Benzene

Endpoint summary

Description of key information

Benzene is absorbed by all physiological routes (inhalation, dermal and oral), the inhalation route is considerably the most important route of exposure (DECOS, 2014). Absorbed benzene is rapidly distributed throughout the body and tends to partition into fatty tissues. The liver serves an important function in benzene metabolism.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - oral (%):

100

Absorption rate - dermal (%):

6

Absorption rate - inhalation (%):

50

Additional information

The toxicokinetics (Absorption, distribution, metabolism and excretion-ADME) of benzene has been extensively studied and was recently reviewed by DECOS (2014). The below summary focuses mostly on the data in humans (if available) as these data are the most relevant for the safety assessment.

Absorption

Benzene is absorbed by all physiological routes (inhalation, dermal and oral), the inhalation route is considerably the most important route of exposure (DECOS, 2014). 

Inhalation

In humans, the reported inhalation absorption ranges from approximately 50-80% (this is influenced by the exposure conditions). The DECOS review cited Pekari et al. 1992, who reported an absorption rate of 50% after 4 hours of exposure to 3.3-33 mg/m³(1-10 ppm) and Srbova et al. 1950 who reported an absorption rate of 80% after minutes of exposure to 150-350 mg/m³(47-110 ppm). However, as values as high as 47 ppm are unlikely in usual settings (for worker or general populations, except for accidents), a value of 50% will be used as a key value for the chemical safety assessment.

Oral

Experimental data on oral absorption of benzene in humans is not available. Benzene appears to be readily absorbed following ingestion as shown by case studies on accidental or intentional oral exposure. In animals, the oral absorption range is 80-97 % (DECOS, 2014). DECOS stated that a study by Parke et al. 1953 showed that rabbits absorbed about 80% of benzene following a 2-3 day exposure at 340-500 mg/kg bw/day and that a study by Sabourinet al. 1987 showed that rats and mice absorbed above 97 % of benzene at 0.5 and 150 mg/kg/bw/day, respectively. Given this information, an oral absorption value of 100 % will be used as a key value for the chemical safety assessment.

Dermal

Dermal absorption is much less significant than oral and inhalation routes as identified in the DECOS (2014) review. The DECOS (2014) review cited Williams et al. 2011 who estimated the dermal absorption rate of benzene to be 0.2 to 0.4 mg/cm²/h (based on collective human and animal data). ECHA RAC (ECHA, 2018), cited Jakasa et al. (2015) who, using a predictive model, showed that exposure to 1 ppm (3.2 mg/m³) of benzene in humans, resulted in a dermal uptake of 5.85%.

Blank and McAuliffe (1985) translated J. Hanke et al. (1961) paper, which concluded that benzene vapour absorption through the skin is negligible. Rauma and co-authors (2013) have estimated that the dermal absorption of benzene vapour is low (about 3% of the total uptake) when compared with the inhalatory uptake.

Therefore, given the aforementioned information, a dermal absorption value of 6 % will be used as a key value for the chemical safety assessment.

Distribution

Upon absorption benzene is distributed to a number of tissues and biological fluids and that the highest concentrations are generally found in lipid-rich tissues in humans (DECOS, 2014).

Metabolism

The metabolism of benzene is inherently complex most of the metabolism is performed by the liver and lungs, with secondary metabolism occurring in the bone marrow (McHale et al., 2012). It has been extensively investigated and was shown to be similar between human and animals (DECOS, 2014)

Firstly, cytochrome P-450 oxidises benzene to benzene oxide by cytochrome P-450 CYP2E1 which is mainly expressed in the liver (DECOS, 2014). Secondly, mostly non-enzymatic pathways are triggered to convert benzene oxide to phenol. Phenol is then oxidised further to catechol or hydroquinone by CYP2E1. Subsequent oxidisation of these molecules forms the reactive species 1,2- and 1,4-benzoquinone, respectively. Benzene oxide can also react with glutathione, which results in SPMA, or can go through iron-catalysis (ring opening) to form trans-muconic acid.

For inhalation exposure, the lung would be a major site of benzene metabolism (Chancy and Carlson, 1995). Furthermore, since CYP2E1 is also expressed in the bone marrow of mice (Bernauer et al., 1999) and in human bone marrow stem cells (Bernauer et al., 2000) it can be assumed that benzene will also be metabolised directly in bone marrow stem cells to toxic metabolites.

Smith (2010) considers that CYP2E1 is the primary enzyme responsible for mammalian metabolism of benzene and that it is reasonable to assume that it is a low-affinity enzyme responsible for benzene metabolism mainly at higher levels of exposure. Smith (2010) further assumes that CYP2F1 and CYP2A13 are reasonable candidates for high-affinity metabolic enzymes, which are active at environmental levels of exposure below 1 ppm. However, there is a lack of scientific evidence for such enzymes (Boogaard, 2017).

Several pathways are involved in the metabolism of benzene oxide:

·      Benzene oxide can undergo conjugation with glutathione (GSH), resulting in the eventual formation and urinary excretion of S-phenylmercapturic acid (SPMA) (Monks et al 2010). The responsible enzyme is glutathione-S-transferase (GST), specifically GSTT1 and GSTM1 for which relevant polymorphisms are reported (see below).

·      Benzene oxide may be further metabolized by epoxide hydrolase (EH) to benzene dihydrodiol and catechol (Meek and Klauning 2010).

·      Benzene oxide spontaneously rearranges to phenol, which subsequently undergoes either conjugation (glucuronic acid or sulfate) or oxidation. The oxidation reaction is catalyzed by CYP2E1 and gives rise to 1,4-hydroquinone, 1,2-hydroquinone (catechol) and further to 1,2,4-benzene triol (DECOS 2014; Monks et al 2010). The enzyme myeloperoxidase (MPO), which is most abundantly expressed in neutrophil granulocytes, a sub-type of white blood cells, metabolises the hydroquinones to their respective benzoquinones. Within this reaction, highly reactive oxygen species (ROS) are formed. In addition, those benzoquinones are very reactive. The conversion from benzoquinones back to the hydroquinones is catalysed by NAD(P)H:quinone oxidoreductase 1 (NQO1) which can lead to further redox cycling (Hartwig 2010). 1,4-Hydroquinone was demonstrated to be clastogenic and aneugenic in vivo and in addition mutagenic in vitro (see DECOS 2014).

·      Benzene oxide equilibrates spontaneously with the corresponding oxepine valence tautomer, which can lead to ring opening to yield a series of six carbon dienes, the most reactive of which is the alpha,beta-unsaturated aldehyde, trans,transmuconaldehyde (Monks et al 2010), further aldehyde metabolites (Meek and Klauning 2010) and finally trans,trans-muconic acid (ttMA) which is eliminated in the urine. Trans,trans-muconaldehyde is a highly reactive di-aldehyde demonstrated in vitro to lead to mutations (Nakayama et al 2004), DNA-protein crosslinks and DNA strand breaks (Amin and Witz 2001). It was also found to induce cross-linking of the gap junction protein connexin43, which seemed to be responsible for inhibition of gap junction intercellular communication (Rivedal et al 2010).

 

Elimination

Elimination is very similar between human and animals, most of the absorbed benzene is metabolised (with phase-II-conjugation being one of the key pathways) and excreted in urine (in the form of sulphates and glucuronides) (DECOS, 2014). Following inhalation exposure, the main route of elimination of unmetabolized benzene is through exhalation (ATSDR, 2007; ATSDR, 2015). Human studies on benzene excretion after oral exposure were not identified. Only a small amount of an absorbed dose is eliminated in faeces. A biphasic pattern of excretion of unmetabolized benzene in expired air was observed in rats exposed to 500 ppm for 6 hours, with half-times for expiration of 0.7 hour for the rapid phase and 13.1 hours for the slow phase. The half-life for the slow phase of benzene elimination suggests the accumulation of benzene (ATSDR 2007).