Photodermatosis refers to the acute and chronic damage caused by light (mainly ultraviolet) irradiation (such as sunburn, photoaging or skin cancer, etc.), clinically, it refers to the narrow sense that people receive too much light energy or due to the body’s own low tolerance to light stimulation of the disease, the broader sense refers to the light can promote or exacerbate the process of the onset and development of certain skin diseases. It is mainly caused by medium-wave ultraviolet (UVB) and long-wave ultraviolet (UVA). (Ultraviolet UV can be divided into long-wave ultraviolet UVA, medium-wave ultraviolet UVB, and narrow-wave ultraviolet UVC according to their wavelengths, with UVA having the longest wavelength of 320-400 nm, UVB having a wavelength of 290-320 nm, and UVC being absorbed by the ozone layer before it reaches the earth.) In addition UV light and photosensitizing substances (including exogenous allergies, photosensitizing foods, photosensitizing drugs) can cause light skin diseases, photosensitizing substances are substances that are good at absorbing ultraviolet rays from sunlight. Exogenous photosensitizers include asphalt, tar, cosmetics, detergents, dyes, food additives, preservatives, anthralin, pyridine, halogenated hydantoin, bleach, sunflower musk, and methylcoumarin, etc. Photosensitizing foods refer to those foods that are likely to cause solar dermatitis. Generally speaking, photosensitive food after digestion and absorption, which contains photosensitive substances will be followed into the skin, if at this time exposure to bright light, and sunlight will react. And then appear exposed parts of the skin redness, swelling, rash, and accompanied by obvious itching, burning or tingling sensation and other symptoms. Will produce photoallergic reaction of food such as: parsley, celery, fennel, parsnip, citrus, lemon, lime, wild chrysanthemum, artemisia, figs, purple cloud grass, wild rape, mustard; quinoa: ashwagandha, beets, dried fungus, yellow mud snail, pippin shrimp, crabs and so on. If eaten in large quantities and exposed to the sun, vegetable sun rash, fruit sun rash, yellow mud snail sun rash will occur. Photosensitive drugs such as: sulfonamides; aromatic amines: long-acting sulfonamides and other antimicrobials; non-aromatic amines: diuretics (thiazides), oral hypoglycemic drugs such as chlorosulfonylurea, glimepiride; anti-arrhythmic drugs: amiodarone, quinidine; non-steroidal anti-inflammatory drugs: ketoprofen, piroxicam; other: ashwagandha, tetracycline family, quinolones, if you take photosensitizing drugs, it may cause photosensitive dermatosis. While we pay attention to photosensitizing substances, sun protection is also essential. Reasonable selection and use of sunscreen cosmetics, to avoid waste to reduce the incidence of adverse reactions, dermatologists suggest that the following criteria can usually be selected and used: 1, indoor workers and housewives in general, the sun protection factor (Sun Protection Factor, abbreviated as SPF, is coated with sunscreen protection of the skin to produce a minimum of erythema required for the energy, with no addition of any Protection Factor (Sun Protection Factor, abbreviated as SPF, is the ratio of the energy required to produce minimal erythema on skin coated with sunscreen protection to the energy required to produce the same degree of erythema on skin without any protection) is 10, and PA (Protection UVA, an indicator of the degree of effectiveness of UVA protection) is +. 2.Outdoor work occupations, outdoor activities and work at noon SPF20, PA + + +. 3, activities under the scorching sun, sea bathing should use waterproof SPF30, PA + + + + products. At present, there is currently no light specifically for photosensitive skin diseases in the case of external commercial drugs, can be commercially available high sunscreen factor cosmetics for photosensitive skin disease patients, usually to choose more than SPF30, PA + + + + + + + + + products appropriate for daily light and as an adjunct to the oral treatment of drugs. In addition, sunscreens, whether for normal sun protection, or for the prevention and treatment of photosensitive skin diseases, should pay attention to the problem of contact allergy for safety reasons, before using a small amount of sunscreen coated on the skin behind the ears or inner forearms, if there is no local adverse reaction after 24 hours, usually can be used without fear.