Mercaptan is an additive that helps in the detection of gas leaks. Mercaptan has a strong sulfur-like or "rotten egg" smell. In your home, a slight odor of mercaptan might simply mean that a pilot light on a stove has been extinguished.

If a pilot light is out, turn off the gas, wait 10 minutes or more while at the same time ventilating the area, then relight the pilot light. A strong odor of mercaptan is your indication that windows should be raised, and that you should not under any circumstances turn on OR off any electrical appliances or lights. Leave the area immediately and call for gas company assistance from a neighbor's or relative's house. Do NOT make the call from a phone or cell phone in the building where the leak is suspected.