What is the primary function of a trap under a plumbing fixture?

The primary function of a trap under a plumbing fixture is to act as a seal against sewer gas. This is essential for health and safety reasons, as sewer gases can be hazardous and unpleasant. The trap, often shaped like a U or S, holds a small amount of water, which creates a barrier that prevents these gases from escaping into the living spaces of a building.

While drainage is certainly an important aspect of plumbing fixtures, and traps must allow for water flow to maintain function, the key role of the trap is specifically in preventing the backflow of harmful sewer gases. This water barrier must remain intact to ensure that the gases do not rise through the plumbing system. This functionality is crucial in maintaining indoor air quality and preventing potential health hazards associated with exposure to these gases.