Gas Odorant: Why Natural Gas Smells Like Rotten Eggs

You've probably smelled it when you turn on a gas stove: that sharp, nasty whiff. It's easy to think that's just how gas smells. But natural gas and LPG (propane) are actually completely odorless, clear like air.

What if gas had no smell at all? A loose pipe could fill your kitchen and you'd never notice. Then one tiny spark of static electricity could set off a big explosion.

So gas companies started giving the invisible danger a smell as its alarm. They mix a distinctive stink, one that human noses pick up very easily, into the clear gas. That additive is the odorant.

Odorants have to meet a tough requirement. If you added strawberry or flower scent, people might not realize gas was leaking. They might even enjoy breathing it in. So utilities deliberately pick an unpleasant smell that anyone reads as a warning.

Typical choices are sulfur compounds: tetrahydrothiophene (THT), which smells like sulfur or garlic, and tertiary butyl mercaptan (TBM), which smells like rotten onions or skunk. People often say gas smells like rotten eggs. That's really a general way of describing a sharp sulfur stink. Our noses are extremely sensitive to sulfur, so even a tiny amount in the air gets noticed right away.

In Korea, standards under the City Gas Business Act and the LPG Act require the odorant to be mixed in strictly enough that an average person can smell a leak at just 0.1% gas in the air. That's tens of times lower than the concentration where gas can ignite (about 5% for methane, about 2% for propane).

So why doesn't the stink keep coming when you cook with gas? The odorant burns along with the gas. In the stove flame, it burns up completely together with the gas, so the smelly compound disappears.

The brief whiff when you light the burner happens because, in the split second before ignition, a tiny bit of unburned gas and odorant drifts into the air. Once the flame catches properly, the smell vanishes, so it doesn't spoil your food or the air in your home.

Only a very small amount of odorant is added, so poisoning from the odorant itself isn't a major worry. A strong smell can still cause headaches or an upset stomach, though. And if leaked gas builds up indoors, it can cause not only explosions but also suffocation from lack of oxygen. If the smell keeps going, gas is still leaking. The basic safety steps are to close the valve and open the windows. Don't touch light switches, exhaust fans, outlets, or other electrical devices, and don't light any flame, because they can create a spark. If the smell continues, go outside and call your gas company or 119 (Korea's emergency number, like 911).