The odour of sewer gas is not just unpleasant, it is dangerous. Fumes are toxic and in high concentrations they are explosive due to their methane content. Ventilate the area right away with fans and open windows until the problem is corrected. The good news is not all sewer gas problems are difficult to correct and some do not require a plumber.
Dry or Unused Pipes
Bends or traps in drain pipes collect water to block sewer gases from coming into the house through open pipes. When water runs through, it is replaced with more water in the same bend. If you do not use a drain for a long period of time, the water in the bend can evaporate which opens the pipe to allow gases to pass through. Flushing the toilet or running water down the drain to refill the pipe with water will correct this problem. Pouring mineral oil down infrequently-used drains also slows evaporation down, suggests expert Tim Carter in the Ask the Builder website.
Failed Wax Toilet Ring
Most common toilets use a thick ring of sticky wax to seal the base of the toilet around the drain pipe. If the wax seal dislodges or breaks, sewer gas can enter the house. Telltale signs are an abnormally low water level in the toilet bowl and water leaking around the base of the toilet.
Replacing the sticky seal is inexpensive but it is also unpleasant. Disconnect the water supply, unfasten the bolts that secure the toilet to the floor and remove the toilet. Toilets are very heavy, so have a helper on hand. Scrape away the old wax from the bottom of the toilet and on the floor at the drain pipe, then place a new wax seal in the same position on the floor. Carefully set the toilet back into place, press down on the bowl to form a tight seal, connect the water supply and refasten the bolts.
Failed Toilet Flange
A toilet flange connects the toilet drain pipe to the floor. A failed flange creates similar symptoms to a failed wax seal, explains the websiteToiletology 101. After removing the toilet, you may notice a disconnected drain pipe or an obvious break in the flange. Without experience, you may need a plumber to replace a bad flange. Always install a new wax seal after replacing the flange.
Broken or Clogged Pipe
A broken pipe lets sewer gas escape and it often requires hiring a plumber. A break may be a literal broken or cracked pipe or it may be failed adhesive at a connection which separates two sections of pipe.
Broken pipes may be difficult to locate but plumbers use special devices to assist. A broken drain pipe may require extensive clean up and treatment to remove leaked material and sanitise the area. The problem may also be a clogged vent pipe, which usually requires a plumber to locate and remedy.