How to Recognize a Natural Gas Leak
Natural gas is a combustible mixture of hydrocarbon gases and in its pure form it is colorless, shapeless, and odorless. The characteristic “rotten egg” odor we are used to is actually an odorant added by the utility company called mercaptan and is added as an aid in detecting leaks.
Leaking natural gas may present hazards including:

• Fire
• Explosion
• Asphyxiation (Because natural gas displaces oxygen in confined spaces)

Some signs of a natural gas leak inside your home include the following:

• Distinctive "rotten egg" odor;
• Hissing or blowing sound.

Some signs of a natural gas leak outside your home include the following:

• Dead or discolored vegetation in an area that has otherwise healthy plants;
• Dirt or dust blowing out of a hole in the ground;
• Bubbles in standing water.

Natural Gas Versus Propane or LPG
Natural gas is not LPG (Liquefied Petroleum Gas). Natural gas is almost 40 times lighter than air but LPG gasses such as propane, are actually heavier than air. As a result, propane gas will collect in low places and natural gas will rise and collect in high places.
Combustibility of Natural Gas
Natural gas by itself cannot burn and must be mixed with air to ignite. But in order to ignite it must also have an ignition source like a pilot light, match or lighter, or in some cases, an electric arc from a light switch, motor or old doorbell.

What to Do if Natural Gas Has Ignited
Interestingly, burning natural gas will not explode.

If you have a situation where natural gas does ignite, it is recommended to let it burn. Do not attempt to put out the flame as this may fill the home or room with natural gas, creating a more dangerous situation.

What to Do if You Smell Natural Gas Inside the Home

Small Leak: If you suspect a small natural gas leak proceed as follows:

• Open all doors and windows.
• If it is possible, check to see if appliance burners are fully off and any pilot lights are lit.
• Call the utility company and follow their directions.

Large Leak: If the gas odor is pronounced and strong and you suspect a possible significant natural gas leak proceed as follows:

• Do not try to find the source of the natural gas leak.
• Do not try to shut off any gas valves or appliances.
• Do not operate any electrical or mechanical devices (including phones or computers).
• Do not start any vehicles or use a garage door opener.
• Leave the home removing all occupants.
• Call the utility company and follow their directions. Do not re-enter the home or return to the area until the utility company employee says it is safe.
• In the case of a suspected gas leak, only turn off the shutoff valve if the meter and shutoff valve are located outside the home, no gas can be smelled at that location and the utility company says it is safe.
• See the tutorial How to Shut Off the Natural Gas Supply Valve for instructions on shutting off a natural gas valve.