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Complex substances such as bitumen do not lend themselves to toxicokinetic analysis as the properties and interactions of the individual constituents will influence the toxicokinetic behaviour. Toxicokinetics of some individual constituents, such as specific volatile organic compound and polyaromatic hydrocarbon (PAH) species have however been studied in more detail (Syracuse Research Corporation, 1985). The main routes for bitumen exposure in humans are inhalation and dermal. The major sites of potential uptake of constituents of bitumen in humans are the lungs and respiratory tract, after inhalation exposure to emissions from bitumen, and the skin, as a result of contact with neat bitumen, cutback bitumen or condensed fumes from bitumen. In general, the individual constituents of bitumen and fumes from bitumen undergo oxidative metabolism, which may lead to bioactivation. Whole body distribution of PAHs has been studied in rodents. These studies have demonstrated that low but detectable levels of PAHs may be found in internal organs, especially adipose tissue which may serve as a storage depot. In general, PAH are eliminated by urinary or biliary excretion of metabolites (ATSDR, 1999; IPCS, 2005). 

 

A limited number of laboratory studies have been done with bitumen solutions, condensed fumes and fumes from bitumens. Also a limited number of studies have been performed in humans, either in volunteers or in workers. From these studies it is clear that some constituents, including PAH, can be absorbed through the skin and be taken up via the lungs. Dermal uptake was shown to be a function of viscosity and the bioavailability of neat bitumen is deemed to be negligible. The bioavailability of PAH from cutback bitumens is expected to be higher, but based on experimental data the amounts of PAH becoming available from the bitumen itself are too low to pose a carcinogenic hazard (Potter et al., 1999; Brandt et al., 1999; Potter et al., 1995). Experiments also show that PAHs in condensed fume from bitumen are bioavailable when the condensate is directly applied to the skin (Roy et al., 2007). Studies in human volunteers showed that under rather extreme experimental conditions, the uptake through the skin of 3- and 4-ring PAH from fumes from bitumen could account for about half of the total exposure (Knecht et al., 2001; Walter and Knecht, 2007). Studies in workers, under normal conditions as they occur during paving, gave evidence of dermal absorption and subsequent systemic availability of 2- to 4-ring PAH through the determination of urinary metabolites.