S-Trap vs P-Trap Diagram

This image shows s-trap vs p-trap. The problem with an S-trap is that they have the tendency to siphon. This will result in an open airway from the sewer lines up through your sink which will allow noxious sewer gases to enter your home. The design of a P-trap will keep the required amount of water is sitting in the trap which act as a barrier to the sewer gases. Even if you have P-traps installed, the water in the trap can dry out over time if the sink is not used. It is recommended to run all plumbing fixtures at least monthly in order to make sure that there traps remain full of water.