prefacing with brief perspectives on medium-specific lead exposures and associated human health hazards. This perspective differs from the previous detailed discussions in the health effects chapters in this book. Chapters 11—19. 

Whether a toxic chemical enters the environment by partitioning into water, soil, or air has profound consequences for exposure. Different environmental media are subject to different physical forces. Chemicals that have partitioned into a particular environmental medium are like hitchhikers that have no choice but to go wherever their ride takes them. Each ride in an environmental medium is a form of what is referred to as advective transport. 

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