Hair bleaching products can damage hair and cause severe burns to the scalp when applied incorrectly or left on too long.Industrial bleaches that work by reduction (such as sodium hydrosulphite) react with a chromophore, the part of the molecule responsible for its color and decrease the number of carbon-oxygen bonds in it, making it uncoloured. This process might be reversed to a certain extent by oxygen in the air, such as yellowing of bleached paper if kept exposed to air. In contrast hydrogen peroxide chemically alters the chromophore so that it increases the number of carbon-oxygen bonds. Due to the relative absence of reducing agents in the environment, chromophores cannot restore themselves as seen in reduction-based bleaches.Bleaching the hair is a gradual process and different colors could be achieved dependent on the original hair color, application time, and strength of the product used. Applied on black hair, the hair will change its color to brown, red, orange, orange-yellow, yellow, and finally pale yellow. Toner
Toning is a common practice after bleaching to mask the undesirable red and orange tones of "brassy" hair using a toner dye. Through toning, the yellow hue of fully bleached hair can be removed to achieve platinum blond hair. The appropriate color of the toner depends on the color of the bleached hair; e.g. to remove the yellow tones a violet toner dye will be needed, but to neutralize red and orange hues a green and blue toner would be more suitable. Tinted shampoos can also be used to tone hair.
Sun exposure
Excessive exposure to the sun is the most common cause of structural damage of the hair shaft. Photochemical hair damage encompasses hair protein degradation and loss, as well as hair pigment deterioration Photobleaching is common among people with European ancestry. Around 72 percent of customers who agreed to be involved in a study and have European ancestry reported in a recent 23andMe research that the sun lightens their hair. The company also have identified 48 genetic markers that may influence hair photobleaching. Medical significance
Drug testing methods using hair samples are found to be disrupted by chemical hair treatments including bleaching. According to a 2019 study bleaching has caused strong chemical degradation on cannabinoids in hair, while permanent colorings in single applications had only negligible effects on cannabinoids. A 2021 study found similar results for methamphetamine concentrations in hair.Persulfate containing products may produce a variety of cutaneous and respiratory responses, such as allergic eczematous contact dermatitis, irritant dermatitis, localized edema, generalized urticaria, rhinitis, asthma, and syncope. References
Further reading
Robbins, Clarence R. (2006-05-26). Chemical and Physical Behavior of Human Hair. Springer Science & Business Media. ISBN 978-0-387-21695-9.