I did a Medline search for the past 5 years and found these 2 interesting articles:
An ultrastuctural study of hair fiber damage and restoration following treatment with permanent hair dye.
Int J Dermatol 2002 Feb;41(2):88-92 (ISSN: 0011-9059)
Ahn HJ; Lee WS Department of Dermatology, Yonsei University Wonju College of Medicine, Kangwon-Do 220-701, Republic of Korea.
INTRODUCTION: Hair fibers are comprised of non-nucleated keratinocytes and are biologically dead. Clinical damage to the hair shaft occurs with the application of hair dye. Whether this damage can be repaired and the time frame involved is of interest and may help to determine the optimal interval between repeat applications. AIMS AND METHODS: Ultrastructural changes of hair shafts after application of permanent hair dye were observed sequentially in a 26-year-old woman. The dye contained p-phenylenediamine, m-aminophenol, resorcin and hydrogen peroxide as an oxidizer. Scanning and transmission electron microscopy was performed immediately before application of the hair dye and again at 30 min, 1 h, 3 h, 6 h, 1 day, 3 days, 1 week, 2 weeks, 4 weeks, 6 weeks and 8 weeks. RESULTS: The hair showed cuticular swelling with focal degeneration. In some places there was exposure of the hair cortex due to extensive cuticular detachment. There were many holes of various size in the endocuticle, small focal lacunae along the intercellular space and an ill-defined A-layer especially beneath the area of cuticular detachment. The exocuticle appeared normal. The findings were most dramatic in the 6 h and 1 day specimens and tended to improve gradually thereafter. The 8 week specimen showed near complete restoration of the hair cuticle and return to the precoloring state. CONCLUSION: Hair returns to its precoloring state and this requires 8 weeks.
J Cosmet Sci 2001 Nov-Dec;52(6):377-89 (ISSN: 1525-7886)
Pande CM; Albrecht L; Yang B Clairol Inc., 2 Blachley Road, Stamford, CT 06922, USA.
We have found that hair dyes protect hair against photodamage. The efficacy of photoprotection of dye products has been measured by monitoring the tensile strength of hair fibers and the integrity of the disulfide bonds in the fibers. Although the hair dyed with permanent dye products are initially weakened due to oxidative chemical damage, they also show a slower rate of degradation upon photoirradiation, compared to the undyed hair. Thus, a less appreciated benefit of using hair colors is their ability to provide protection against sun damage.