The Environment and Your Nose

It is often said that a picture is worth a thousand words.  Equally common are the phrases hearing is believing and touching is believing, both used to suggest sensing something firsthand is convincing.  What seems to be missing in the English idiom is a phrase to account for smell.

Interestingly, smell is a particularly acute sense that ties directly to the thalmus, amygdala, and other parts of the limbic system.  It is through the sense of smell that long term memories are established and emotions are locked into the mind.

The sense of smell can be thwarted through environmental compounds able to affect the human sensory equipment called the glomeruli.   The question to be answered is whether the glomeruli and its’ intricate biochemical processes can be permanently affected causing a general loss of use and resulting in permanent loss of smell.

What would a life be like without the ability to sense the world through smell?

The sense of smell, or olfaction, is a vital sensory system that allows us to detect and interpret odors in our environment. It plays a significant role not only in the perception of food flavors but also in various other aspects of daily life and well-being.

The Olfactory System: The primary organs for detecting smell are the olfactory bulbs.  They are a pair of small, elongated structures located just above the nasal cavity and below the frontal lobe of the brain. They play a crucial role in the sense of smell, serving as the primary neural processing centers for olfactory information.

Structure:  Each olfactory bulb is composed of several layers of different types of neurons, including mitral cells, tufted cells, periglomerular cells, and granule cells.  The bulbs receive direct input from the olfactory sensory neurons located in the olfactory epithelium of the nasal cavity.

Function:  When odor molecules bind to the olfactory receptors in the nasal cavity, they stimulate the olfactory sensory neurons, generating electrical signals.  These signals travel through the olfactory nerve (cranial nerve I) to the olfactory bulb.  Within the olfactory bulb, the signals are processed and refined. This involves distinguishing between different odors and amplifying or reducing certain signals to enhance odor perception.

Olfactory Glomeruli:  The axons of olfactory sensory neurons converge in the olfactory bulb to form spherical structures called olfactory glomeruli.  Each glomerulus receives input from olfactory sensory neurons that express the same type of olfactory receptor protein. This means that each glomerulus is dedicated to processing signals from a specific type of odor molecule.

Projection to Higher Brain Centers:  The processed signals from the olfactory bulb are transmitted to various regions of the brain for further interpretation and response. These regions include the olfactory cortex, the thalamus, the amygdala, and other parts of the limbic system.  The direct connection between the olfactory system and the limbic system (involved in emotion and memory) is why smells can evoke strong emotional reactions and memories.

Plasticity:  The olfactory system, including the olfactory bulbs, exhibits a high degree of plasticity. This means that it can adapt and change in response to experiences and environmental factors. For example, exposure to new odors or prolonged lack of exposure to certain odors can lead to changes in the structure and function of the olfactory bulbs.

Health and Disease:  Changes or damage to the olfactory bulbs can lead to a range of olfactory disorders, including anosmia (loss of the sense of smell) or hyposmia (reduced sense of smell).  Some neurodegenerative diseases, like Alzheimer’s and Parkinson’s, are associated with changes in the olfactory bulbs and can lead to olfactory dysfunction.  The olfactory bulbs can also be affected by tumors, infections, or traumatic injuries.

The sense of smell, while understandably complex, is marvelously hidden from view in day-to-day life.  It is most often used without a thought.  Here are a few instances of how the sense of smell enters day-to-day living.

Interplay with Taste:  Olfaction is closely tied to gustation (sense of taste). Together, they create the perception of flavor. This is why when the sense of smell is impaired (e.g., during a cold), food can taste different or bland.

Memory and Emotion:  The olfactory system has a direct connection to the limbic system in the brain, which is involved in emotion and memory. This connection is why certain smells can evoke strong emotions or memories. A particular scent might remind someone of a specific event, place, or person from their past.

Social and Reproductive Behavior:  Smell plays a role in social interactions and mate selection. While its role in human attraction and social behavior is still a topic of research, some studies suggest that humans can pick up on certain pheromones or body odors that can influence attraction or social behavior.

Danger Detection:  Olfaction can alert us to dangers in our environment. The ability to smell smoke, spoiled food, or gas leaks is crucial for survival.

Mood and Well-being:  Certain smells can influence mood and well-being. For instance, aromatherapy uses essential oils and aromatic plant compounds to promote relaxation, alleviate stress, and improve mood.

Health Indicators:  Changes in the sense of smell can be indicative of certain health conditions. For example, a sudden loss of smell can be a symptom of viral infections, including COVID-19. Neurodegenerative diseases like Parkinson’s and Alzheimer’s can also affect the olfactory system.

Influence on Behavior and Choices:  Smells can influence behavior and decision-making, from the products we buy (perfumes, foods, etc.) to the places and environments we prefer.  Ever wonder why the odor of cinnamon is used in stores selling christmas products.

In the olfactory system, each glomerulus in the olfactory bulb receives input from olfactory sensory neurons (OSNs) that express the same type of olfactory receptor (OR). When an odorant molecule binds to a specific receptor in the nasal epithelium, it triggers a signaling pathway, which results in a signal being sent to the corresponding glomerulus in the olfactory bulb.

In cases of strong or persistent odors, it is conceivable that there might be heightened or sustained activation of specific olfactory sensory neurons, leading to increased signaling to their corresponding glomeruli. This increased signaling can be thought of as a form of “overstimulation.”

However, the concept of “overstimulation” in this context doesn’t imply damage or harm to the glomerulus. Instead, it suggests an increased or heightened neural activity in response to the odorant. The olfactory system, like other sensory systems in the body, has adaptive mechanisms that can adjust sensitivity based on the level and duration of stimulation. For instance, prolonged exposure to a particular odor can lead to “olfactory adaptation” or “olfactory fatigue,” where the intensity of the perceived smell decreases over time even though the odorant is still present.

In practical terms, while a particular glomerulus might experience increased activity from a strong odorant that matches its corresponding olfactory receptor, this doesn’t necessarily translate to any harmful overstimulation of the glomerulus or the olfactory system in general. However, as mentioned earlier, certain strong or irritating odors can lead to discomfort, but this isn’t solely due to the activity of the glomeruli but rather a more complex interplay of factors in the olfactory system and other parts of the nervous system.

Exposure to certain toxins or chemicals can lead to hyposmia (reduced sense of smell) or anosmia (complete loss of smell). The duration and intensity of exposure, as well as the specific toxin involved, will influence the severity and persistence of the olfactory loss.

Many of the toxins associated with loss of smell can be found in homes or office settings.  Some can be found in agricultural venues and other public areas like schools and transportation vehicles.   Review the toxins identified below and see if your environment might be one in which these toxins are found.

Zinc-based Nasal Sprays: Some over-the-counter nasal sprays that contain zinc have been associated with loss of smell. The FDA issued a warning about certain zinc nasal products due to reports of anosmia.

Solvents: Prolonged exposure to solvents like turpentine, benzene, toluene, and others used in industrial settings can lead to olfactory dysfunction.

Pesticides and Insecticides: Chronic exposure to certain pesticides has been associated with a decrease in olfactory function.

Heavy Metals: Prolonged exposure to heavy metals like cadmium, lead, and mercury can impact the sense of smell.

Smoke and Particulates: Prolonged exposure to smoke from fires or certain work environments, as well as fine particulates in polluted air, can cause or exacerbate olfactory loss.

Chemical Irritants: Chronic exposure to irritants like ammonia, sulfur dioxide, chlorine, and others can damage the mucous membranes in the nose and lead to olfactory issues.

Formaldehyde: This chemical, used in various industrial processes and found in some building materials, can affect the olfactory system with prolonged exposure.

The duration over which a toxin affects the sense of smell can vary. Some chemicals might cause immediate and acute olfactory dysfunction after a single, intense exposure, while others might require chronic, prolonged exposure over months or years to have a noticeable effect.

The loss might be temporary in some cases, with the sense of smell returning after the cessation of exposure and recovery of the olfactory system. In other cases, especially with long-term exposure or if there’s significant damage, the loss might be permanent.

If someone suspects that their sense of smell has been affected by exposure to a particular toxin or chemical, they should seek medical attention. A healthcare professional can help ascertain the cause, provide advice on avoiding further exposure, and might offer treatment or rehabilitation options.

Several studies have investigated the potential effects of pesticides on the olfactory system. Chronic exposure to certain pesticides can impact olfactory function, though the exact mechanisms may vary. Some pesticides that have been associated with changes in olfactory function include:

Organophosphates: These are a class of insecticides known to affect the nervous system. Chronic exposure to organophosphates can impact various neural functions, including olfaction. Examples include malathion, parathion, and chlorpyrifos. The exact mechanism isn’t fully understood, but it might relate to their primary action of inhibiting acetylcholinesterase, an enzyme crucial for neurotransmission.

Many of these chemicals are for commercial use.  However, that doesn’t mean exposure is limited.  Public areas including schools can be treated by professionals who commonly use these chemicals due to their effectiveness.  Agricultural spraying can also result in over spray or just airborne aerosol generation that can waft into communities or businesses.

Pyrethroids: These are synthetic chemicals modeled after pyrethrins, natural insecticides from chrysanthemum flowers. Some studies have suggested that pyrethroids might affect the olfactory system, though the data is less conclusive than for organophosphates.

Note: the pyrethrin family of pesticides are very common and usually considered safe for humans.  Current research is finding that this may not actually be the case.  When using an insecticide in or around the home environment be certain to wear breathing filtration equipment and avoid exposure to other mucus membranes.  Long sleeved shirts are always a good idea during application, and then removed following application.

Paraquat: This herbicide has been associated with various health concerns, including a potential increased risk for Parkinson’s disease. Given that Parkinson’s can lead to olfactory dysfunction, there’s interest in understanding whether paraquat directly affects the olfactory system or whether its potential to contribute to neurodegenerative processes is the primary concern.

Maneb: Like paraquat, this fungicide has been studied in the context of Parkinson’s disease risk. Chronic exposure might indirectly affect the olfactory system through broader neurological impacts.

It’s important to note that the degree to which a pesticide affects the sense of smell can depend on factors like the intensity and duration of exposure, the specific chemical involved, and individual susceptibility. Furthermore, the majority of these findings are based on occupational or high-dose exposures. The risk for the general public, especially when these chemicals are used according to labeled guidelines, might be different.

Understanding the enormous pressure that most farmers live under is the first step in working with these fine people.  If you are concerned with a farmer spraying crops, then go visit with them and strike up a conversation.

I have yet to meet a farmer that wouldn’t give his shirt to help someone in need.  Ask the farmer what the schedule is for spraying.  Tell them of your concern and that you want to work with them to keep your family away from the chemicals that will be used.  Tell them you want to take a day trip on the day set aside for spraying.  Most farmers don’t have a specific day, but between the two adults an email or a sign can be posted on the day of spraying.

Bear in mind that a fresh baked apple or fruit pie can go a long way to making friends with a neighbor farmer.

Protecting and preserving your sense of smell involves taking precautions against factors that can damage or diminish it. Here are some steps to protect your olfactory function:

Avoid Prolonged Exposure to Irritants: Chemicals like solvents, pesticides, and certain household cleaning agents can adversely affect your sense of smell. If you must use them, ensure proper ventilation, and consider using protective equipment like masks.

Avoid Smoking: Smoking damages the sensitive lining of the nose and can lead to chronic issues with your sense of smell. Avoiding smoking or quitting if you currently smoke can help protect your olfactory function.

Protect Against Trauma: Since head injuries can lead to olfactory loss, always use protective gear when participating in activities that might result in head trauma, like cycling, skating, or certain contact sports.

Treat Sinus and Respiratory Infections: Chronic sinusitis or respiratory infections can block or damage the olfactory epithelium. Seek treatment promptly if you suspect an infection.

Regularly Clean Your Nose: Use saline nasal sprays or nasal irrigation (like a neti pot) to keep your nasal passages clean, especially if you’re exposed to dust or other particulates.

Minimize Exposure to Air Pollutants: If you live in an area with poor air quality, consider using air purifiers in your home and avoid outdoor activities during peak pollution times.

Practice Good Hygiene: Handwashing and practicing good hygiene can reduce the risk of viral infections, some of which might lead to a temporary loss of smell.

Be Cautious with Medications: Some medications have been linked to olfactory dysfunction. If you notice changes in your sense of smell after starting a new medication, consult your healthcare provider.

Stay Hydrated: Keeping the mucous membranes in your nose moist can help in maintaining a good sense of smell.

Engage in “Olfactory Training”: If you’ve experienced a decrease in your sense of smell, some evidence suggests that regularly exposing yourself to a range of scents can help in regaining olfactory function. This is like “physical therapy” for your nose.

Regular Health Check-ups: Some conditions like diabetes, hypothyroidism, or nutritional deficiencies can affect the sense of smell. Regular health check-ups can help in early detection and management of such conditions.

Avoid Zinc-based Nasal Sprays: As previously mentioned, certain over-the-counter nasal products containing zinc have been associated with olfactory loss.

Stay Informed: Be aware of potential environmental or occupational hazards that might affect olfaction and take necessary precautions.

If you notice a significant or sudden change in your sense of smell, it’s important to seek medical advice. Early detection and management of any underlying issues can be crucial in protecting and restoring olfactory function.

Some over-the-counter (OTC) nasal sprays containing zinc have been associated with a potential loss of smell. The connection between zinc nasal products and olfactory dysfunction led the U.S. Food and Drug Administration (FDA) to issue warnings about certain zinc-containing nasal products.

For example, in 2009, the FDA advised consumers to stop using three zinc-containing intranasal products (Zicam Cold Remedy Nasal Gel, Zicam Cold Remedy Nasal Swabs, and Zicam Cold Remedy Swabs, Kids Size) due to reports of anosmia (loss of the sense of smell). These products were subsequently recalled by the manufacturer.

It’s essential to note that not all products containing zinc have been associated with this risk. The problem seems to be specific to certain zinc-based nasal preparations.

As always, if anyone is considering using an OTC product and has concerns about potential side effects, they should consult a healthcare professional. It’s also a good practice to regularly check for FDA warnings and other health advisories related to OTC products.