A lot, in fact. Whether it is cheese, wine, truffles, or durian, smells define them all. When we lose our sense of smell (olfaction), we could be in trouble. The COVID-19 pandemic brought the underrated sense of smell to the forefront. But there is more to the smell. Let鈥檚 dig into the science of it.
The science behind smell
Evolution has taught us how to detect specific smells or odors. Odors helped us to survive as well as reproduce. We detect odors through both the nose and the mouth. When we inhale, these odor molecules attach to our sensory neurons, which then send signals to the olfactory bulb in the brain, which analyzes and results in smell. That鈥檚 how we smell our environment, from the kitchen to the toilet.
Our body gives off odor all the time. Often, quite a specific scent. It could be through perspiration (sweat), breathing (exhale), spitting (saliva), and/or through excretion (urine). Body odors can be genetic or can vary depending on environmental or internal body conditions. Odor molecules originate from volatile organic compounds (VOCs). Think about perfume. What we smell as sweet, floral, or nutty fragrances from others鈥� clothing in a social gathering are VOCs. Like perfume, we continuously produce hundreds of VOCs in different quantities due to biochemical reactions (metabolism) within our body cells. These compounds can diffuse from the cells of origin into the bloodstream and into the air in the lungs as volatiles while exhaling or through the skin as sweat. When our cells become unhealthy due to disease, specific changes happen to these VOCs. For example, someone with diabetes can make breath smell like nail polish remover (acetone), or liver failure can give off musty fishy odor (methyl mercaptan, dimethyl disulfide). In fact, VOCs are like a molecular fingerprint. Clinical researchers identify these characteristic scents as biomarkers. This is how the Breathalyzer test works. After a few drinks, when alcohol (ethanol) becomes acid (acetic acid) due to oxidation, the Breathalyzer quickly captures the electrical conductivity changes. Credited to Robert Borkenstein, the Breathalyzer is now used by police in cities worldwide.
Smell is critical 鈥� how is so?
Even though humans are good at detecting odor at low concentrations, some insects and animals are far better than we are. Female mosquitoes have bitten all of us. They can find us from meters away. It is carbon dioxide we exhale, the odor we exude, the heat we transmit; mosquitoes sense them all. On the other hand, honeybees can detect the scent of diseased breath (VOCs) and identify the presence of tuberculosis (TB) and lung cancer. Not only insects, but also animals such as the African giant pouched rat can identify tuberculosis (TB). TB is prevalent in low-income countries. Lower-cost diagnosis in real-time, while being easy to use in a non-invasive manner in remote areas, is the key to screening the population. A rat鈥檚 nose is sensitive and faster at identifying TB bacterium than a lab technician looking for TB bacterium in sputum under a microscope. These rats are now trained to positively confirm TB in places like Tanzania, Mozambique, and Ethiopia by smell. We have seen dogs sniffing for drugs or explosives in the airports. Dogs can smell and detect traces and trails of scents (VOCs), due to their powerful and millions of olfactory receptors (approximately 300 million, compared to 5 million in humans). Even dogs have been trained to identify sources of C. difficile contamination (a cause of gastroenteritis) in hospitals. Nowadays, different types of cancers have gotten the most attention. Being non-invasive (via breath, saliva, sweat, or urine), research on animal-based diagnostics is on the rise. Prostate cancer is known to be the second leading cause of cancer death in men. Research has shown that trained dogs correctly detected (sensitivity) Gleason 9 prostate cancer from a urine sample 71% of the time. The problem is that dogs cannot be used as scalable diagnostic sensors. Moreover, animals have their own behavioral flaws, such as whether the animal is well-trained, well-rewarded, or under stress. Then there are odor issues, which can be a mixture of several volatile organic compounds. Identifying disease-specific biomarkers (olfactory signature) is critical. However, the urine scent (VOCs) signature through machine learning can have a significant impact on non-invasive diagnostics. The key to odor diagnosis is the Sensitivity and Specificity. Sensitivity correctly identifies people who have the disease, and specificity correctly identifies those who do not have the disease. Research continues with smart sensors and machine learning for the non-invasive science of olfaction.
Simple non-invasive screening over an invasive one
Oral diseases caused by bacteria can compromise overall health and reduce the quality of life. We like it or not, that鈥檚 why we visit dentists. Nearly 90% of the foul odor comes from the oral cavities, and 10% originates from the digestive and respiratory systems. Saliva contains almost 98% water and numerous organic compounds, including hormones, peptides, enzymes, proteins, urea, uric acid, lactate, and creatinine. Monitoring VOCs in saliva, produced by oral bacteria, can help identify several oral diseases, including gingivitis, periodontal disease, dental caries, and oral cancer.
A recent study even explained how VOCs in exhaled breath can help identify blood cancers. Then there are mental health challenges as our life expectancy increases. Let鈥檚 not kid ourselves. Alzheimer鈥檚 Disease (AD) has become a cause for genuine concern. Often, the process of amyloid 尾 and p-tau protein accumulation starts decades before the onset of AD. Interestingly enough, Japanese researchers have found promising results from screening odors in distinguishing during the transition from normal to the development of amyloid structures.
MRI studies have shown that the posterior cingulate cortex (PCC) region of the brain shrinks in individuals with AD. Can we boost our brain size with smell? Recently, Kyoto University and University of Tsukuba researchers figured that out with 28 women who wore rose-scented oil on their clothes for a month. In comparison, 22 women wore clothes without any scent. The verdict: volume of gray matter in the PCC increased in those who wore rose-scented oil for a month.
Does this mean scents could keep AD at bay? We are not there yet, but let鈥檚 keep smelling.
Prithu Mukhopadhyay, Ph.D.
Editor-in-Chief
Journal of Vinyl & Additive Technology
Montreal, Quebec, Canada
