In what situation is an air gap most commonly used?

An air gap is primarily utilized to prevent backflow, which can occur when there is a change in pressure in the system that might cause contaminated water to flow back into the potable water supply. The connection from a faucet to a dishwashing machine is a common application for an air gap because it ensures that any wastewater from the dishwasher cannot siphon back into the faucet and contaminate the drinkable water.

This installation maintains a physical separation between the clean and used water, complying with health and safety standards to protect against contamination. The design leverages gravity and air pressure to eliminate the possibility of cross-contamination.

In other contexts, such as connecting two pieces of plumbing pipe or in a waste disposal unit, the design intent does not require the same level of protection against backflow, as these do not involve the potential contamination of drinking water. Similarly, while outdoor hose connections might have their own preventive measures, they do not generally employ air gaps as a standard practice for contamination prevention in the same critical manner.