Histological effects long-term

In a long-term inhalation study in rats, exposure to 1,4-dichlorobenzene at air concentrations of 490-499 ppm 5 hours per day, 5 days per week for 76 weeks resulted in an increase in absolute liver weight throughout the study in males and at weeks 27 and 112 in females (Riley et al. 1980). This effect was not accompanied by histological alterations or by increased serum transaminase activities. No hepatic effects were noted at 75 ppm. None of the adverse hepatic effects reported at lower concentrations of 1,4-dichlorobenzene for shorter durations (Hollingsworth et al. 1956), as described above, were identified in the 76-week study. Based on the NOAEL of 75 ppm for lack of hepatic effects, a chronic-duration MRL of 0.1 ppm was calculated as described in the footnote to Table 2-1 and in Appendix A (Hollingsworth et al. 1956). 

Histologic improvement seen in 60% of treated patients, but long-term effect unknown... 

Bhawan, J., Short- and long-term histologic effects of topical tretinoin on photodamaged skin, Ini. J. Dermatol., 37, 286, 1998. 

Phenol is not considered to be carcinogenic. On the contrary, long-term histological studies show that phenol (even when the formula contains croton oil) has a protective effect against skin cancers. A logical explanation of this protective effect may lie in the fact that the cells that take the brunt of UV rays are the keratinocytes closest to the stratum corneum, the ones that form, for example, the top of the dermal papillae. On the other hand, the keratinocytes that are in the lower regions of the dermal papillae are relatively protected compared with those higher up the UV photons have been partially absorbed or diffracted by the epidermal layers above them. In addition, the sebo-cytes or keratinocytes that make up the shaft of the hair are anchored more deeply in the skin, at a depth where the harmful effects of the sun are not as severe. 

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