Studies on the dermal and systemic bioavailability of polycyclic aromatic compounds in high viscosity oil products

The assessment of skin penetration by viscous oil products is an important element in the risk assessment of these materials where skin contact is lik ely. Systemic bioavailability (body uptake) is viewed as a good indicator o f skin penetration following cutaneous exposures. The results of this study provide quantitative information on the influence of viscosity on the bioa vailability of a specific polycyclic aromatic compound (benzo(a)pyrene) in base oils, residual aromatic extracts and bitumens following skin exposures to mice. The materials studied were a base mineral oil (viscosity 32 cSt a t 35 degrees C), a 1:1 blend of the mineral base oil and a residual aromati c extract (198 cSt), several residual aromatic extracts (ca. 5000 cSt, 35 d egrees C) and a range of bitumens (0.65-69 x 10(6) cSt, 35 degrees C). Thes e were each spiked with 0.1% radiolabelled benzo(a)pyrene, as a representat ive carcinogenic polycyclic aromatic compound, then used for cutaneous expo sures to mice. The results indicate that as viscosity increased in the rang e ca. 30 to 5000 cSt (base oil to residual aromatic extract) the uptake of the radiolabelled benzo(a)pyrene into blood was reduced by ca. fivefold. Fu rther increases in viscosity from ca. 5000 to 69 x 10(6) cSt (i.e. residual aromatic extract to bitumen) resulted in a further but smaller (ca. twofol d) reduction in uptake. The relationship between the amounts of free benzo( a)pyrene measured in blood and viscosity showed the same trend. This trend was also mirrored by the degree of binding of benzo(a)pyrene metabolites to DNA in skin. The findings in mouse skin in vivo indicate that viscosity ca n significantly affect skin penetration and systemic bioavailability of pol ycyclic aromatic compound components of oil products. Results obtained with viable human skin in vitro also showed that the bioavailability of benzo(a )pyrene was reduced by the viscosity of the oil product matrix. It is thus necessary to take account of physical properties such as viscosity in the o verall risk assessment of viscous oil products, particularly in the case of very viscous materials such as bitumens. The significantly reduced bioavai lability of hazardous compounds from undiluted materials is thus an importa nt factor to consider when assessing the risks from dermal exposures.