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In common usage, a gas leak refers to a leak of natural gas, from a pipe or other containment, into a living area or any other area where the gas should not be. As natural gas may explode when exposed to flame or sparks, this situation is dangerous.
Pure natural gas is colorless and odorless. Scents in the form of traces of mercaptans are usually added, to assist in identifying leaks. This odour commonly takes the form of rotting eggs. Persons detecting the odor must evacuate the area and abstain from starting fires or using sparking electrical equipment.
As a result of the Pipeline Safety Improvement Act of 2002 passed in the United States, Federal Safety Standards require companies providing natural gas to conduct safety inspections for gas leaks in homes and other buildings receiving natural gas. The gas company is required to inspect gas meters and inside gas piping from the point of entry into the building to the outlet side of the gas meter for gas leaks. This requires entry into private homes by the natural gas companies in many cases.
Leak grades 
Most state and federal agencies have adopted the Gas Piping and Technology Committee (GPRTC) standards for grading natural gas leaks
A Grade 1 leak is a leak that represents an existing or probable hazard to persons or property, and requires immediate repair or continuous action until the conditions are no longer hazardous.
Examples of a Grade 1 leak are:
- Any leak which, in the judgment of operating personnel at the scene, is regarded as an immediate hazard.
- Escaping gas that has ignited.
- Any indication of gas which has migrated into or under a building, or into a tunnel.
- Any reading at the outside wall of a building, or where gas would likely migrate to an outside wall of a building.
- Any reading of 80% LEL, or greater, in a confined space.
- Any reading of 80% LEL, or greater in small substructures (other than gas associated sub structures) from which gas would likely migrate to the outside wall of a building.
- Any leak that can be seen, heard, or felt, and which is in a location that may endanger the general public or property.
A Grade 2 leak is a leak that is recognized as being non-hazardous at the time of detection, but justifies scheduled repair based on probable future hazard.
Examples of a Grade 2 Leak are:
- Leaks Requiring Action Ahead of Ground Freezing or Other Adverse Changes in Venting Conditions. Any leak which, under frozen or other adverse soil conditions, would likely migrate to the outside wall of a building.
- Leaks Requiring Action Within Six Months
- Any reading of 40% LEL, or greater, under a sidewalk in a wall-to-wall paved area that does not qualify as a Grade 1 leak.
- Any reading of 100% LEL, or greater, under a street in a wall-to-wall paved area that has significant gas migration and does not qualify as a Grade 1 leak.
- Any reading less than 80% LEL in small substructures (other than gas associated substructures) from which gas would likely migrate creating a probable future hazard.
- Any reading between 20% LEL and 80% LEL in a confined space.
- Any reading on a pipeline operating at 30 percent SMYS,[expand acronym] or greater, in a class 3 or 4 location,[clarification needed] which does not qualify as a Grade 1 leak.
- Any reading of 80% LEL, or greater, in gas associated sub-structures.
- Any leak which, in the judgment of operating personnel at the scene, is of sufficient magnitude to justify scheduled repair.
A Grade 3 leak is non-hazardous at the time of detection and can be reasonably expected to remain non-hazardous.
Examples of a Grade 3 Leak are:
- Any reading of less than 80% LEL in small gas associated substructures.
- Any reading under a street in areas without wall-to-wall paving where it is unlikely the gas could migrate to the out-side wall of a building.
- Any reading of less than 20% LEL in a confined space.
Other contexts 
If working with other dangerous gases (such as in a lab or industrial setting), a gas leak can refer to a leak of some other gas. The identity of the gas in question is usually obvious from context, and requires the same response: evacuate, and seek help from someone equipped to deal with that particular gas. Other details depend on the gas in question.
See also 
- Trevor A. Kletz (2001). Learning from Accidents. Gulf Professional Publishing. ISBN 075064883X.