Smell is a chemical sensation, also known as olfaction. It occurs when airborne chemicals are detected by chemical receptors that line the nasal cavity. We can smell substances when these receptors send signals to the brain.
Smell can alert people to environmental dangers, such as smoke. We also use it to help determine what substances are, or whether or not something is safe to eat or drink.
Various factors or conditions can cause a person to lose their sense of smell. This loss is called anosmia. Loss of smell can be temporary or permanent, full or partial. The limited ability to detect smells is called hyposmia.
Humans are able to detect up to 400 different odors, far fewer than the number of odors they are presented with. A person smells via a process called chemoreception.
The olfactory system is responsible for chemoreception. This system is made up of receptor cells, or olfactory sensory neurons. These are located high up in the nose, in the epithelium, or outer layer, of the nasal cavity. These cells number in the millions and are connected to the brain. They are able detect the airborne and waterborne chemicals in a person’s surroundings.
Smells reach the olfactory system through the nostrils, or through the channel that connects the throat to the nose. When neurons detect molecules released from other substances, they send messages to the brain. The brain then registers the response as a specific smell. Each smell activates a unique set of receptors, so the brain can identify a specific smell by the combination of receptor cell responses.
Sensitive nerve endings on the moist areas of the eyes, throat, mouth, and nose also play a part in smell. They are able to sense whether or not certain substances are irritating.
The sense of smell is important for maintaining a healthy diet, as well as assessing our environment.
Because smell plays a major role in a person’s ability to taste food, a loss of smell can cause a person to alter eating habits. In many cases, people with anosmia will lose weight because eating is less enjoyable. In other cases, people eat too much and gain weight, or add salt to food to increase the flavor. This can cause or exacerbate problems with the heart, blood pressure, and thyroid.
Anosmia is sometimes an early sign of conditions like multiple sclerosis, Alzheimer’s disease, and Parkinson’s disease.
The inability to detect odors can be the result of any condition that blocks the nasal passage. This can be temporary or permanent, full or partial. A common temporary cause of loss of smell is the nasal congestion that accompanies allergies or colds. This usually clears up on its own.
Aging can cause a loss in the ability to smell. This loss can be partial or complete. Men are more susceptible to impairment, along with pregnant women (Progress in Molecular Biology and Translational Science).
Smoking cigarettes can cause a decrease in the ability to smell.
Irritation or destruction of the mucous membranes that line the inside of the nose can cause a permanent or temporary loss of smell. This is usually caused by sinus infections, hay fever, the common cold, or the flu. Non-allergic rhinitis can also cause similar damage.
Physical obstructions in the nasal passage can also affect smell. These can include tumors, nasal polyps, or nose deformities. An injury or surgery to the mouth, nose, or brain can cause problems with smell, as can a tumor or growth in the brain or elsewhere in the head. These changes can be permanent.
Because the sense of smell is the result of receptor neurons sending messages to the brain, any brain or nerve damage can affect smell. Conditions such as Huntington’s disease, Alzheimer’s disease, multiple sclerosis, and diabetes can affect smell. Brain aneurysms, brain injury, or brain surgery can also affect smell.
Certain medications can also create problems with smell, including amphetamines, estrogen, and long-term use of nasal decongestants. Radiation therapy can also affect smell.
Some smell disorders cause a change in how odors are registered by the brain. Other complications with smell can be caused by:
- hormone disorders
- nervous disorders
- exposure to insecticides
- exposure to harmful chemicals
Many smell disorders cannot be treated. Loss of smell that is caused by aging cannot be treated.
Temporary smell disorders that are caused by nasal congestion, usually the result of allergies or a cold, usually go away on their own. Antihistamines, nasal decongestants, and nasal sprays can help to speed this up.
Problems caused by other blockages in the nasal pathways, like a tumor, can go away when the underlying cause is treated. Smell issues caused by brain damage or heavy damage to the olfactory system may not be reversible, even after treatment.
Some loss in the ability to smell is a natural part of aging for many people. This is usually permanent. Loss of sense of smell caused by smoking cigarettes may be partially reversible if smoking is ceased.
People with smell disorders that are permanent will experience changes in the way that they taste food. Adding salt and other seasonings to food can boost its flavor, and may also stimulate remaining taste sensations (U.S. National Library of Medicine).
At home, people with impaired smell should use electrical devices instead of gas-operated devices, because they will be unable to smell gas leaks. They should also install smoke detectors.