Despite our best efforts at cleanliness, houses occasionally produce smells of unknown origin. Here are some common conditions that may explain the mystery:

Bathrooms

If the bathroom sink is rarely used, the trap (the P-shaped drain under the sink) may be dry. The trap holds a pool of water so that gases in the drain cannot enter the house. Running water will refill the trap and should stop the odor.

• If your trap looks like an "S" instead of a "P," the water may be sucked out of the drain (siphoned) when the sink drains. If so, you will need to consult a plumber to rework your drain line.
• In the back of many sinks, you will see holes or slots that keep the sink from overflowing. The channel from the sink to the drain can collect debris and smell. Try flushing the channel with bleach or a strong cleaner.
• Odors around the toilet may be caused by a hole in the wax ring that connects the toilet to the drain. Check to see whether your toilet is firmly connected to the floor by carefully moving the toilet front to back and side to side. If your toilet moves, your wax ring and seal may be defective.

Kitchens

• Kitchen-sink odors often come from a garbage disposer. Try chopping up a lemon rind or pouring some vinegar in the disposer. If this does not work, clean the black rubber at the top of the drain. Debris can collect and smell.
• If your hot water smells like rotten eggs (sulfur), it may be the water heater. The heater's sacrificial anode is a magnesium rod that is meant to corrode, instead of the tank corroding. Unfortunately, it can smell. Flushing your tank is usually the best solution. It is recommended to turn off the gas or electricity for the heater first. Flush the tank until the water runs clear and refill.

Musty Odor

• Musty or moldy odors are typically caused by fungus. Since fungus requires moisture, halting the moisture (leak) will usually kill it.