Natural gas is both odorless and colorless. An ingredient called mercaptan is added to it so you can easily detect its presence. If an odorant wasn’t added, it would be difficult to know when natural gas is leaking or if an unlit valve was left on by mistake. So mercaptan’s rotten-egg smell is a very valuable safety feature.
One thing is clear -- mercaptan stinks. It takes only a few parts per million of mercaptan for the average person to wrinkle a nose and say, “What is that smell?”
If you smell gas you need to act immediately.