Sulfur compounds can produce gases that are both odorous and potentially dangerous. Under normal conditions, these gases are kept safely contained underground, but occasionally, the gases can escape to above ground parts of your home. The familiar rotten egg smell associated with sulfur gas compounds is often the clearest indication of the presence of these gases.
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Sewer gas is made up mostly of hydrogen sulfide, together with smaller concentrations of methane, ammonia, carbon dioxide, sulfur dioxide and nitrous oxides. When wastes break down inside your sewage system, sewer gases are a natural byproduct, but are kept safely contained inside the sewage systems. Mechanical failures of plumbing components or water appliances can provide the gases a means to escape and make contact with humans.
The list of possible causes for a sewer gas smell outside is a long one, but if the smell only occurs when you flush the toilet, the likely explanations are more limited. If your home uses a septic system, drain field failure, a full septic tank or another type of septic system, failure could cause sewer smells to back up into the yard. If you have recently installed a new toilet, an incorrectly installed toilet gasket could allow sewer gases to escape as well, but this is generally an interior smell.
Addressing the Problem
The first step when you smell sulfur is to keep smelling. Follow your nose to try and trace the exact location of the smell. For septic systems, check the land above your septic drain field and your septic tank vents to see if the odor is stronger there. Smell around the gasket between your toilet and your floor to see if the smells are emanating from there. If they are, simply install a new gasket per the manufacturer's instructions. If the source of the smell appears to be part of your plumbing or septic system, have a plumber inspect and repair it right away.
The health effects of sewer gas exposure range from the minor (eye and respiratory irritation, nervousness, nausea) to the more serious and even deadly. Even if large concentrations of sewer gas are escaping onto your lawn when you flush the toilet, your family is unlikely to encounter the gases in large enough concentrations to actually cause these problems. Still, it is always best to err on the side of caution by preventing people and pets from frequenting the lawn until repairs are complete and fresh air has diluted the sewer gas smell.