What is a common cause of surfaced stains on nails?

Surface stains on nails often occur due to external factors, and smoking is a well-known contributor. Tobacco products contain tar and nicotine, which can leave a yellowish or brown stain on the nails and surrounding skin. This staining occurs because the substances in smoke can adhere to the keratin in the nails, leading to discoloration over time.

Other factors can also affect the appearance of nails, but smoking is particularly distinct due to the specific chemicals involved that actively stain the nail surface. It is important to note that while poor hygiene, excessive moisture, and frequent nail enhancements can lead to various nail health issues, they are less directly associated with the specific type of staining described in the question. Poor hygiene might lead to other nail problems, excessive moisture could result in conditions like onychomycosis, and frequent nail enhancements could strip away natural nail oils, but these factors do not typically cause the surface discoloration attributed to smoking.