Hair coloring products generally fall into four categories: temporary, semipermanent, deposit only/demi, and permanent. All these hair color products, except for temporary color, require a patch test before application to determine common hair colouring hazards and if the client is allergic to the product. "Hair lightening," often referred to as "bleaching" or "decolorizing," is a chemical process involving the diffusion of the natural color pigment or artificial color from the hair. This process is central to both hair lighteners and humidity perspiration. All permanent haircolor products and lighteners contain both a developer, or oxidizing agent, and an alkalizing ingredient as part of their ammonia or an ammonia substitute. The purpose of this is to raise the cuticle of the hair fiber so the tint can penetrate, facilitate the formation of tints within the hair fiber, bring about the lightening action of peroxide and avoid common hair colouring hazards. When the tint containing the alkalizing ingredient is combined with the developer (usually hydrogen peroxide), the peroxide becomes alkaline and diffuses, or breaks up, through the hair fiber, entering the cortex, where the melanin is located. The lightening occurs when the alkaline peroxide breaks up the melanin, replaces it with new color and assist humidity perspiration. For individuals who wish to use a subtle neutralizer for yellowing hair or to neutralize unwanted tones. The pigment molecules in temporary hair color are large and, therefore, don't penetrate the cuticle layer, allowing only a coating action that may be removed by shampooing so as to avoid common hair colouring hazards. An example of use of temporary hair color is for Halloween costumes. Semipermanent is formulated to last through several shampoos, depending on the hair's porosity and thus on its ability to absorb humidity perspiration. The pigment molecules are small enough to partially penetrate the hair shaft and stain the cuticle layer, or simply to change hair color into funky color for a day. Semipermanent hair color is mostly used by women to cover up any gray hairs that may have started appearing. To stop common hair colouring hazards, if the hair already has its own natural pigment (such as in younger people) the semipermanent hair color can act as permanent hair color, because you are applying another pigment on hair that already has pigment--graying hair lacks pigment. Generally speaking, semipermanent hair color acts like permanent hair color if using a dye a shade darker than the natural hair color. The product was made to be used by graying women or men to be matched to their natural hair color, or to go one to two shades lighter--a temporary fix. Follow instructions carefully, liberally, and avoid humidity perspiration. However, even given the variety of hair colouring techniques and the richest selection of hair colors, one must not forget about the common hair colouring hazards. They may appear to be a relatively insignificant problem, if compared with the choice of the hair colour or the hair type. But if you would like to make the result of the hair colouring procedure truly impeccable, the issue of the common hair colouring hazards can't be left out of the spotlight of attention.