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A faucet that supplies clean drinking water is fitted with a flexible hose to fill a mop bucket. A careless employee leaves the end of the hose in a bucket of dirty mop water overnight. Due to changing air pressure within the building's plumbing system, the dirty mop water moves up the hose and into the faucet - contaminating the clean drinking water. What is this type of contamination called?

A Cross-connection

Explanation: The term cross-connection is sometimes confused with cross-contamination. Although both terms are related to contamination, cross-connection has a specific definition.Cross-connection happens when a clean water supply is accidentally connected to a dirty water supply, usually through a hose or an incorrectly installed faucet.For example, if a faucet is installed too close to the sink so that when the sink is full of water, the end of the faucet is submerged in the sink water. This can cause contamination of the water within the faucet. Changing air pressure can cause the dirty sink water to move up and into the clean faucet water.The best way to prevent this is an "air-gap" or a "backflow prevention device".An air-gap is simply ensuring that there is sufficient space between the faucet and the sink so that the faucet cannot become submerged in the sink water.A backflow prevention device is a fixture with valves that keep water moving in only one direction and prevent it from going backward. A thermocouple is an example.Both of these preventative measures can be used for faucets, hoses or any other potential water-to-water connection.