A breather filter is used to protect the contents of:

A breather filter is specifically designed to protect the contents of storage tanks by preventing the ingress of contaminants while allowing for the expansion and contraction of air within the tank. These filters serve a critical role in maintaining the quality of the materials stored inside tanks—such as fuels, oils, or other liquids—by filtering out particulate matter and moisture from the air that might otherwise enter the tank. This helps to reduce the risk of contamination that could compromise the integrity of the tank's contents.

In contrast, air ducts are primarily concerned with air distribution and would use different types of filters aimed at maintaining air quality, while heating systems might have specific filters designed for filtering air to improve efficiency and indoor air quality. Water reservoirs utilize different mechanisms for keeping water clean instead of breather filters, as they often rely on physical barriers or different types of filtration systems altogether.