The Ameren Illinois Utilities work hard to make natural gas safe and dependable. We have an extensive maintenance program to keep gas mains and services in the best possible shape. However, gas leaks do occasionally occur. Natural gas is injected with a substance that gives it a distinctive, "rotten egg" type of odor. For your safety, assume that such a smell is a gas leak unless you know otherwise. Never smoke, light a match, use the telephone or turn on a light or electrical switch when a natural gas odor is present. Leave the building and call the Ameren Illinois Utilities immediately.