The smell of sewage in a home bathroom is usually due to the presence of methane gas that is leaking from the sewage system and into the home. Methane gas is flammable in high amounts while also being highly toxic in small rooms or spaces.
The smell of sewage in a bathroom from a leaking sewage system can also carry gases other than just methane. The combination of gases, such as carbon monoxide, hydrogen sulfide and methane, can cause serious respiratory issues or even death if left unchecked. Clogged pipes are the most common cause of leaking gases and should be checked by a plumber or sewage expert immediately.