Strange smells coming from plumbing fixtures are more than just an annoyance. They’re often signs of something wrong—something that, left unchecked, can lead to damage, health risks, or expensive repairs. Every odor has a cause, and knowing what that smell means can help diagnose the problem and take the right steps to fix it.
One of the most recognizable odors is the rotten egg smell. This is usually caused by hydrogen sulfide gas, which forms when sulfur bacteria grow in warm, stagnant water. These bacteria are often found in drains, water heaters, or other places where water sits for long periods. As they break down organic matter, they release gas that smells like sulfur. The odor can be isolated to a single drain or spread throughout the plumbing system, depending on where the bacteria are growing.
Another common problem is sewer gas. This has a musty, decaying odor that may be constant or appear in waves. Sewer gas contains methane, ammonia, and other compounds that come from decomposing waste. In a healthy plumbing system, sewer gas is vented safely through the roof via a system of pipes. But when those pipes are blocked, broken, or improperly installed, the gas can escape into the building instead. Dry traps are a frequent cause, especially in fixtures that haven’t been used in a while. When water in the trap evaporates, it no longer forms a barrier between the living space and the sewer system.
Moldy or earthy odors under sinks or behind walls are usually caused by slow leaks. These leaks create damp, enclosed spaces where mold and mildew thrive. Over time, the moisture seeps into cabinets, drywall, and flooring, spreading the odor and causing structural damage. Leaks might come from loose fittings, worn washers, or corroded pipes, and they can go unnoticed for weeks or months before the smell becomes strong enough to signal a problem.
Chemical smells near drains or basins can come from a buildup of cleaning products or reactions between different substances in the plumbing. In some cases, poorly vented plumbing can create air pressure issues that cause gases from the drain system to bubble back into the room. These smells may be sharp, sour, or artificial and can be worsened by using too many chemical cleaners or pouring substances down the drain that don’t belong there.
In kitchens, odors often come from the garbage disposal or grease traps. Food scraps, oils, and grease can build up in the disposal unit or within the drain line, where they become breeding grounds for bacteria. If left untreated, the buildup hardens and begins to emit foul smells. Regular cleaning and proper use of the disposal help reduce these issues, but once the smell sets in, deeper cleaning or disassembly may be needed.
Laundry rooms can develop a distinct sewage odor during wash cycles. This often points to problems in the washing machine standpipe. Lint, soap, and dirt collect in the pipe and begin to decompose. Without proper venting or if the pipe is partially clogged, the smell can escape every time the machine drains. Flushing the pipe and checking the trap and venting system can help correct this issue.
Water filtration systems and softeners can also contribute to odor problems. If filters aren’t replaced regularly, or if bacteria get into the system, the result can be water that smells metallic or like sulfur. Maintenance of these systems is key, and using the correct replacement parts helps avoid contamination.
In multifamily buildings, plumbing systems are often interconnected. A problem in one unit can affect others. Odors can travel through shared vent stacks or back up into common drain lines, making it harder to trace the source. In these cases, smoke testing and video inspections are useful tools for identifying cracks, loose fittings, or other defects.
Odor issues aren’t always caused by major system failures. Often, it’s a matter of maintenance. Keeping traps filled with water, avoiding overuse of harsh chemicals, and addressing minor leaks before they spread can prevent many common plumbing smells. Routine checks of all plumbing connections—especially those hidden behind fixtures or under appliances—can reveal issues before they lead to damage.
Seasonal changes can also influence odors. In warmer months, bacteria multiply faster, and evaporation from traps can occur more quickly. During the winter, frozen vent stacks can cause pressure imbalances that lead to odors inside the home. Awareness of how temperature and humidity affect the plumbing system helps prevent unexpected surprises.
No one wants to live with strange smells coming from sinks, tubs, or drains. But those odors are important warning signs. They point to issues with ventilation, drainage, sealing, or cleanliness. Paying attention to them and responding promptly can save time, money, and frustration.
Understanding what causes plumbing odors is the first step in solving the problem. Every smell tells a story. Knowing how to read those signals makes all the difference when it comes to maintaining a clean, functional, and safe plumbing system.