Question: what is the difference between sewer gas and natural gas?
Answer: Like natural gas, sewer gas has a sulfuric odour. However, sewer gas generally smells more like rotten eggs and less like a skunk. The problem comes from a backup of sulfides, ammonia, methane, and other inorganic compounds. In low doses (and in most homes), sewer gas is harmless enough—even though it's unpleasant.