In an unfortunate turn of events, you reach over a candle and singe the hair on your arm. Or perhaps your curling iron starts to smoke as it burns a section of the hair on your head (as it did in the case of this young girl, whose hair trouble went viral on YouTube last year). Once you’re over the initial shock of the unexpected fire hazard, what lingers is the distinct, sharp odor of burnt hair. Thanks to reader MuradResurgence, the weasel decided to explore the cause of this funky phenomenon.
The Chemistry of Burnt Hair
Human hair is made up of keratin, a fibrous structural protein. Keratin is also a main component in skin and nails. This particular type of keratin is alpha keratin or a-keratin, and it contains significant amounts of cysteine, which acts as a cross link for keratin’s polypeptide chains. This cysteine essentially works as the glue that holds the structural elements of keratin together. As a sulfhydryl group, or an amino acid containing sulfur, cysteine is ultimately to blame for the foul smell of burnt hair.
Sulfur: The Stinky Element
There are many well-known smells of the world that can blame sulfur for their notoriety. The stench of odorized natural gas (which without the addition sulfide to aid in its detection is naturally odorless), skunk spray, and garlic can all blame their sulfuric compounds. Perhaps the most famous form of odorous sulfur is that which is attributed to rotting eggs. But give sulfur a break. Not all forms of sulfur stink. With enough oxygen, sulfur can take the form of sulfate, which is generally not as offensive as its hydrogen sulfide counterpart, the unfortunately stinky variety found in hair.
Activating the Smell
With all this chemistry in mind, the next most logical question might ask why hair doesn’t always produce that icky, sulfuric smell if its key components remain the same whether wet, dry, or burnt? Here we must refer to another of the weasel’s articles on smelly smells, “What is With That Wet Dog Smell?” to explain the basics of your olfactory system and concentrations of smelly molecules in the air. Similar to the root causes of wet dog smell, the noxious smell of burnt hair must be activated. In dogs, water acts as the amplifier of the smell known as wet dog, whereas heat releases the noxious sulfuric smell of burnt hair. And because of these smelly sulfuric compound’s lack of volatility (meaning that they are relatively slow moving through the air), burnt hair smell can be one of those lingering smells – clinging to the nostrils even after the culprit has been discarded.
Beyond it’s chemical components, one might also argue that our extreme sensitivity to the smell of burning or burnt hair is a result of some survival mechanism – alerting us immediately to the danger posed to our safety. It’s an interesting concept. Either way, we’d suggest being careful around fire and women’s hair styling devices, if only to avoid the smell.