Which disinfectant property is NOT affected by the number of usages?

Shelf life refers to the time period during which a disinfectant remains effective and safe to use before it deteriorates or becomes compromised in quality. The shelf life is primarily determined by the chemical stability of the disinfectant and the conditions under which it is stored, such as temperature and exposure to light.

Unlike concentration level, effectiveness, and physical appearance, the shelf life does not change or diminish with each use. Concentration level can be affected by dilution with water or other substances during application, while effectiveness can decline as the disinfectant is used multiple times and becomes contaminated. Physical appearance may also change depending on factors related to usage and storage conditions. Therefore, shelf life is the correct answer as it remains constant regardless of the number of times a disinfectant is used.