The Chemistry of Smell

We all learned about our five senses in grade school—sight, hearing, touch, taste, and smell. And as we got older, we learned more about what actually causes these senses to be activated. When light bounces off of an object, our eyes receive that light and we call it sight. When vibrations travel through the air, our eardrums vibrate too and our brain interprets the sound. Touch and taste are equally straightforward; when we brush up against something or put food in our mouth we touch and we taste. But what causes smells? We could just say aromas are in the air. But what is it that actually triggers our brain to recognize a scent?

The answer is small molecules and a little bit of chemistry.

Olfaction, A.K.A. Smell

The scientific word for smell is olfaction, and this process is facilitated by the olfactory system. You’ve probably noticed that “olfact-” is code for “has to do with smell;” we’ll keep using this code word as we talk about what triggers our sense of smell.

In any case, there are a lot of parts to the olfactory system. The most interesting of these parts are the olfactory receptors. These receptors are part of specialized brain cells that span from your nasal cavity, across your olfactory epithelium (a type of skin), and directly to your brain. Humans have about 12 million of these receptors while animals known for their sense of smell—like dogs—have 1 billion or more.

Courtesy of Rice University

These olfactory receptors are what receive smells and allow your brain to interpret them. But, this still doesn’t answer the question of what actually causes smells.

Small Molecules, Big Smells

When we sense a smell in the air, we’re actually sensing the presence of small, vaporized molecules. So, when you smell a flower, baking bread, or your smelly gym socks, you’re actually sensing molecules that have evaporated from the surface of those objects. These molecules move through the air and into your nose. Then, they bind with a specific set of olfactory receptors and generate a signal in your brain, triggering you to recognize that specific smell.

Courtesy of eSchoolToday

Not all materials can vaporize easily. That’s why materials like metal tend not to have a strong smell. There are also some molecules that humans don’t have odor receptors for. Gases like carbon monoxide and natural gas are odorless not because they’re not vapors, but because we simply don’t have the receptors to smell them.

All Molecules Are Not Created Equal

Not surprisingly, there are many, many different kinds of odor-causing molecules. There are also about 340 different types of odor receptors in humans that work together in different combinations to identify specific smells. Even in your home, there can be many different odor-causing molecules floating around. However, there are a few key molecules that when filtered out of your air can greatly improve your indoor air quality.

Ammonia

Ammonia and ammonia-related compounds have a characteristic, pungent smell. And in large quantities ammonia can be dangerous. But in homes, this gas often comes from a variety of minor sources and is an annoyance rather than a danger. Ammonia can come from cleaners, cat urine, older refrigerators, and fertilizers used near your home.

Formaldehyde

Formaldehyde levels in homes are typically low, but the smell of formaldehyde can be noticeable even at these low levels. This gas makes its way into homes through particleboard and plywood, cigarette smoke, and some dyes and preservatives. It is also a main ingredient in some insulation found in homes from the 1970s and 1980s.

Sulfur Compounds

Sulfur compounds, specifically hydrogen sulfide, are responsible for rotten egg smells in your home. If you’ve ever lived in a home that used well water you’re probably very familiar with this smell. While hydrogen sulfide is not dangerous at low levels—and the human nose can detect very low levels of this gas—it’s definitely a nuisance.

Homeowners have options when it comes to eliminating smells in their homes. Candles and air fresheners are often the first line of defense against home odors. But, now that we know that odors are actually caused by certain molecules in the air, there are some better options.

Rather than cover up odors the best option is to eliminate odor-causing molecules altogether. The easiest way to do this is to simply add odor-molecule trapping capabilities to the filters you already use in your home. Serionix manufactures filters like this, made to fit easily into your home furnace system. These filters were originally designed for cleanrooms and are incredibly effective at removing airborne chemicals. Using a filter like this gives you the cleanest possible air for your home!

Sources

1. http://www.rsc.org/education/eic/issues/2009March/smell-chemical-molecule-receptor-shape.asp
2. https://cnx.org/contents/5nOdwC6i@5/Taste-and-Smell
3. http://www.ehow.com/info_8783314_could-cause-smell-ammonia-home.html
4. http://www.idph.state.il.us/envhealth/factsheets/formaldehyde.htm
5. http://www.ehso.com/ehshome/hydrogensulfide.htm

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