Ultraviolet light is a general term for radiation with wavelengths from 10 nm to 400 nm in the electromagnetic spectrum, which cannot cause people to see. 1801 German physicist Ritter discovered the existence of ultraviolet light when he found that the outer section of the violet end of the daylight spectrum could make photographic negatives containing silver bromide sensitive. According to the intensity of ultraviolet rays can be divided into three categories: UVA that is ultraviolet long wave, is the strongest kind of ultraviolet rays, regardless of the season, cloudy or sunny and morning or evening, can pass through the glass, clothing and daily protective equipment will sunburn the skin, resulting in premature aging of the skin; UVB that is ultraviolet medium wave, will form damage to the bare skin, making the skin dark, red and swollen and even leave spots; UVC that is ultraviolet short wave, almost no damage to the skin. UVC, or short-wave UVB, will hardly damage the skin. UVB is strongest in the summer when it first clears after a rain; it is strongest between 10:00 a.m. and 2:00 p.m. during the day. What is UV allergy? UV allergy is an abnormal metamorphic reaction caused by the action of sunlight on the human body; photomorphic reaction is a kind of immune reaction, as long as there is a small amount of photosensitive substances in the human body, the reaction will occur after exposure to UV light, manifested as erythema, papular rash, pemphigus-like or blistering rash on the face, neck, forearm extensor, back of the hand and other exposed areas, after sunlight exposure, the skin lesion is obviously aggravated and the itching sensation is increased. The rash is often recurrent and may develop moss-like changes and increased pigmentation over time. The symptoms are especially severe in spring, summer and autumn; it is also called solar dermatitis. After exposure to the sun in summer, erythema and blisters appear on the exposed skin of the neck, face, arms, and back of the shoulders, with unbearable itching and pain like burning. This process is an acute damage reaction of the skin under strong sunlight, i.e. sunburn. Repeated sunburns can also lead to photosensitivity of the skin, and then, when exposed to sunlight again, a photosensitive reaction (solar dermatitis) is produced. In Chinese medicine, this disease is called sunburn, which is caused by the invasion of Yang heat and poisonous evil into the surface of the body, which is embedded in the skin and burns the skin and hurts the muscle; if the poisonous heat is mixed with summer dampness or wrestles with internal dampness and soaks the skin, swelling and blisters will appear, and after breaking, vesicles and ooze. Prevention of ultraviolet allergy 1. Apply sunscreen to the face and exposed parts, and when outdoors, play sunscreen umbrella, wear long-sleeved outerwear and wide brimmed protective cap or umbrella to cover up. 2.Prevent prolonged exposure to the sun. Exposure to sunlight at noon in summer should not exceed 30 minutes to protect the skin from damage caused by various wavelengths of ultraviolet rays and visible light. 3.Eat more food containing vitamin A and fresh vegetables and fruits to maintain the normal function of the skin. 4.Some photosensitive substances that can induce UV allergic dermatitis, such as oilseed rape, spinach, lettuce, figs, etc., should be eaten sparingly or not at all.