Odor Fade
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The odor of propane may be diminished or absent under a variety of conditions. Issues begin long before propane is delivered to the consumer (see Matters of degree).Research shows that the mercaptan can chemically react and be absorbed by other materials, significantly reducing the level of odorant in the gas.
STEEL TANKS AND LINES
Odorant can be faded by the presence of air, water or rust inside a tank or cylinder. Mercaptan chemically reacts with steel and rust in tanks and fuel lines, neutralizing the odorant over a period of days or weeks. The odorant in a common 20-pound cylinder can be completely depleted in five to seven days, resulting in gas that is essentially deodorized.
Often the tanks are new or have not been used continuously. If an empty propane tank is allowed to remain open to the atmosphere, a layer of rust can form on the inside that will result in odor fade when the tank is filled again.
The risk of odor fade varies depending on the materials and methods used in manufacture of tanks. A Canadian oil company found that the odorant in gas stored in cylinders from one manufacturer was stable, but cylinders from another manufacturer showed “rapid and complete fade” of odorant.
The odor in propane can be faded in brand new cylinders. In a survey of 1-pound cylinders purchased in retail stores, the odorant was found to be significantly depleted in six of nine cylinders.
The propensity for odor fade can vary from refill to refill. The chart below shows that in one 20-pound grill-type cylinder, the level of mercaptan dropped to zero the first and third time it was filled, but not the second or fourth time.
CONCRETE, MASONRY AND POROUS MATERIALS
Propane odorant is readily absorbed by concrete, masonry, drywall and other porous materials used in foundations and basements.
In a test done for the U.S. Consumer Product Safety Commission, a concrete block was placed in a chamber representing the dimensions of a typical basement, which was then filled to an explosive level with propane. In four hours, the level of ethyl mercaptan fell below the limit of detection. Two hours later, no ethyl mercaptan remained in the gas.
Ethyl mercaptan is also effectively stripped from propane gas as it flows through soil, as from a leaking buried pipe. Percolating propane through soil can remove some or all of the odorant from propane, similarly to running water through an activated charcoal filter.
Olfaction
The acuity of the sense of smell varies widely among apparently healthy people, and diminishes with age. The older we get, the less we are able to detect odors. The ability to smell is affected by colds and allergies, smoking, and conditions such as Alzheimer’s disease. Millions of Americans are unable to smell mercaptan — many of whom are unaware of that fact.
More than half of people over 60 don’t recognize the smell of gas, according to a survey by Monell Chemical Senses Center. The average 70-year-old’s threshold for detecting ethyl mercaptan is 10 times higher than that of the average 18-year-old, according to investigators at the John B. Pierce Foundation Laboratory.
Smells can also be masked by fumes, solvents or grease, smoke, cooking and other odors.
What this means: You can’t rely on the odor of propane to warn of a leak. The odor may be very faint, or you may smell nothing unusual at all.
Read more about propane safety.