Pitch, petroleum, arom.

Endpoint summary

Description of key information

PAH are absorbed rapidly through the pulmonary tract, the gastrointestinal tract, and to a much lesser extent through skin. Metabolism is complex resulting mostly in hydroxylated species which in part may further be conjugated. Excretion is via urine, bile and faeces. Conjugates excreted into bile can be hydrolysed in the gut and be reabsorbed.
Absorption through human skin will be not more than 2 % within and after 8 h of exposure. Permeation through rat skin is much more pronounced (ca. 8 fold) compared to human skin.

Key value for chemical safety assessment

Bioaccumulation potential:

low bioaccumulation potential

Absorption rate - dermal (%):

2

Additional information

The substance pitch, petroleum, arom. (petroleum pitch / petro pitch / PP) [CAS no. 68187-58-6] is a UVCB and consists of a complex and variable combination of polycyclic aromatic hydrocarbons (PAH). Substance matrix is composed of in part highly condensed ring aromatic systems because the substance is obtained as residue from a distillation process at temperatures up to > 400 °C.

The substance fraction that can be distilled from the material between 230 and 500 °C is only 23 %. The percentage of substances that can be identified by GC analysis is only about 4 %. These components are mainly PAH consisting of 4 and more condensed aromatic rings (pyrene, fluoranthene, benzanthracenes, benzopyrenes). Concentrations range from < 0.05 to ca. 0.5 % (w/w).

The fate and properties of petro pitch as well as adverse effects in biological systems will be characterised on the one hand by the PAH present as constituents in the pitch, on the other hand, by the inert matrix of the pitch associated with low solubility in aqueous and in organic media. The poor bioavailability of the pitch matrix will reduce effects observed with neat individual PAH, hence the toxicokinetic characteristics of single PAH can be taken for granted as "worst case".

Toxicokinetics

As petro pitch is basically composed of PAH, the toxicokinetics of this substance class determines the toxicokinetic properties of petro pitch to a certain extent. Toxicokinetic characteristics of PAH are reported based on a review of the WHO 1998/2003.

Absorption

PAH are absorbed through the pulmonary tract, the gastrointestinal tract, and the skin. The degree of absorption may be different for individual substances. The rate of absorption from the lungs depends on the type of PAH, the size of the particles on which they are absorbed, and the composition of the adsorbent. PAH adsorbed onto particulate matter are cleared from the lungs more slowly than free hydrocarbons.

Gastrointestinal absorption in rodents has been reported to be high. Fractional absorption of lower-molecular-weight PAHs, such as two-ringed naphthalene, may be more complete than that of higher-molecular-weight PAHs, such as five-ringed benzo[a]pyrene. Effective absorption by the different routes is also evidenced by observation of systemic toxicity following exposure by the different routes. Absorption from the gastrointestinal tract occurs rapidly in rodents, but metabolites return to the intestine via biliary excretion.

Distribution

PAH are widely distributed throughout the organism after administration by any route and are found in almost all internal organs, but particularly those rich in lipids. Intravenously injected PAH are cleared rapidly from the bloodstream of rodents but can cross the placental barrier and have been detected in foetal tissues.

Studies with 32P-postlabelling for the detection of DNA-adducts after percutaneous absorption of mixtures of PAH in rodents showed that components of the mixtures reach the lungs, where they became bound to DNA.

Metabolism

The metabolism of PAH to more water-soluble derivatives, which is a prerequisite for their excretion, is complex. In general, parent compounds are converted into intermediate epoxides (a reaction catalysed by cytochrome P450-dependent mono-oxygenases), which are further transformed by rearrangement or hydration to yield phenols or diols. In following steps conjugation by glutathione, sulphate or glucuronic acid may occur. Alternatively or in addition, a second oxidation at another position of the aromatic system is possible to yield tetrols, which can themselves be conjugated with sulphuric or glucuronic acids or with glutathione. Most metabolism results in detoxification, but some PAH are activated to DNA-binding species, principally diol epoxides, which can initiate tumours.

Excretion

PAH metabolites and their conjugates are excreted via the urine and faeces, but conjugates excreted into the bile can be hydrolysed by enzymes of the gut flora and be reabsorbed. It can be inferred from the available information on the total human body burden that PAH do not persist in the body and that turnover is rapid. This inference excludes those PAH moieties that become covalently bound to tissue constituents, in particular to nucleic acids, and are not removed by repair.

References

WHO (1998). Selected non-heterocyclic polycyclic aromatic hydrocarbons. Environmental Health Criteria 202, Geneva, Switzerland, WHO (World Health Organisation) 1998

WHO (2003). HEALTH RISKS OF PERSISTENT ORGANIC POLLUTANTS FROM LONG-RANGE TRANSBOUNDARY AIR POLLUTION, JOINT WHO/CONVENTION TASK FORCE ON THE HEALTH ASPECTS OF AIR POLLUTION. WHO Regional Office for Europe, World Health Organization 2003

Dermal absorption

In synopsis of observations from comparative in-vivo and in-vitro studies (human vs. rodent) on spiked creosote, it is expected that not more than 2 % of a dermal dose of a tar oil will be absorbed through human skin within and after 8 hours of exposure (Fasano 2007a, b). The conversion factor human vs. rat skin was found to be 0.12, which means that the dermal dose absorbable within 8 hours is about 8-fold higher in rat than in human skin. As petro pitch contains an inert matrix, skin absorption is expected to be even lower than demonstrated with the test substance creosote.

Studies of Van Rooij 1995 and Sartorelli 1999 show that lower molecular weight PAH are absorbed faster than higher molecular weight PAH. For absorption through pig ear (Van Rooij) and full thickness skin from the abdomen of monkeys (Ceropithecus aetops) (application in artificial sweat) (Sartorelli), differences in absorption rates from 103 ng/(h*cm²) (phenanthrene) to 0.8 ng/(h*cm²) (benzo[a]pyrene) and from 24 ng/(h*cm²) (fluorene) to 0.4 ng/(h*cm²) (benzo[a]pyrene) were observed.

Taking into account the PAH composition of petro pitch (mostly higher size PAH) it can be estimated that human dermal absorption of petro pitch will very likely be considerably lower than for creosote (Fasano, 2007a, b).