What makes gas so smelly?

There have been neighborhood reports of gas smells in the weeks before the San Bruno explosion. Unknown at this time is whether such a smell was a precursor of the gas line break.

NTSB vice chair Chris Hart mentioned at a press conference referenced in the LA Times online that not all transmission lines for gas are required to have the odorant added beforehand.

Odorant is there mainly to flag leaks in the distribution lines. As far as I know the utilities are responsible for adding it, not pumping stations upstream. But the possibility that the pipeline gas was odorized and a leak was missed needs to be checked out. PG&E may well have had other precursors that the line (built in the 1950s) needed attention.

The odorant is commonly methanethiol (aka methyl mercaptan), smelly down to a few parts per billion. Since most natural gas as extracted is odorless -- one exception is sour gas, from hydrogen sulfide -- the thiols help people detect gas leaks before the cloud a flammable or explosive mixture with air.

A 1937 explosion caused by leaking, un-odorized raw gas at a public school in New London, Texas, helped firm up the practice of adding odorants to gas. This blast, which killed almost 300 people, launched the news career of Walter Cronkite.

Many biological processes produce methanethiol, such as the human digestion of broccoli. Another is the decomposition of algae. During WWI, there was a crash effort to obtain phosphates by harvesting and digesting giant bladder kelp off the California coast. The breakdown of kelp produced a smell so atrocious that one worker called it "enough to drive a dog off a gut wagon." Colorful expression! A gut wagon was an operation that sold outdated, funky sandwiches to workers during lunch breaks.