The most common source of a rotten egg smell in residential structures is a natural gas leak. Though natural gas itself is composed primarily of methane, which has no odor, gas companies use an additive known as mercaptan to provide an early warning for homeowners
Though inhaling the methane found in natural gas is not harmful, natural gas is highly flammable and much denser than the air we normally breathe. It will displace oxygen and can cause slow suffocation of anyone inside a confined space. Natural gas emitted from a leak can be ignited by virtually any heat source or spark.