Natural gas is odorless and colorless, so gas companies add a harmless chemical called mercaptan to give it its distinctive “rotten egg” smell. Mercaptan is an organic substance made of carbon, hydrogen, and sulfur, and is found naturally in living organisms, including the human body where it is a waste product of normal metabolism. It is often added to natural gas to make it easier to detect leaks, as it can be detected by most people in extremely small quantities, less than one part per million. The odor makes natural gas easy to detect and can alert the public of a natural gas leak, helping to avoid a potentially dangerous situation.