Every sink and drain in your house has a p-trap underneath it that drains into your sewer system. The p-trap is filled with water that creates an air tight seal that prevents sewer odor from coming back through the drain pipe. Sewer odor will occur when the water in the p-trap dries out. P Trap Pal check valves will prevent sewer odor from entering your home even if the p-trap has dried out!
The most common way a p-trap can dry out is from evaporation as a result of inactivity. There are a number of other ways a p-trap can lose water including improper ventilation, wind oscillation, capillary action, and siphoning. With a P Trap Pal installed, sewer gas will not enter your home even when your p-trap has dried out!