Regulations require that all gas in distribution systems must be odorized so that leaks will be noticed quickly. The odorant used is mercaptan, which is a harmless chemical that has a smell similar to rotting eggs. Odorant is monitored at all interconnection points and more is injected if needed. Odorant must also be monitored and injected at storage withdrawal, and sometimes at other key points in the distribution system. Components of an odorization system include the tank that holds the mercaptan, a pump system that injects the mercaptan into the gas stream, controllers, monitors, and filters.