Problems with smell happen for many reasons, some clearer than others. Loss of smell may be permanent or temporary, depending on the cause. Effects of Aging As with vision and hearing, people gradually lose their ability to smell as they get older. Smell that declines with age is called presbyosmia and is not preventable. Age is only one of the many reasons that problems with smell can occur. Most people who develop a problem with smell have recently had an illness or injury. Common Causes The common causes of smell disorders are -  sinus and upper respiratory infections   - aging   - smoking  - nasal polyps   - head injury   - allergies  - hormonal disturbances  - dental problems  - exposure to certain chemicals   - numerous medications   - radiation for treatment of head and neck cancers  - diseases of the nervous system. sinus and upper respiratory infections aging smoking nasal polyps head injury allergies hormonal disturbances dental problems exposure to certain chemicals numerous medications radiation for treatment of head and neck cancers diseases of the nervous system. Sinus and Upper Respiratory Infections. The most common causes of smell problems are the common cold and chronic nasal or sinus infection. Respiratory infections such as the flu can lead to smell disorders. Swollen sinuses and nasal passages often result in problems with smell because the odors in the air do not have good access to the olfactory sensory cells. These conditions may cause total or partial loss of smell. The problem usually diminishes or goes away when the underlying medical condition clears up. Aging. Problems with smell become more common as people get older. A person's sense of smell generally declines when he or she is over 60. Smoking. Tobacco smoking is the most concentrated form of pollution that most people are exposed to. It impairs the ability to identify and enjoy odors. Nasal Polyps. Loss of smell can be caused by nasal polyps, which are small, non-cancerous growths in the nose or sinuses that can block the ability of odors to reach olfactory sensory cells high up in the nose. In rare cases, benign non-malignant tumors grow on the olfactory nerves leading to the brain, causing a loss of smell. Previous surgery or trauma to the head can impair your sense of smell because the olfactory nerves may be cut, blocked, or physically damaged. Automobile accidents are among the most frequent causes of trauma to the face and head. Allergies. If your smell disorder is caused by allergies and seasonal nasal congestion, you should avoid allergens, such as ragweed, grasses, and pet dander. Exposure to Certain Chemicals. Sometimes exposure to certain chemicals, such as insecticides and solvents, can permanently damage the sense of smell. Medications.  If you are taking certain medicines, you may notice a change in your ability to smell. Certain medications, including some common antibiotics and antihistamines, can cause problems with smell. The sense of smell usually returns to normal when you stop taking the medicine. Radiation Treatment.  People with head and neck cancers who receive radiation treatment to the nose and mouth regions commonly experience problems with their sense of smell and taste as an unfortunate side effect. Older people who have lost their larynx or voice box commonly complain of poor ability to smell and taste. Diseases of the Nervous System. Sometimes a problem with smell can be a sign of a more serious health problem. This might include diseases of the nervous system, such as Parkinson's disease or Alzheimer's disease. Loss of smell may be the first sign that something is wrong. Check with your doctor if you've been experiencing a problem with your sense of smell for a while. You may be able to prevent or get early treatment for a more serious health problem.