The component of natural gas that makes it dangerous is methane, which is extremely flammable. A natural gas leak can cause a fire or an explosion. It's not easy to detect a natural gas leak because methane, and the carbon monoxide that it releases, is odorless, tasteless and colorless. Many gas companies add mercapton or some other sulfide-based substance to natural gas. This is what gives it that rotten egg smell and makes leaks easier to detect.