Summarize this article:

There are numerous potential causes of a loss of sense of smell. These include damage to the mucous membranes lining the inside of your nose and obstructions to your nasal passage. Damage to mucous membranes can occur when you have a cold, flu, or suffer from hay fever or sinusitis. These are the most common causes of a loss of smell and are generally temporary.  Obstructions, such as nasal polyps, can cause problems with smell and may in some cases require surgery. Damage to your brain or nerves can also affect your sense of smell. A head injury can result in a loss of sense of smell. Before you consider going to the doctor, you can ask yourself some questions to begin the process of evaluating the deterioration of your sense of smell. The answers to these questions will help the diagnosis if you do go to a doctor. Start by asking when you first became aware of the loss of smell, and then ask yourself about the conditions under which this occurred.  Was this a one off experience or does it recur? If so, what links the times when it does recur? Were you suffering from hay fever at the time? Did you have a cold or flu at the time? Had you suffered a head injury? Were you exposed to pollutants or substances such as dust to which you may have had an allergic reaction? Short-term changes in your sense of smell are relatively common, if you have a cold for example, but if the problems persist and your smell doesn't return you should make an appointment with your doctor. If necessary your doctor will refer you to a specialist who will test you and give you a diagnosis. You may be asked to smell specific odours in a paper booklet, and the specialist may perform an endoscopic examination of your nose.   It may not seem the most serious health problem, but your sense of smell is important and you should visit the doctor if you have concerns. If you are unable to smell be especially careful with any gas appliances and be sure not to eat any food that has passed it's best before date. Problems with your chemical senses can be an early sign of more serious conditions including Alzheimer's disease, Parkinson's disease and multiple sclerosis. Smell disorders can also be related to hypertension, obesity, malnutrition and diabetes.

Summary:
Understand the causes of a diminished sense of smell. Evaluate your sense of smell. Know when to visit a doctor.