source of the leak to an affected tree. Harm to animals Methane is an asphyxiant gas which can reduce the normal oxygen concentration in breathing air. Small animals and birds are also more sensitive to toxic gas like carbon monoxide that are sometimes present with natural gas. The expression "canary in a coal mine" derives from the historical practice of using a canary as an animal sentinel to detect dangerously high concentrations of naturally occurring coal gas. Greenhouse gas emissions Methane, the primary constituent of natural gas, is up to 120 times as potent a greenhouse gas as carbon dioxide. Thus, the release of unburned natural gas produces much stronger effects than the carbon dioxide that would have been released if the gas had been burned as intended. Leak grades In the United States, most state and federal agencies have adopted the Gas Piping and Technology Committee (GPTC) 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 foreign sub-structure. 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