What common feature is used to prevent cross-connection in plumbing systems?

An air gap is a critical feature in plumbing systems designed to prevent cross-connections, which occur when there is a potential pathway for contaminants to enter potable water supplies. Specifically, an air gap provides a physical separation between the end of a water supply pipe and the flood level rim of a fixture, such as a sink or toilet. This gap ensures that water cannot flow back from the fixture into the supply lines due to back-siphonage, which might otherwise happen during negative pressure situations.

By maintaining this physical barrier, the air gap effectively eliminates the risk of non-potable water mixing with potable water, ensuring that drinking water remains safe and uncontaminated. This is particularly important in applications where the water coming out of a fixture might contain pollutants, such as in dishwashers or kitchen sinks.

While the other options may have their own specific safety functions within plumbing systems, such as trap seals helping to prevent sewer gases from entering buildings, they do not directly prevent the type of cross-connections that air gaps are designed to address.