Currently, the hair dyes used by professional hairdressing institutions and sold in the retail market are basically chemical hair dyes. So, what are the dangerous elements contained in these hair dyes? Paraphenylenediamine: Paraphenylenediamine is a known sensitizer. In 2002, there was a report of acute poisoning of hair dye production workers exposed to high doses of p-phenylenediamine. 78% of the patients’ blood was filled with methemoglobin, confirming the clinical characteristics of acute poisoning. China’s current regulation “Cosmetic Hygiene Code” stipulates that the maximum permissible concentration of p-phenylenediamine in hair dyes is 6%, while the European Union has recently announced the relevant regulations, which have reduced the permissible concentration of p-phenylenediamine in hair dyes from the original 6% to 2%. Under the normal use of this dose, it is obvious that there is no dose condition for acute poisoning to occur. There is a consensus that p-phenylenediamine has been defined as a strong allergen in 1939 and can cause hair dye dermatitis that cannot be ignored. It has been reported that about 5% of individuals in the general population experience varying degrees of localized skin erythema, flaking, itching or edema after hair dyeing. Therefore, hair dyes containing p-phenylenediamine should state that they contain this substance, and consumers are advised to do allergy tests before use, reduce the contact area with the scalp as much as possible during the hair dyeing process, and hairdressers are advised to wear gloves during the dyeing process to prevent skin contact. Because p-phenylenediamine is an aromatic amine, researchers have been closely monitoring its potential carcinogenicity for decades. The results of the study showed that p-phenylenediamine oxidation products can induce cellular transformation, suggesting that oxidative hair dyes are potentially carcinogenic. The researchers also suggested that because the in vitro experiments were conducted at the cellular level, animal carcinogenicity experiments are necessary to determine whether the transformed cells form tumors in animals in order to clarify whether they are carcinogenic. Some researchers also believe that many of the safety evaluation studies conducted now have operating conditions that differ from the way and dosage people are actually exposed to hair dyes, which makes it easy to overestimate the harm of hair dyes to humans. Experimental designs should be improved to simulate the conditions under which people are actually exposed to hair dyes in order to draw more scientific conclusions. In addition to p-phenylenediamine, there are three other hazardous elements in hair dyes, including heavy metals, hydrogen peroxide, and ammonia. Heavy metals: The heavy metal elements lead, arsenic and mercury are toxic substances in hair dyes. Mercury ions can interfere with the process of tyrosine becoming melanin in the skin, mercury and mercury compounds can cross the barrier of the skin, causing mercury poisoning and a large number of dark spots; arsenic and its compounds are toxic, skin such as a large amount of absorption of arsenic can cause dermatitis, pigmentation and other skin diseases, and eventually lead to skin cancer; excessive lead absorbed by the skin can harm human health, such as affecting the hematopoietic system, nervous system, kidney Gastrointestinal tract, cardiovascular and other functions. Hydrogen peroxide: Hydrogen peroxide is also called a color developer or oxidizer. This raw material is available in different forms and strengths, and it can catalyze color formation and produce longer-lasting colors. The greater the amount of color developer, the more sulfur will be removed from the hair. Without sulfur, the hair becomes harder and lighter. Therefore, long-term use will cause hair to become yellow and split ends, and can even cause severe hair loss. This causes a serious loss of hair quality and the hair will not be able to return to a healthy state again. Ammonia: Most oxidative hair dyes have a very irritating smell, this is because most alkalinizing agents use ammonia, and long-term exposure can cause symptoms such as skin pigmentation or finger ulcers. Ammonia can enter the body through the scalp hair follicles, and long-term use can cause relatively great harm to human health. Therefore, in China’s standard GB7916 “cosmetic hygiene standards” on the use of ammonia limits are clearly defined.