Although our olfactory sense is constantly operating, and we rely on it every day, it is also the most mysterious sense, in that most people don’t fully understand how it works. Beyond that, many people don’t quite understand why things “smell” in the first place. Why do some pleasant smells, like roses, disappear the moment we step away from the flower, while other unpleasant smells, like cat urine or a dirty sponge, hang around in the air for hours?
The most notable mystery for many smell-obsessed investigators is the tendency of certain smells to stick on our hands, even after we’ve thoroughly washed and scrubbed. After smoking a cigarette, the stench of tobacco can linger for hours on your fingers. If you cut some onions or garlic, those pungent scents seem to soak in and set up residence in your hands. The question is… why do the smells of certain foods linger on the skin?
Short Answer: Depending on the size and composition of the molecules from that food, as well as the oil content of your skin, some foods are more likely to “stick” around and keep your hands a bit stinky.
The Science of Smell
We often ask why certain things smell like they do, or why our body perceives a scent in a certain way, but we don’t often take one step further back – why do things have a “smell” at all?
Well, over the course of evolutionary history, chemical compounds developed that have a certain aroma, fragrance or flavor, and these are known as aroma compounds. Naturally speaking, these aroma compounds often develop during the ripening of fruits and vegetables, and are widely found in foods, spices, wine, oils, essential oils. There are also synthetically developed aroma compounds to flavor food, or even to add to dangerous chemicals to warn people not to consume them.
Whether these aroma compounds are natural or man-made, they play a major role in how we perceive and experience the world. Most of these chemical compounds have a molecular weight of <300, meaning that they can hang and linger in the air, be swept along by the breeze, and (in nature, at least) attract pollinators or other consumers.
When these volatile molecules are detected by the olfactory receptors in the nose, they trigger a familiar pattern of nerve activity, which our brains recognize as a particular smell. So, when you are chopping onions or sniffing a rose, the brain almost instantly registers what is lying right in front of you, since those strong-smelling plants are packed and surrounded by aroma compounds that your brain recognizes.
However, it is important to remember that not all aroma compounds are created equal. For example, esters are most commonly found in fruit, while linear-terpenes have a woody or sweet smell that often hangs in the air. Cyclic terpenes include many scents of flowers, while aromatic varieties include the powerful active ingredient smells of clove, cinnamon, anise and thyme. Alcohols, ketones, aldehydes and thiols are even more varieties of aroma compounds, each with a unique chemical composition that determines how they behave. Their chemical structure is very important in determining how long they last, and where they can stick!
Why Do My Hands Still Smell?
When you crush or process certain foods and other substances, certain aromatic compounds are activated or released. For example, when you begin cutting garlic to put in your favorite dish, garlic’s active ingredient is released, which is called allicin. It almost immediately breaks down into four other stinky sulfuric compounds, including allyl methyl sulfide, which is what gives you that characteristic bad breath. Additionally, it permeates the skin, gets into your bloodstream, and can make your urine, hair and sweat have that hint of garlic for more than 48 hours.
Sulfuric compounds are particularly pervasive, and are commonly found in onions and garlic, two of the food smells that people most often complain of “lingering” when they’re not wanted. Many of the stinkier aroma compounds, including sulfuric compounds and thiols, are hydrophobic, meaning that they repel or fail to mix with water. However, this gives them a particular affinity for oils, which they can find readily available on our skin. Certain aroma compounds are lipophilic, meaning that they are attracted to other oils, and will therefore be more difficult to get off of your hands, even if you wash and scrub them. Chemical compounds like this can penetrate into the skin, so a simple hand-washing won’t eliminate them. Once the body metabolizes them, there is a better chance of being odor-free.
Fortunately, there are some methods for removing these stuck-on smells, particularly those of garlic and onions. It has been shown – anecdotally – that chromium in the stainless steel reacts with sulfuric compounds and neutralizes the smell, rather than letting those compounds bind with your hands. If you rubs your garlic or onion-smelling hands against a stainless steel pan (no soap required), you can quickly get rid of the stench.
Tobacco is another culprit that plagues people’s nostrils; even if they wash their hands, their fingers still maintain that foul smell. All of the added chemicals and compounds in cigarettes make the smoke particularly sticky and packed with aroma compounds. These will bind with your skin, shirt, walls, carpet, furniture…. anything and everything. In a similar way to sulfuric compounds in garlic, or the acidic compounds in stool and urine, these aroma compounds will permeate your skin, particularly through extended, direct physical contact (smoking a cigarette), making it very difficult to eliminate them. Washing your hands thoroughly with warm water and a bit of baking soda (sodium bicarbonate) can be a very effective way to unbind those tobacco and resinous compounds. You certainly don’t want to be rubbing your eyes with those fingers!
As a whole, whether a smell sticks to your skin or not depends on the physiochemical properties of the aroma compound, and whether it is prone to binding with the oils on your skin.