Final answer:
The emergency air supply line in plumbing prevents negative pressure, which can cause backflow and contaminate the water supply, by acting as an air cushion to absorb pressure shocks.
Step-by-step explanation:
The emergency air supply line in plumbing systems, often found near water faucets, is a crucial component designed to prevent negative pressure within the pipes. This negative pressure can potentially cause backflow, contaminating the water supply with dirty water or chemicals. By incorporating an air-filled tube, these systems provide a buffer against sudden changes in water pressure. When there is a sudden drop in pressure on the supply side (due to a burst pipe or a high demand elsewhere in the system), the air-filled tube, by being compressible, absorbs the shock and prevents any dirty water from getting back into the clean water system. It acts much like an air cushion or shock absorber, maintaining a consistent and safe water pressure level at the faucet.