Uptake, Bioaccumulation, Excretion

Uptake and Excretion of Benzene, Toluene, Xylenes and Styrene by Man and Land Mammals 

About half of the inhaled benzene is exhaled again in less than 20 minutes [11, 12, 136, 196, 197, 199]. Given longer exposure times, the resorption rate is initially higher and then decreases [197]. Depending on the individual, resorption can be as high as 80% [195]. The resorbed benzene is deposited by the blood in the central nervous system and the bone marrow (18 times the blood concentration), in the fatty tissue of the peritoneum (10 times the blood concentration), in the heart (5 times the blood concentration) and in the brain (2.5 times the blood concentration [12, 136,197]. The distribution coefficient between air and blood is about 1 7 [197]. The benzene retention time in the system is quite long [197]. Approx. 80% have been oxidized and removed after three days, but even after 5-7 days traces of benzene can still be detected - for example, in the bone marrow, testicles, and the skeletal muscles [197]. The resorbed benzene is excreted almost entirely in the form of 

Similarly to toluene, xylenes are taken up by the lungs, through the skin or the mouth and pass into the lipid-rich tissues [197]. They are oxidized to toluic acids and xylenols, which are excreted with the urine in various forms - possibly as conjugates - with some of the fractions requiring somewhat longer than in the case of toluene [197]. 

When styrene is inhaled, about two-thirds are resorbed it can also be absorbed through the skin [197]. It accumulates in all organs, but principally in the liver, kidneys and blood and is also found in faeces [197]. It is oxidized at the double bond and decomposed to mandelic acid and phenyl glyoxylic acid which are easily identifiable in the urine [195]. Benzoic acid has also been detected in rabbits [197]. As with the xylenes, excretion takes about three days [197]. If guinea-pigs are exposed to only about 1 ppm styrene (0.005 mg/1), however, no mandelic acid is detected in the urine - only at higher concentrations [197]. 

Mathematical models concerning the uptake, transport and effect of nonionic poisons (benzene in particular) have been developed and compared with the results of experiments [136]. These revealed that the concentrations are purely additive [136]. 

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A very large amount of information is available on the levels of total copper in various compartments of the environment, but little information on copper speciation (WHO 1998). Copper is transformed in the environment to forms that are either more or less bioavailable, depending upon the physical and chemical conditions present in the environment of interest. The net uptake of copper by microorganisms, plants, and animals from the surrounding environment (water, sediment, soil, and diet) is defined as bioaccumulation . The species of copper present in environmental media and its associated bioavailability, together with differences in plant and animal uptake and excretion rates, determine the extent of bioaccumulation. 

Thus, overall, the bioaccumulation of PCBs is a function of the uptake of PCBs from water and food, and the losses due to metabolism, excretion, and growth dilution all are a function of time. A general model for estimating the concentration of PCBs (Cflsh) in fish is ... 

As mentioned previously and shown in Fig. 6.8, movement of a chemical in a system containing plants or animals may involve exchange with the air, soil, and water phases. Bioaccumulation results when the plant or animal uptakes the contaminant. In plants, the contaminant may be, in turn, released to the air by means of volatilization or to the soil with subsequent accumulation by phytostabilization, adsorption, and sedimentation or transport to the aqueous phase by advective, diffusive, or dispersive processes. Excretion of the contaminant or a metabolite from animals may also be received by any compartment. 

Expected to be readily biodegradable. The estimated log Kow value is > 6.2, which indicates a high potential for bioaccumulation, however, their uptake into fish is expected to be hindered by their large molecular size. If taken up by fish, these carboxylate ester-containing substances are expected to be readily metabolized and excreted. Thus, the bioaccumulation potential is expected to be much lower than predicted from log Kow alone. 

Bioaccumulation results when uptake of chemicals by dietary and nondietary pathways exceeds metabolism and excretion. Rates at which substances are absorbed, altered, and then excreted are relatively important. Bioaccumulation may involve sequestration mechanisms, such as the deposition of polychlorinated biphenyls (PCBs) in fat, or the incorporation of lead in the mineral portion of bone. Incorporation into fat is dependent on the lipophilicity of the compound. The most commonly performed test of lipophilicity involves experimental determination of the equilibrium partitioning of a test compound between octanol, a nonmiscible organic solvent, and water, often expressed as the log10 of the ratio or the octanol/ water partition coefficient (log K ). Organic compounds in which the log Kow value is less than 3.5 do not appreciably accumulate in the lipids of mammals [5], Because energetic compounds have relatively low log Kow values (Table 10.1), bioaccumulation cannot be explained solely by lipophilicity. 

The term bioaccumulation factor (BAF) is a coefficient used to quantify the net body burden of chemicals (given uptake, distribution, metabolism, and excretion rates) and is expressed as the ratio of the concentration of a compound in the organism (or normalized to the lipid weight of the organism) to the concentration in its food or, for benthic or soil invertebrates, in the sediment or soil [9] (also see Chapter 6). Although by definition, BAF is measured at steady state, due to the complex nature of multiplepathway exposure, steady-state measures of BAF are often difficult to obtain. 

Fish bioaccmnulation and biomarkers in environmental risk assessment have been reviewed by Oost et al. [360]. Fish bioaccmnulation markers may be applied in order to elucidate the aquatic behavior of enviromnental contaminants and to assess exposme of aquatic organisms. The feasibility of PAH tissue concentrations in marine species as a monitoring parameter for PAH exposme depends on their uptake, biotransformation and excretion rates. Since it remains hard to accmately predict bioaccumulation in marine species, even with highly sophisticated models, analyses of tissue levels are required. The main problem is that PAHs do not tend to accumulate in fish tissues in quantities that reflect the exposme. The analysis of PAH metabolite levels in fish bile can be used to assess the actual PAH uptake, rather than the analysis of the non-hydroxylated PAHs content [328,361]. A number of sentinel fish species have been proposed to asses pollution by PAHs [325,326], as well as several mussels [322,323,326,352]. Several studies have also correlated the high levels of 1-OHPy and B(a)Py metabolites found in the bile of cat-shark with contamination sources such as boat traffic and combustion-based industries present in the sampling area [362]. 

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