Mold and mildew are microscopic fungi that feed on organic material such as plant or animal matter. Mold and mildew are identical except mildew usually refers to mold that can be seen (e.g., spores, fuzzy growth, discoloration). Not all mold is visible. 

There may be over 100,000 mold varieties that survive both outdoors and indoors by spreading spores (tiny reproductive cells). Mold spores spread easily through the air. When spores are inhaled they can cause allergic rhinitis (hay fever). Because of their size, they can also reach the lungs. These airborne spores can produce mild to severe symptoms in people who are sensitive to these allergens. Allergies to mold are also responsible for a large number of severe asthmatic conditions.  

Mold and mildew allergies are usually triggered by inhaling mold spores into the lungs but may also be triggered by contact with the skin, mouth, nose and eyes. Eating foods contaminated with mold or mildew may also trigger an allergic reaction and may affect the digestive system.

Molds play a vital role in nature by decomposing leaves, wood and animal matter. Some molds are cultivated for food and medicine. Cheese and penicillin, for example, are produced by specific molds in controlled environments.

Mold can be odorless or have a musty odor or earthy smell – particularly with large mold or mildew infestations. Some molds cannot be seen because the infestation is small. When it is visible, mold commonly has a black or greenish color. Mold and mildew can be any color, however, including brown, orange, yellow, violet, blue and white.   

Unlike other airborne allergens, such as pollen, mold does not have a specific season. Instead, mold concentrations are affected by weather conditions such as wind, humidity, rain and temperature.

Mold and mildew can survive outside in freezing temperatures to eventually thrive again in warmer weather. Outdoor mold spores generally begin to appear in the spring, with levels peaking in July in warmer areas of the country and October in colder areas. Mold spores can be found outdoors year-round in many states, particularly those in the South and on the West coast. Mold can be found indoors throughout the year.

Mold allergies commonly develop in patients who are allergic to other airborne allergens, such as animal dander and pollen. The condition is also common in those who have relatives with airborne allergies. People with certain occupations are also at an increased risk for developing mold allergies due to a high level of exposure to mold. Those at risk include:

• Farmers
• Dairymen
• Loggers
• Bakers
• Mill workers
• Carpenters
• Greenhouse employees
• Wine makers
• Furniture repairers

Toxic mold (or black mold) refers to large accumulations of mold that are hidden or concealed. This mold is usually Stachybotrys chartum (Stachybotrys atra) but can also be other forms of mold. It may grow in areas that cannot be easily detected, such as in air ducts, remote basement or attic spaces, or in the hollow spaces of walls. 

Many experts link this type of mold infestation with extreme allergic reactions, birth defects, miscarriages, lung diseases, respiratory illnesses, various infections, cancer, memory loss, brain damage and death (a possibility for infants, elderly and people who are already ill). However, other experts doubt the existence of toxic mold syndrome (illnesses caused by exposure to mold). 

Research into the effects of toxic mold is ongoing and how this type of mold affects the body is not completely understood. For instance, it is not known if all reactions to toxic mold are allergy related or if there is another body response involved. However, a recent study suggests that mold growing in homes is much more likely to trigger an allergic reaction than a toxic one. Patients are encouraged to discuss their concerns with a physician.

People with allergies to mold will need to limit their exposure to these allergens as much as possible. Pollen and mold counts may be helpful. Many news sources in major cities count mold spores by taking a sample of particulates in the air and then counting and identifying the specific mold spores in the sample. Depending upon the weather, however, the amount of airborne spores in the air can change rapidly. Counts may also differ between day and night. In addition, the specific mold that causes a person's allergic reactions may not be counted separately. As a result, a mold count may not relate directly to the person's condition. Knowing the general count, however, can help a person decide whether to stay indoors or not.

Diagnosing and treating mold and mildew allergies involve the same methods as most other allergies. A physician will conduct a physical examination and obtain the patient’s medical history. If a mold allergy is suspected, the physician will perform an allergy skin test or blood test.

Once diagnosed, the best treatment strategy for mold and mildew allergies is avoidance. However, this is not always possible. Therefore, allergy medications, such as antihistamines, decongestants and corticosteroid nasal sprays, may be prescribed to help treat symptoms. Physicians may also recommend allergy shots (immunotherapy) for persistent or severe symptoms. This type of therapy works by regularly exposing an individual to very small amounts of mold allergen through a series of injections. By increasing their exposure to the allergen over a period of months or years, some individuals are able to build up a tolerance to the allergen. This can lessen the severity and frequency of allergic reactions in some people.