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Hair coloring has been practiced for thousands of years, with the most famous early practice being the use of henna preparations by the early Egyptians. This natural dye imparted redorange high-lights to hair, and still today henna (Lawsonia inermis), henna extract and the actual active color ingredient of henna, (2-hydroxy-1,4-naphthoquin-one), are recognised cosmetic ingredients. However, natural colorants make up only a miniscule part of the hair color market, due to the far superior performance and versatility of synthetic colorants. The development of modern hair colorants began in the late 19th century when synthetic organic compounds were first used for this application. Modern products allow the user to achieve everything from temporary color addition through to more durable color highlights and grey coverage to lasting color changes. Estimates for the usage of hair color throughout industrialised countries range upward to 50%.

Uniqueness of hair dyeing
The dyeing of 'living' hair fibres offers a singular challenge to the colorist, and several factors set this technology quite apart from other dyeing procedures. The option, indeed the necessity, of periodic reapplication is notable. This is less to restore faded color than to apply color for the first time to new growth hair. This constant development of new substrate is perhaps the most peculiar feature of this dyeing category. Repeated application does, however, allow the industry to use dyes with lower fastness than in other categories. Hair fibres as a dyeing substrate are rather complex. The fibres themselves are heterogeneous cross-sectionally and even, to some extent, from root to tip (see Figure 1). The outer layer, or cuticle, consists of overlapping cells, generally five to seven layers, while the inner cortex has longitudinally arranged keratinous fibrils within elongated corticular cells. It is the cortex which contains the natural melanin colorant. New growth hair near the root has a tight corticular structure while the weathered and treated tip ends are far more porous, thus making uniform dyeing quite challenging under the mild conditions of product use. Additionally, hair may vary substantially in both chemical and physical composition, depending on race, hair color, environment and a variety of other factors. Conditions of application are quite restrictive allowing very limited ranges of pH and temperature and, to a lesser extent, time. Only physiologically accept-able environments are used, and limited adsorption and/or diffusion at ambient temperature precludes the use of many traditional dyestuffs. Dyebath exhaustion is practically non-existent. The process may be more accurately described as staining, since significant amounts of dyestuffs are washed away and not utilized. Finally, the exposure of the user to both dye precursors and finished dye-stuffs demands rigorous toxicological testing and acceptable profiles for all materials.

Types of hair colorants
There are three general categories of hair colorants, defined by the duration the color remains on the hair. However, the distinctions between these various categories have been somewhat blurred in the past 1015 years, due to new dyestuffs and improved formulations.

Temporary coloring
Temporary colorants are removed by the first shampooing. These products are usually called 'color rinses' and use high molecular weight ionic colorants which are deposited on the surface of the hair with no penetration into the cortex. These are acid dyes with minimal affinity for the overall anionic nature of hair, and are generally used for a specific event or occasion. A second class of temporary colorants are large cationic basic dyes with at least an ionic attraction for hair, thus somewhat better fastness, but still only surface deposition. Products formulated solely with D&C† and/or FD&C†dyes do not require a preliminary patch test, usually performed 48 h prior to application, to check for allergic response. This test is required for other hair colorant products, and not having to perform it makes these products simpler and more appealing to use. Their limited efficacy, however, means that users often move onto a different, longer lasting hair colorant type.

These are colorants certified by the US Food and Drug Administration (FDA) for use in drugs and cosmetics (D&C) or foods, drugs and cosmetics (FD&C).
Each potential FDA-approved colorant is tested for minimum assay and maximum impurity levels, which vary for each compound, and are certified to meet these standards.

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