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Natural Gas Odor Fade & Purging

Eversource would like natural gas contractors to keep these important natural gas safety messages regarding odor fade and purging in mind.

Odor Fade

Even though odorant is added to natural gas, contractors should not rely solely on the sense of smell to determine if natural gas is present in the ambient air of a work space.

This is because it may be possible that:

  • Some individuals suffer an impaired sense of smell (chronic or transient) and cannot detect the odorant;
  • The odorant can at times be disguised by other odors present on the job site or naturally occurring in the environment;
  • Some individuals who have worked around natural gas odorant for an extended period of time may suffer from odor fatigue and may be unable to recognize the presence or change in odor levels; and
  • In some rare cases, odor fade (loss of odorant) may occur making it difficult to detect the presence of natural gas in the air. In general, odor fade occurs when physical and/or chemical processes cause the level of odorant in the gas to be reduced. Odor fade can occur in both existing pipe and new installations but is most likely to occur in new steel pipe of larger diameters and longer lengths. Odorant fade can also occur in plastic pipe and in smaller and/or shorter pipe installations.

In addition to pipe age, length and composite, odor fade may also be the result of the:

  • Configuration of a customer's piping and gas related facilities;
  • Existence of rust, moisture, and/or liquids within the piping system; and
  • Pressure and flow levels of natural gas through the piping.

Given the possibilities of odor fade and odor fatigue, Eversource reminds all contractors to consider the need to condition all new pipeline installations and additions of new piping segments before placing them into service.

Likewise, contractors are reminded to utilize properly calibrated combustible gas indicators when working in an environment where a natural gas leak may be present or occur.


On February 4, 2010, the U.S. Chemical Safety and Hazard Investigation Board (CSB) approved urgent safety recommendations on gas purging. In summary, the CSB has recommended that during the purging of fuel gas piping at industrial, commercial, and public facilities:

  • Purged fuel gases shall be directly vented to a safe location outdoors, away from personnel and ignition sources
  • If it is not possible to vent purged gases outdoors, purging gas to the inside of a building shall be allowed only upon approval by the authority having jurisdiction of a documented risk evaluation and hazard control plan (i.e. local fire marshal, building inspector or other governmental agency). The evaluation and plan shall establish that indoor purging is necessary and that adequate safeguards are in place such as:
    • Evacuating nonessential personnel from the vicinity of the purging;
    • Providing adequate ventilation to maintain the gas concentration at an established safe level, substantially below the lower explosive limit; and
    • Controlling or eliminating all potential ignition sources.
  • Combustible gas detectors are used to continuously monitor the gas concentration at appropriate locations in the vicinity where purged gases are released; and
  • Personnel are trained about the problems of odor fade and odor fatigue and warned against relying on odor alone for detecting releases of fuel gases.

Please be advised that Eversource provides this safety update solely as a customer service. By providing this update, Eversource does not assume any duty or responsibility to ensure that your activities are code compliant and reminds each contractor that it is responsible for understanding and complying with the most recent building, safety and fuel gas codes and all related laws and regulations when working on or near natural gas equipment.

Though Eversource may, from time-to-time, provide you with updated information about best practices, these updates are not intended to substitute your own obligations to learn and comply with current code and safety requirements.

Though safe practices are everyone's concern, the responsibility for monitoring and complying with changes in relevant codes, laws and regulations rests solely with the person performing the work.