For a majority of Americans, natural gas is a clean, efficient source of heat. It is generally safe, provided you properly use, monitor and maintain your gas appliances and pipelines. However, the threat of fire and explosion makes it essential to recognize the symptoms, or signs, of natural gas leaks. Although natural gas is nontoxic, larger indoor leaks may result in health symptoms relating to depleted oxygen, including headaches, dizziness, fatigue and nausea. In severe cases, natural gas may cause asphyxiation.

...

Sulfurous Odor

...

The mercaptans added to natural gas give this normally odorless gas a sulfurous, rotten-egg odor that people with a properly functioning sense of smell can detect at very low levels. This strong, offensive odor is what usually alerts people to the possibility of a gas leak, especially if they have become familiar with the odor as a result of educational programs by their local gas company.

Malfunctioning Gas Appliances

...

A leak in the interior of a home may produce abnormally high pilot or burner flames on stoves and other gas appliances. The leak may also cause these appliances, or their fittings or lines, to make unusual noises. One or both of these signs, in combination with the mercaptan smell, provide very strong evidence of a leak. If you observe these signs, you should immediately leave the building and call your gas company from an outside phone. Do not use your telephone indoors if you suspect a gas leak.

Dead Vegetation Near Pipeline

...

An outdoor natural gas leak may discolor, wither or kill vegetation near the leaking pipeline or tank. In the absence of some other obvious explanation, a natural gas leak may well be the source of this damage. Other possible signs of outdoor leaks include bubbling from wet ground, and dirt being blown into the air. If you suspect an outdoor gas leak, call your gas company immediately, and ask them to send a representative to check the area with a gas-detection device.