In six of nine cylinders, no measurable ethyl mercaptan remained in the propane. The mercaptan odor of propane can also be absorbed by soil and porous building materials. Arthur D. Little Co. did an experiment in which a concrete block was placed in a chamber, simulating the dimensions of a typical American basement, which was then filled with propane. Six hours later, no mercaptan could be detected in the gas. Absorption by mortar and soil are particular concerns because propane acts much differently than natural gas when released into the air.
Natural gas is lighter than air and tends to dissipate if leaked.
Propane, however, is heavier than air and tends to collect in low-lying places such as basements and cellars where soil, concrete and drywall are often found. Keltie's Bum Steer, for example, was built over a swimming pool used as a foundation. According to fire officials, this was the area into which the propane leaked. Absorption by soil is also a concern because large propane tanks are often buried underground at homes where the fuel is used for heating and cooking. Propane-fueled appliances and tanks are required to have labels warning consumers what do if they smell leaking gas, but do not inform consumers that they may not smell the gas.