Even though a distinctive odor is added to natural gas to assist in the detection of leaks, you should not rely on just your nose to alert you to the presence of natural gas. There are a number of reasons why your sense of smell may not always let you know that gas is leaking or has been released. And, certain conditions in pipe and soil may cause odor fade - the loss of odorant so that it is not detectable by smell.
Odor fade (loss of odorant) refers to the phenomenon in which physical and/or chemical processes cause the loss of odorant in natural gas so that its distinctive odor may no longer be detectable by smell.
Learn more about odor fade.