JOURNAL OF COSMETIC SCIENCE 52 and indoxyl. Indoxyl is the precursor to indigo the intermediate indoxyl will dye keratin. The blue indigo molecule does not bind with keratin it will wash out of hair. The vashma indigo powder is mixed with water to make a paste and applied to hair before contact with air changes the indoxyl to indigo. The indoxyl molecules will migrate from the indoxyl-rich paste into the keratin and bind with it. If the indoxyl molecule is con- verted to indigo before it has a chance to bind to and stain keratin, it will not bind, it will wash out of the hair. This chemical change happens rapidly so once water is stirred into the powder, it has to be applied to hair before it can change to the indigo molecule (15). Indigo dye is not as permanent as henna, but the full length of the hair color remains stable for years with daily shampoo as long as the intermediate indoxyl is bound to the hair. Only the intermediates in these plant powders bind permanently and effi ciently with keratin. Henna stains hair when wetted with an acidic liquid. Cassia stains hair when wetted with an acidic liquid. Vashma indigo stains hair when wetted with a neutral or slightly alkaline liquid. Henna and cassia require time (usually 8 h at 70°F) to release the dye molecules to stain hair. The fermentation of vashma indigo pre-releases the dye mol- ecule vashma indigo must be used immediately after mixing. For these fundamental reasons, pre-mixed henna powders and pastes are unsatisfactory for dyeing hair, and adul- terants are added to compensate for their poor outcomes. The plant powders must be mixed separately and added together at the time of application. The compound henna additives cross-react adversely with oxidative hair dyes. The pure henna, indigo, and cas- sia powders can be applied over oxidative hair dye without an adverse chemical reaction. This is why the future growth of non-coal-tar-derivative hair dye hinges on understand- ing and optimizing a centuries-old hair dye technology and methodology. DYNAMIC LEARNING: AN ESSENTIAL PROCESS FOR HENNA Modern industry has an underlying assumption that it is possible through science to tinker with a product until it suits consumer notions of convenience and expediency to compete for market placement. With henna, indigo, and cassia, people engaged with the physical requirements of natural plant dyes over centuries and gradually constructed a socially and personally positive practice of hair dye. If the hair dye industry is going to transit sensitized clients to henna, cassia, and indigo, it will be informative to look at the historical and anthropological records of how people successfully used these dyes, and create processes that duplicate these activities. Before the 20th century, the understanding of henna hair dye process was transmitted from person to person at the village bath a person who wanted to cover gray hair had the When I. tinctoria leaves are fermented in an alkaline vat, the indoxyl molecule breaks from the indican mol- ecule. The indoxyl molecule changes to the blue indigo molecule in contact with oxygen.

EPIDEMIC OF PARA-PHENYLENEDIAMINE SENSITIZATION 53 cumulative expertise and assistance of all who went to the community bath (16). Now that people bathe privately, they are cut off from the group knowledge of the henna pro- cess. The chemistry of henna, indigo, and cassia hair dye is simple, but requires experi- ence to mix and apply, and adjustments by each person for optimal results. It is not possible to market henna successfully with brief, static instructions the consumer cannot be a passive recipient of goods. Marketing henna will require education and interaction with consumers. The subtleties of hair type, water supply, mixing times, and application time are each signifi cant variables unique to each consumer. These cannot be learned from brief, static instructions. Web 2.0, the change from static web pages to dynamic or user-generated content and the growth of social media can serve as a bridge among people who need to reconstruct community knowledge through shared experience. Combining the informa- tion transmission of hair dye methods from before the 20th century and using parallel methods through the “electronic village bath” creates a convivial educational and social support group for henna. Expert customer service engagement with clients through elec- tronic interactive media is crucial to the success of henna in the 21st century, just as visits to the village bath and learning from other bathers was crucial to successful henna out- comes in the previous 400 centuries. Using dynamic electronic support, we have success- fully transitioned over 50,000 severely sensitized people from oxidative hair dye to henna, indigo, and cassia in the last 3 years alone (17). For the cosmetic industry to recapture hair dye clients lost to PPD sensitization and tran- sition them to henna, a box of powder with one paragraph of written instructions will not be suffi cient. A commitment to long-term education of and engagement with clients will be as necessary as developing the product infrastructure. Re-educating stylists about how to incorporate henna into salons will be crucial. Engaging with stylists who have had to abandon their careers because of chemical sensitization, and establishing salons for “plant- only” technologies serving highly sensitized patrons would be ideal. If, in 2030, 16% of graying clientele cannot use oxidative hair dye, the establishment of an alternative should be not only economically viable but essential to growth of the hair dye industry. REFERENCES (1) V. Smith, S. Clark, and M. Wilkinson, Allergic contact dermatitis in children: Trends in allergens, 10 years on. A retrospective study of 500 children tested between 2005 and 2014 in one U.K. centre. British As- sociation of Dermatologists’ Annual Conference. Leeds Teaching Hospitals NHS Trust, Leeds, UK (2015). (2) Cosmetic, Toiletry & Perfumery Association director-general, Dr. Christopher Flower, and Dr. Anjali Mahto, spokesperson for the British Skin Foundation (2016). Henna techniques are best learned interactively and dynamically