Hair coloring is an ancient beauty technique that dates back 4,000 years. With the continuous improvement of living standards, both young and old people are increasingly using hair dye to improve their quality of life and meet aesthetic needs. At the same time, hair dyes vary in quality and lack strict quality management standards. As a result, adverse reactions caused by hair dyeing have become more common. Hair dyes can be divided into temporary hair dyes, semi-permanent hair dyes and permanent hair dyes according to the duration of the dyeing effect. Permanent hair dyes are the most widely used and the key substances are p-phenylenediamine [PPD] and p-toluenediamine [PTD], 0.1%~0.5% of which may be absorbed by the skin during the dyeing process. Contact dermatitis is mainly a delayed allergic reaction caused by hair dye sensitization. It is usually confined to the contact area and manifests as scalp and facial edema, and may even involve the ears, neck and shoulders, but is usually not life-threatening. The incidence of delayed allergic reactions caused by hair dyes is about 1 per 100,000, but the actual incidence may be higher, and most of them occur after consumers dye their own hair. Animal studies have shown that aromatic amine chemicals are carcinogenic, but according to the available research data, there is no sufficient evidence that hair dyes are carcinogenic when used in humans. Only isolated cases of non-Hodgkin’s lymphoma, multiple myeloma, acute leukemia and bladder cancer have been reported that may be related to the long-term use of hair dyes in extra-large doses. We suggest that before hair dyeing, you should pay attention to: 1. Perform a skin sensitivity test of hair dye 48 hours before hair dyeing by placing a drop of the dye on the same spot on the inside of your wrist and waiting for it to dry naturally. If redness, swelling, itching, etc. occur during the process, the product should not be used further and a dermatological consultation should be made. If a slight adverse reaction occurs on the consumer’s scalp during the hair coloring process, the hair dye should be removed immediately. If consumers are sensitive to a particular hair dye material, they should use a hair dye that does not contain that allergen. But still pay attention to the possibility of cross allergic reaction and false negative. 2. Don’t dye your hair if you have scratches on your scalp Keep your hair clean and dry before dyeing, and don’t do special care such as h oil a week before dyeing; it is best to wash your hair a day before dyeing, for one thing, the oil secreted by your scalp has a protective effect on your scalp, and for another, you can avoid accidental scratches during the shampooing process. Do not use styling products on the day of hair dyeing. 3. You should apply lotion to your hairline and behind your ears before dyeing your hair to prevent damage caused by hair dye staining the skin. 4.Pregnant women and nursing mothers are not suitable for contact with chemical dye products. 5.Smooth out your hair before dyeing When dyeing your hair, massage your head thoroughly with your fingers until the hair dye cream is evenly distributed on each hair strand. After the hair dye is evenly distributed on the hair strands, do not cover or wrap the hair, but let it spread out freely. Please do not heat your hair when dyeing, as this may result in blurred hair color. 6. Do not repeat hair dyeing within three months because the decolorizing agent contained in the hair dye can damage the hair scales, and the damaged scales will become fragile and breakable, so if you repeatedly dye your hair within three months, the damaged scales will be damaged again before they recover.