Sewer gasses not only smell unpleasant, but they can also be toxic to humans. In addition, methane may be present in sewer gasses, which is highly flammable. Some toilets may smell because they have not been cleaned in a while, but if your bowl is clean, then you need to find the source of the smell.
Frequency of Use
How often you use a toilet can affect whether or not sewer gasses can actually come up the toilet's drain system and out the toilet's bowl. On the bottom of the toilet itself is a curvy drain pipe called the trap. The trap is designed to keep water inside its curves, which will block the smell of sewer gasses from coming up through the toilet's drain pipe. If you ever remove a toilet from its mount, you will notice the smell coming from the pipe in the bathroom floor. When you do not use a toilet for an extended period of time, the water will eventually evaporate out of the toilet's trap and allow the gasses to pass through.
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A broken seal between the bottom of the toilet and the drain pipe that sits in the bathroom's floor may lead to sewer gasses leaking out. Two different pieces make up the seal between the toilet and the large drain pipe. The first piece is a flange, which is anchored by a series of screws into the bathroom floor. The flange also provides an anchor for the closet bolts, or the bolts that you see in the base of the toilet. The second piece of the seal is a wax ring, which sits between the flange and the toilet's base. An improperly placed wax ring or a loosely mounted toilet will lead to the seal breaking and sewer gasses coming through.
Another possibility is the presence of sewer microorganisms in your toilet's bowl, which you will smell each time you flush the toilet. The microorganisms normally stay at least on the other side of the toilet's trap, unable to make their way through the trap and into the bowl. During hot and humid weather, however, the sewer microorganisms can make their way through the water in the trap, because of its elevated temperature, and into the bowl. Pouring bleach down the overflow tube inside the toilet's tank will cleanse the bowl of the microorganisms, although you may need to use several cups of bleach to clean out all of them.
The vent pipe introduces air into the plumbing system, acting more as an air intake, not venting gasses or air from the pipes. A blockage in the bathroom's vent pipe will result in the toilet's water level dropping as air is sucked from other plumbing devices when one is used. Sometimes the blockage is easy to remove, like a bird's nest, while others require a plumber's assistance to remove. A plumber can also send simulated fog through the vent pipe to see if there are any leaks or sections of the pipe missing.
Overtightened nuts on the toilet's base bolts, pouring hot water in the bowl or sudden blows to the toilet can all result in the porcelain cracking. If you use an auger or snake to clear out a clog, doing so too forcefully can crack the toilet as well. Once the toilet cracks, it is only a matter of time before water or sewer gasses begin to leak out.