Asphalt, oxidized

Endpoint summary

Description of key information

Studies have shown that PACs present in bitumen fume condensates can penetrate the skin. The rate of flux and the total amount absorbed was however low.

Key value for chemical safety assessment

Bioaccumulation potential:

low bioaccumulation potential

Additional information

Complex substances such as oxidized asphalt 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 exposure in humans are inhalation and dermal. The major sites of potential uptake of constituents in humans are the lungs and respiratory tract, after inhalation exposure to emissions from oxidized asphalt, 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). 

In an in-vitro dermal absorption study, 200 ul aliquots of neat bitumen fume condensate, containing low concentrations of PACs, was applied to the surface of human skin samples fixed in a Franz Diffusion cell. Samples of the receptor fluid were analysed at time intervals over a 48hr period following application. Results show that PACs present in bitumen fume condensate can penetrate the skin.

The rate of dermal flux (ng/cm²/hr±SD) for selected PACs was:

anthracene  6.5 ± 0.9;  fluoranthene-pyrene 1.8 ± 0.3; whilst the rate for total 3-6 ring PAC was 120 ± 30.  

The amount absorbed over a 48hr period was relatively low; measured values compared to applied dose were anthracene 5.3%, fluoranthene-pyrene 3.3% and  3-6 ring PAC 1.8%.  

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.