If you smell natural gas in a localized area near a natural gas appliance, check the pilot light. Most modern automatic equipment, like water heaters and furnaces, have safety shut-offs to control the escape of natural gas if the pilot goes out. Manually controlled appliances, like a natural gas range, may have a pilot light that does not turn off, but can be safely re-lit. All appliances should have a panel with the lighting instructions attached. If you can't determine the source of a natural gas odor, and it is localized around an appliance, turn the natural gas to the appliance off at the shut-off valve and get a professional to look at the appliance.
If the source of the odor can't be accounted for or controlled, you have an emergency. As with any emergency, stay calm. Leave the building or area and call 911.
Do not activate ignition sources.
Ignition sources can be matches or lighters, and also electrical switches. If a switch is on, leave it on; if it's off, leave it off. Either operation can cause a spark. Keep in mind that flashlights, doorbells, and telephones can be ignition sources, too.
Don't take chances! If you smell a natural gas odor, leave the premises immediately and go to a safer location to call 911 and the gas company.