Odor

An odor, often spelled odour, is a smell or scent caused by volatilized chemical compounds. These compounds are usually found in very low concentrations in the air. Humans and many animals can perceive these chemicals through a biological system called the olfactory system.

While the word "odor" can describe any smell, including unpleasant ones, other terms are often used for pleasant scents. The food and cosmetic industries frequently use words like "aroma," "fragrance," or "scent" to describe floral smells or perfumes. Most odors consist of organic compounds, but some simple compounds without carbon, such as ammonia or hydrogen sulfide, are also odorants.

The process of smelling is a complex physiological chain. It begins in the nasal cavity, where a small patch of tissue called the olfactory epithelium is located. This tissue contains millions of olfactory receptor (OR) cells, which are specialized neurons. Each of these neurons has tiny projections called cilia that stay in direct contact with the air. When odorous molecules enter the nose, they bind to receptor proteins extending from these cilia. This binding acts as a chemical stimulus that triggers an electric signal. The signal travels along the axons of the olfactory nerve toward the brain. Once the signal reaches a specific threshold, the neuron fires, sending the message to the olfactory bulb.

The olfactory bulb is a part of the brain's limbic system. It serves as a relay station that connects the nose to the olfactory cortex. Once the signal reaches the bulb, the brain begins to interpret the smell. It does this by relating the scent to past experiences and the specific substances inhaled. The information is then forwarded to the central nervous system (CNS). The CNS is responsible for controlling emotions, behaviors, and basic thought processes. This is why a smell can instantly trigger a memory or a specific feeling. The system does not just interpret one single compound; instead, it identifies the entire odorous mix.

Our ability to recognize specific smells depends on how our receptors work. A single odorant is usually recognized by many different receptors. Conversely, different odorants are identified by unique combinations of receptors. The specific patterns of these neuron signals help the brain identify the smell. Odor sensation also depends heavily on concentration, which is the number of molecules available to the receptors. Scientists use specific methods to measure this. For example, they can find a detection threshold, which is the concentration where 50% of a population can distinguish a smell from a clean reference. They can also find a recognition threshold, which is usually two to five times higher than the detection threshold.

Research has explored the genetic and chemical makeup of these receptors. Most olfactory receptors are seven-helix-turn transmembrane proteins. Scientists have proposed that these receptors might be metalloproteins, which are proteins that contain metal ions. In 1978, a researcher named Crabtree suggested that copper might be a likely candidate for the receptor site of strong-smelling volatiles. In 2012, researchers Zhuang, Matsunami, and Block tested this in mice. They used a chemical to block copper in a mouse's nose and found the mice could not detect certain sulfur-containing compounds without it. This confirmed that copper is essential for detecting specific molecules like thiols.

Human smell varies significantly based on age and gender. Generally, the ability to identify odors decreases as people get older. Sensitivity often begins to decline during the second decade of life and deteriorates more noticeably after age 70. Regarding gender, a 2019 meta-analysis suggested that women have a small advantage in odor discrimination, though some studies claim a male advantage. Additionally, pregnant women often experience increased smell sensitivity, which can lead to food cravings or aversions. Even though humans have only 350 functional olfactory receptor genes—much fewer than the 1,300 found in mice—we are still quite capable. Studies suggest humans can distinguish roughly one trillion unique aromas.

Smell is a primary evolutionary sense that helps with survival. It can induce pleasure or subconsciously warn an animal of danger, such as a predator. It also helps organisms locate food or mates. One interesting phenomenon is habituation, or adaptation. This happens when a person is exposed to a continuous odor, such as their own body odor. The sense of smell becomes fatigued, and the brain begins to ignore the constant stimulus to focus on new changes. This allows the brain to stay alert to different or new scents in the environment. While most people only get emotional information from smells, experts like perfumers can identify individual chemicals within complex mixtures.