Odor
Overview
Odor (臭氣, odor) is a sensation interpreted by the brain when volatile chemical compounds floating in the air bind to olfactory receptor cells in the olfactory epithelium inside the nose. Humans possess approximately 400 functional olfactory receptor genes, enabling the discrimination of thousands of odors. Odor has a powerful impact on survival (detecting spoiled food, warning of danger), emotion (pleasure from perfume or food), and memory (the phenomenon where a specific scent evokes the past). Scientifically, it is understood as a process where chemical signals are converted into electrical signals, and recently, olfactory artificial intelligence and electronic nose technology are rapidly advancing.
Main Content
Physiological Mechanism of Olfaction
Odor recognition occurs in three main stages. First, about 10 million olfactory nerve cells in the olfactory epithelium inside the nose come into contact with odor molecules in the air. Each nerve cell expresses one type of olfactory receptor protein, which binds to specific chemical structures. Second, upon binding, a G-protein cascade is activated, increasing intracellular cAMP concentration, opening ion channels, and generating an action potential. Third, this electrical signal is transmitted via the olfactory nerve to the olfactory bulb, then distributed to various brain regions such as the amygdala (emotion processing), hippocampus (memory storage), and thalamus, forming a comprehensive odor perception. Notably, the olfactory pathway has a unique structure that connects directly to the cerebral cortex without passing through the thalamus, unlike other senses, explaining why odors instantly stimulate emotion and memory.
Classification and Characteristics of Odors
Odors are categorized by chemical structure. Representative categories include esters (fruity scents), terpenes (pine, citrus), aldehydes (soap, grass), sulfur compounds (rotten eggs, garlic), and amines (fishy smell). Due to differences in the expression of olfactory receptor genes (OR gene family), individuals may perceive the same odor differently, explaining personal variations in sensitivity to specific smells (e.g., body odor, urine odor). Additionally, the pleasantness of an odor can change with concentration; for example, skatole is perceived as floral at low concentrations but as fecal at high concentrations.
Odor and Culture/Society
Odor carries different meanings across cultures. In the West, the perfume and deodorant industries are highly developed, while in East Asia, there is a strong tendency to view "body odor" negatively. In Korea, the smell of "doenjang" (fermented soybean paste) is positively associated with home-cooked meals, whereas in the West, it may be rejected as a strong fermented odor. Odor is also linked to social class and hygiene concepts; in 18th-century Europe, the perception that "stench = disease" drove public health policies (sewers, bathhouses). In modern times, "odor discrimination" has emerged as a social issue, sparking controversies over workplace body odor regulations and perfume use.
Measurement and Technology of Odor
Objectively measuring odor is challenging. Traditionally, sensory tests (human panels evaluating odor intensity and pleasantness) have been used, but recently, electronic nose (e-nose) technology has advanced. An e-nose consists of an array of metal oxide semiconductor sensors that learn patterns of electrical resistance changes for each odor molecule through deep learning to identify odors. As of 2024, e-noses are being practically applied in food quality control (freshness detection), medical diagnosis (analyzing breath odor of lung cancer and diabetes patients), and environmental monitoring (tracking odor emission sources). Additionally, olfactory virtual reality (VR) technology is being developed and applied in gaming, education, and therapy (alleviating odor associations in patients with post-traumatic stress disorder).
Latest Trends
As of 2024-2025, odor research and industry are undergoing rapid changes. First, AI-based olfactory prediction models are gaining attention. In September 2024, a joint research team from Google Research and MIT published a graph neural network (GNN) model that predicts odor solely from molecular structure, classifying over 5,000 odors with 95% accuracy. This is expected to revolutionize perfume development and food additive design. Second, e-nose technology has been miniaturized, with smartphone-integrated sensors entering the commercialization phase. In early 2025, Samsung and Apple each filed patents for "Galaxy Smart Sensor" and "iPhone Air Sniffer," announcing plans to incorporate features for user health monitoring (early diagnosis of respiratory diseases) and food safety (expiration date warnings). Third, olfactory marketing is becoming more sophisticated. The global perfume market is projected to grow to $80 billion by 2025, with an increasing number of hotels and retail stores introducing signature scents. Additionally, privacy concerns regarding "odor data" have emerged, with the EU discussing a draft regulation in 2024 to classify olfactory data as biometric information. Finally, research has begun on the impact of climate change on the odor ecosystem. Reports indicate that rising temperatures alter the emission of floral scent components (terpenes), affecting the behavior of pollinating insects.
Related Topics
- [[Olfaction]]
- [[Electronic nose]]
- [[Perfume]]
- [[Sense]]
- [[Chemoreception]]
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