Skin barrier function: The stratum corneum of the skin, located in the outermost layer of the epidermis, was once thought to be a structure formed by useless dead cells. However, recent studies have revealed that keratinocytes and their extracellular components are closely embedded with each other, forming a special brick-wall structure that provides a barrier function for the skin. This structure provides the body with an osmotic barrier that prevents the loss of water and electrolytes within the skin, allowing survival in a dry environment; it also provides an anti-microbial barrier that inhibits the growth of disease-causing microorganisms without interfering with normal flora colonization. A variety of skin disorders have been found to be associated with abnormal skin barrier function, and disruption of skin barrier function may not only be one of the clinical manifestations of certain skin diseases, but also serve as an important initiating factor. Rosacea: Rosacea is a chronic inflammatory skin disease involving the vasculature of the facial skin and the sebaceous glandular units of the hair follicles, and occurs in people aged 30-40 years and occasionally in children. The prevalence rate reported abroad is 0.5%-22%, with a male to female ratio of approximately 1:3; no large-scale epidemiological data are available in China to provide a specific incidence rate. The clinical manifestations are transient and persistent erythema, capillary dilation, papules, pustules mainly in the middle of the face, and orange-yellow patches or even nasal flaps in some patients with longer disease duration due to excessive sebaceous gland hyperplasia. Based on the type of lesion, the National Rosacea Society’s expert committee in 2004 classified patients as erythematous capillary dilation, papulopustular, nasal flaccid, and ophthalmic. Over the years, there have been many different opinions on the etiology of rosacea, such as the trichophyton theory, the Helicobacter pylori theory, the seborrhea theory, the environmental theory, and the vascular abnormality theory, but the conclusions of each study are often very different. Since the pathogenesis of rosacea has not been clear, there is a lack of specificity in treatment. Patients with rosacea are more sensitive to external stimuli, and a variety of endogenous and exogenous triggers can cause exacerbation of rosacea, including diet, alcohol consumption, systemic and topical medications, temperature, climate, mood, and activity. A prospective epidemiological study found that patients with rosacea were more likely to have nonspecific contact allergic reactions to contacts. A skin irritation study showed a 100% positive rate in patients with the erythematous capillary dilated phase of rosacea and a 68% positive rate in patients with the papulopustular type, compared to 19% in normal controls. In a study by Wu Yan et al, 66.7% of patients with rosacea complained of burning pins and needles, 66.7% indicated the presence of dry skin, and the positive rate of lactic acid irritation test was as high as 46.7%. The above findings suggest that patients with rosacea may have abnormalities in barrier function.