Hair growth is divided into three phases, namely the anagen phase, the regression phase and the resting phase. The anagen phase, which is the growth phase of the hair, lasts about 2 to 6 years, with an average of 3 years, and only during this period the hair can grow longer. The regressive phase, which lasts about 3 weeks, is the period when the hair begins to degenerate. The resting phase, which lasts about 3 months, is the stage of hair shedding. So our human hair, too, goes through an evolutionary process from birth, development, aging to death. Just as animals recede, human hair is constantly growing and shedding. About 90% to 95% of all our hairs are in the anagen phase, and the remaining 5% to 10% are in the resting phase. 50 to 100 hairs will be shed every day, while an equal amount of hair will be regenerated. So if you see the phenomenon of hair loss, to judge correctly, not all are pathological hair loss, most people belong to the normal physiological phenomenon, no need to panic. The growth rate of hair is about 0.4mm per day, which means about 1.2cm per month, and about 15cm per year. The growth rate of hair also has individual differences, mainly related to genetics, health conditions, hormone levels, drugs and climate. For example, systemic lupus erythematosus, syphilis, baldness, high fever, post-surgery, hypothyroidism, dieting, etc. can cause hair loss and slow down the growth rate. Another example is that anti-tumor drugs and retinoids often cause premature hair loss. There are also certain scalp lesions that can cause hair loss, such as discoid lupus erythematosus, flat moss on the scalp, fungal infections of the scalp, and perifolliculitis. When abnormally large amounts of hair loss occur, the cause needs to be determined. In the case of physiological alopecia areata, the lost hair can be completely regenerated. However, certain pathological hair loss can cause permanent and irrecoverable hair loss. Don’t panic when you have hair loss, but see your doctor, find the cause, and treat it with medication intervention if necessary.