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As long as the P-trap contains water, sewer gases and odors have a harder time moving up through the drain and into the living space.
The water seal also helps reduce the chance of insects or other unwanted pests traveling through the drain opening. In other words, the P-trap is a simple but important line of defense.
Why the Water Seal Matters
The water inside a P-trap may not seem important because it stays hidden beneath the sink or inside the plumbing system.
However, that small pool of water performs a major protective function every day. It helps keep the air inside the home separate from the air and gases in the sewer line.
If that water disappears, the protective barrier disappears with it. Once the trap dries out, odors from the plumbing system can move more easily through the drain.
This is why some unused bathrooms, laundry areas, basement drains, or spare sinks may develop unpleasant smells after sitting dry for a long time.
The issue is not that the drain is dirty in the usual sense. The problem is that the water seal has weakened or vanished because the drain has not been used often enough.
Why People Think the Paper and Glass Trick Helps
The viral paper-and-glass hack is based on a legitimate plumbing idea. If a drain is covered, evaporation may slow down, and the opening may be partially blocked.
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