1,2,4-trimethylbenzene

Endpoint summary

Description of key information

1,2,4- Trimethylbenzene has low systemic toxicity in rats when administered by inhalational or oral routes. No significant treatment-related effects were observed up to oral doses of 600 mg/kg/day or inhaled concentrations of up to 1800 mg/m3. Mild pulmonary lesions occurred, possibly as a result of respiratory irritation.

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Endpoint conclusion

Dose descriptor:

NOAEL

600 mg/kg bw/day

Repeated dose toxicity: inhalation - systemic effects

Endpoint conclusion

Dose descriptor:

NOAEC

1 800 mg/m³

Additional information

Several repeated dose studies have been conducted to evaluate both oral and inhalation exposures to trimethylbenzenes. Typically liver and kidney effects are reported at high doses together with some acute central nervous system (CNS) effects. The focus in this evaluation is on 1,2,4-trimethylbenzene where data is available, but read across from other isomers is considered where specific data is unavailable. The trimethylbenzene isomers have similar toxicological profiles (Firth 2008). There are no repeat dose toxicity data available in humans.

Oral

A 28-day study in rats was conducted on 1,2,4-trimethylbenzene (Biosafety Research Center,1996). Administered doses were 0, 30, 100, 300 and 1000 mg/kg/day with primary effects reported at the top two doses including increased liver and kidney weights and increased salivation. Weight gain was suppressed at the highest dose level. No haematological changes or abnormalities at necropsy were reported. The NOAEL was 100 mg/kg/day. 

A 90-day oral study is not available for 1,2,4-trimethylbenzene, but a study is available on the isomer 1,3,5- trimethylbenzene. In this oral gavage study, rats were dosed at 0, 50, 200, and 600 mg/kg/day, with a recovery group being dosed at 600 mg/kg/day and observed for 28 days post-treatment. In the 600 mg/kg/day group an increase in serum phosphorus levels (males and females), liver weight increases (absolute and relative weight increase in females; relative increase only in males) and kidney weight increases in males were reported. In the recovery group, no apparent effects were reported at the end of the 28-day recovery period.    

Based on the recovery group data, the effects reported at the highest dose were considered reversible and were attributed to an adaptive response (enzyme induction) to the test substance. Liver growth is likely to be due to microsomal enzyme induction whilst the increased kidney weights in male rats are likely to be due to alpha-2µ-globulin related nephropathy. With respect to the elevated serum phosphorus levels observed, these levels were not statistically different at the end of the 28-day recovery period and no associated histopathological effects were reported which could be correlated with the increased serum phosphorus levels. The NOAEL for this study was therefore concluded to be 600 mg/kg/day.

Dermal

There are no repeat dose dermal studies available but testing via the dermal route is not required as human skin contact is unlikely and in accordance with column 2 of REACH Annex VIII, testing shall be performed using the most appropriate route of administration.

Inhalation

The key study is considered to be a 12 month chronic inhalation toxicity study with high flash aromatic naptha (a 50:50 blend of Shellsol A and Solvesso 100, which contains 44.81% total trimethylbenzene, all isomers) in rats (Clark et al, 1989). Rats were exposed to the aromatic solvent blend at concentrations of 0, 450, 900 or 1800 mg/m3 6 h/day, 5 days/week for 52 weeks. An initial reduction in body weight gain occurred in both male and female rats at the higher exposures. Various statistically significant haematological changes were transiently seen in males up to six months, but were not considered biologically significant. High exposure male liver and kidney weights were increased at 6 and 12 months but, in the absence of histopathological changes, were considered to be physiological adaptive responses. Some rats in all exposure groups and the control group showed mild pulmonary macrophage infiltration and alveolar wall thickening however there was no appreciable difference between the groups and no evidence that exposure had influenced the incidence or severity of the changes. Overall, no treatment-related histopathological abnormalities were found. It was concluded that chronic exposure to this high aromatic naphtha was without systemic toxicity in rats under the conditions of this study with a NOAEC of 1800 mg/m3. The test material contained a mixture of di- and tri-alkyl benzenes (including methyl-ethylbenzene isomers and other trimethylbenzene isomers) possessing presumably similar physico-chemical and toxicological properties therefore no adjustment was made to the NOAEC to account for the specific trimethylbenzene content.

These data are supported by the studies of Korsak et al (1997, 2000 a & b). In these studies, rats were exposed to nominal vapour concentrations of 0, 123, 492 or 1230 mg/m3 of 1,2,4- trimethylbenzene or 1,3,5- trimethylbenzene for 90 days (6h/day, 5 days/week). In all 3 studies, all animals survived and there were no effects on body weight and no clinical signs of toxicity. The 90 day studies (Korsak 2000 a & b), resulted in an overall low degree of systemic toxicity. Adaptive effects of exposure to 1,2,4- trimethylbenzene and 1,2,3 - trimethylbenzene included slightly increased sorbitol dehydrogenase activity and some blood changes (decreased red and increased white blood cell counts), observed at 1230 mg/m3.  Pulmonary lesions, comprising an increase in peribronchial, lung parenchymal and perivascular lymphocytic infiltration, were described by the authors at 492 and 1230 mg/m3 for both isomers, however the pathology was poorly reported. Since no comparable findings were reported by Clark et al. (1989) in a study of longer duration it is concluded that these findings are of limited relevance to hazard identification. There were no significant histopathological changes in the upper respiratory tract or in any other organs examined, demonstrating the lack of systemic effects. The overall systemic NOAEC from these investigations by Korsak et al. is therefore considered to be 1230 mg/m3 for both isomers.

In the supporting respiratory irritation study (Korsak et al, 1997), bronchiolar lavage fluid (BAL) was collected at termination (24 h after the last exposure) and BAL cell smears were stained and examined by light microscopy. Differential cell counts were made and cell viability was assessed using the trypan blue test. There was an increased number of cell macrophages, polymorphonuclear leukocytes and lymphocytes at all three test concentrations compared with the controls. Total protein lactate dehydrogenasee (LDH) and acid phosphatase activity in BAL were significantly increased in all exposed groups. However, the observed changes were not concentration-dependent, showing no progression of effects even with an exposure 10 times higherTherefore, although respiratory irritation effects were observed in the BAL even at the lowest concentration of 123 mg/m3, there were no toxicologically significant clinical effects at concentrations up to and including 1230 mg/m3.

Conclusions

1,2,4- Trimethylbenzene has low systemic toxicity in rats when administered by inhalational or oral routes. An oral NOAEL of 600 mg/kg day was established from a 90 day study on 1,3,5-trimethylbenzene and an inhalational NOAEC of 1800 mg/m3 has been established from a 12 month study on high flash aromatic naphtha.

References

SCOEL (1994) Scientific Expert Group on Occupational Exposure Limits for Trimethylbenzenes. SEG/SUM/34

Justification for classification or non-classification

1,2,4-Trimethylbenzene has low oral and inhalational sub-chronic toxicity and dermal exposure is unlikely. It does not therefore warrant classification under DSD or CLP.