Improper fitting affects corneal ventilation Contact lenses can be divided into hard contact lenses and soft contact lenses by material. Soft contact lenses replaced high oxygen permeability hard lenses (RGP) because they were safe and comfortable to wear. Soft lenses were later found to be less breathable and prone to eye complications, so people turned to wearing RGP again. The impact of contact lenses on eye health is mainly on the cornea, which is the outermost structure of the eye and plays a major role in refraction, accounting for 70% of the total refractive power of the eye, and is the most important and precise refractive structure in the eye. Although the cornea is only about 1 mm thick, it can be divided into 5 layers from the outside to the inside depending on the composition of the cells. When corneal endothelial cells are damaged for various reasons, corneal endothelial cell loss can occur, resulting in permanent corneal edema and herpetic keratopathy, and even blindness, necessitating corneal transplantation. RGP, on the other hand, does not cause hypoxia due to mild movement in the cornea. The shorter the wearing time, the less damage is done. We know that there are no blood vessels in the cornea. The cells exchange nutrients, oxygen and metabolic waste through the atrial fluid on the inside of the cornea and the tear fluid on the outside. Wearing contact lenses prevents this exchange from taking place effectively, and the cells may be damaged by lack of oxygen. At the same time, carbon dioxide produced by endothelial cell metabolism cannot be excreted in time and is retained, which may have toxic effects on the cells. However, the reduction in corneal endothelial cell density is significantly less in people who wear high oxygen permeability rigid contact lenses (RGP) than in those who wear soft lenses. The study also found that the shorter the duration of daily contact lens use, the less likely corneal endothelial cell damage occurred. In people who used lenses for less than 12 hours per day, corneal endothelial cell density was greater than 2,000 cells per square millimeter. Day and night wear can cause corneal edema and hypoxia Contact lenses require contact with the cornea, which does cause some damage to the corneal epithelium and has been reported to cause corneal epithelial detachment. However, corneal epithelial cells can regenerate after damage and do not leave a scar, so proper, intermittent wear rarely results in serious consequences. However, contact lenses of good quality and high oxygen permeability are, after all, foreign objects that cover the cornea and separate it from the air. If you wear contact lenses continuously for a long time, the cornea will be in a constant state of oxygen deprivation, causing corneal epithelial edema and reduced resistance. At this time, if microorganisms such as bacteria or viruses are present, infection can easily occur, leading to corneal inflammation. A study in Hong Kong showed that for keratitis caused by microbial infections, the annual incidence was about 0.63/million in the general population, 3.09/million in those who wear soft contact lenses every day, 9.30/million in those who wear soft lenses continuously overnight, and only 0.44/million in those who wear gas permeable hard lenses. Long-term wear leads to keratitis Soft contact lenses due to spongy structure after a long time in the lens residual protein antigenic substances, wear a long time after the occurrence of chronic immune conjunctivitis, prompting inflammation of the cornea. In particular, cosmetic lenses and soft contact lenses should be worn sparingly.