Odors & Sense of Smell

Overview

Odors are a common complaint regarding indoor air quality and virtually all spaces will have some degree of odor presence, even if it is only detectible by a few persons.

Understanding what odors are, where they come from, and how they effect us is important when striving to improve indoor air quality. 
 

Advertisers have played a role in developing our perceptions of odors and the number of products containing a ‘fragrance’ is staggering. 

But odors are very subjective – what might be pleasant to one person maybe be objectionable to another. 

Regardless of a person’s feelings about a particular odor, it’s a fairly common belief that nice odors are healthy or harmless and bad odors are unhealthy.  Unfortunately, nothing could be further from the truth. 

People are led to believe that if something smells  “fresh & clean” it is fresh and clean. 

However, if clean means “without contamination by any substance”, including chemicals, then clean has no smell!
 

Pleasant to repulsive, all odors have one thing in common: they are chemicals.

When we ‘smell’ something, we are detecting the presence of chemical molecules in the air.  They may be chemicals that have volatilized (evaporated) into the air, aerosols, or substances adhering to airborne particles. 

However, not all chemicals produce detectible odors.  Some, like carbon monoxide, are odorless even at lethal levels.

Suffice to say, if there is an odor present, there are chemicals present.  Sometimes a single compound but more likely a mix of chemical substances (perfume can have hundreds of different ingredients).
 

Scientists are gaining insights into the sense of smell, but it still represents one of the most complex and puzzling physiological processes.

In simple terms, a smell begins when chemical molecules (odorants) dissolve in the nasal mucus and bind with receptors in specialized tissue located in the upper-rear of the nasal cavity.

The receptors transmit signals to the olfactory bulb located in the front of the brain.  The olfactory bulb, in turn, transmits signals to the cortex which distributes information to other parts of the brain.

While we understand the basic detection and signaling mechanisms, processing of the information is a bit more mysterious.

Why, for example, can a particular odor trigger a long lost memory or emotional response? And how on earth can we catalog and identify thousands of different substances (reported to be as high as 10,000 for mammals)? 

A very important aspect of our olfactory system is that it can be tricked.  The olfactory system can acclimate to certain odors – a condition called ‘nasal fatigue’ – where you get used to the odor.  The chemical is still present but you cannot smell it. 

In addition, other odors can overwhelm the sensor making the new odor predominant and the original odor less noticeable (common technique of many “air fresheners”). 

Lastly, some chemicals will effectively deaden the sense of smell so that you don’t notice them, or any others.
 

The ability to detect an odor varies dramatically between individuals and species (which is why dogs can be used to sniff out certain substances at levels we cannot detect). 

Because of the individuality involved, published data on chemical odor thresholds is not absolute and is often presented as averages, ranges, or lowest detectable levels discovered. 

The chemical itself and the amount present is obviously a controlling factor.  For a given substance, the concentration in the air, typically measured in parts per million or billion (ppm/ppb) by volume, becomes the focal point.

Some substances are not detectable at virtually any concentration, some well below cautionary levels, and some well above levels of concern.

Secondly, the agreeability of the odor has little to do with health concerns.  For example, most people find the smell of vinyl chloride to be pleasant but very high levels can cause you to pass out and extreme levels cause death.   

So does your nose really know?  When it comes to “good or bad”, probably not.
 

Odors may originate from biological sources such as bacteria and molds.  When these organisms consume nutrients and amplify (grow), volatile organic compounds (VOC’s) are liberated in their metabolic processes. 

Other sources include building materials & systems, paints, carpeting & wall coverings, furnishings, cleaning/maintenance products, textiles, personal care products and the occupants themselves.  Always remember combustion processes generate airborne chemicals. 
 

The health effects of odors are dependent on the chemicals involved, their concentration, and time of exposure. 

Adverse symptoms can range from mild irritation to death for many chemicals.  Some are known or suspected carcinogens.  It is advised to research the health information by chemical substance involved.
 

Finding out what fragrance chemicals might be in a product can be difficult because industry lobbied successfully to keep proprietary information from being revealed. 

Product regulators (FDA. EPA, CPSC, etc.) have lists of what cannot be used but beyond those substances, the manufacturer is permitted to add ingredients without disclosure under the fragrance category provided they are “generally regarded as safe” (GRAS). 

Unfortunately, GRAS-status does not necessarily mean human exposure testing has been performed on the substances...
 

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