Indoor allergies

Alternative names
Allergy to mold - dander - dust; Pet allergies


Allergy-related symptoms can occur after exposure to mold, particles of animal skin usually found on animal hair (dander), dust, or other substances. These substances are usually in found indoors and do not cause symptoms for most people. (See also asthma and allergic rhinitis.)

The symptoms are identical to those of hay fever sufferers, who are sensitive to pollen.

Causes, incidence, and risk factors

Allergies are caused by an oversensitive immune system, leading to a misdirected immune response. The immune system normally protects the body against harmful substances such as bacteria and viruses. Allergy occurs when the immune system reacts to substances (allergens) that are generally harmless and in most people do not cause an immune response.

An allergy to airborne particles most commonly leads to allergic rhinitis (symptoms in the eyes, nose, and throat). Hay fever is the term used for an allergic reaction to pollen. However, people may develop an allergic reaction to other particles in the indoor or outdoor environment that leads to symptoms virtually identical to hay fever.

  • Mold - Mold spores are carried in the air and may be present all year long. Mold is most common indoors in damp locations such as swamp coolers, basements, bathrooms, or washrooms. Fabrics, rugs, stuffed animals, books, or wallpaper can harbor mold spores if they are frequently in contact with water or kept in a damp place. Outdoors, mold lives in the soil, on compost, and on damp vegetation.  
  • Animals - People who are allergic to certain animals are, in fact, rarely allergic to the animals’ fur or feathers themselves. They are actually allergic to the small scales of skin (dander) that the animal sheds. Some people are allergic to the animal’s saliva, particularly cats (whose saliva contains a protein known to cause allergy). Saliva exposure occurs if the animal licks the person. It may also occur from petting the animal after it has groomed itself or from touching an object that the animal has recently licked or chewed.  
  • Dust - House dust contains microscopic particles of pollen, mold, fibers from clothing and other fabrics, detergents, and microscopic insects (mites). Dust mites, including small fragments of dead mites, are the primary cause of dust allergy and are found in the highest numbers in bedding, mattresses, and boxsprings.

A few people develop allergies to other irritants in the environment, including smoke, fumes from industries or cleaning products, tobacco, powder (face powder, baby powder, and so on), laundry detergents, and other common substances.

When an allergen enters the body of a person with a sensitized immune system, it triggers antibody production. Histamine and other chemicals are released by body tissues as part of the immune response. This causes itching and swelling in affected tissues, mucus production, and in serious cases, hives and rashes, as well as other symptoms.

Symptoms vary in severity from person to person. Most environmental allergens contact the skin or eyes, or are inhaled. Therefore, most symptoms affect the skin, eyes, or the breathing passages.

Many disorders are associated with, triggered by, or worsened by allergies. These include eczema and asthma.

Allergies are relatively common. Heredity, environmental conditions, number and type of exposures can affect a predisposition to allergies. For reasons that are not fully clear to scientists, allergies are on the rise, particularly in so-called “Westernized” regions such as the United States and Europe.


  • runny nose  
  • tearing eyes  
  • itching of the nose, eyes, throat, or skin  
  • sneezing  
  • wheezing  
  • coughing  
  • difficulty breathing  
  • redness in the eyes  
  • hives (rarely)  
  • skin rashes  
  • sinus pressure

Signs and tests

The history of the person’s symptoms is important in diagnosing allergies, including whether the symptoms vary according to time of day or the season and possible exposures such as having a pet in the household.

Allergy testing is done to see what is cuasing the symptoms (i.e., to identify the specific allergens):

  • Skin testing is the most common and useful method. This may include intradermal, scratch, patch, or other tests.  
  • Less commonly, the suspected allergen is dissolved and dropped onto the lining of the lower eyelid as a means of testing for allergies. (This test should only be done by a physician, never the patient, since it can be harmful if done improperly.)  
  • In individuals that cannot undergo skin testing (as determined by the doctor), a RAST blood test (to look for IgE antibodies to a specific allergen) may be helpful.

Having allergies may also alter the results of the following tests:

  • a WBC count, particularly eosinophil count  
  • serum immunoelectrophoresis


The goal of treating allergic rhinitis is to reduce allergy symptoms caused by the inflammation of affected tissues.

The best “treatment” is to avoid what causes the allergic symptoms in the first place. (See Prevention, below.) It may be impossible to completely avoid all allergens to which you are sensitive, but you can often take steps to reduce exposure.

Medication options include the following:

  • Short-acting antihistamines, which are generally over-the-counter (non-prescription), often relieve mild to moderate symptoms but can cause drowsiness. A pediatrician should be consulting before using these medicines in children as they may affect learning. One formerly prescription medication, loratadine (Claritin), is now available over the counter. It does NOT tend to cause drowsiness or affect learning in children.  
  • Longer-acting antihistamines cause less drowsiness and can be equally effective, and usually do not interfere with learning. These medications, which require a prescription, include fexofenadine (Allegra), cetirizine (Zyrtec), and desloratadine (Clarinex).  
  • Nasal corticosteroid sprays are very effective and safe for people with symptoms not relieved by antihistamines alone. These prescription medications include fluticasone (Flonase), mometasone (Nasonex), and triamcinolone (Nasacort).  
  • Decongestants may also be helpful in reducing symptoms such as nasal congestion, but should not be used for long periods.  
  • Cromolyn sodium is available as a nasal spray (Nasalcrom) for treating hay fever. Eye drop versions of cromolyn sodium and antihistamines are available for itchy or bloodshot eyes.  
  • Leukotriene Inhibitors - montelukast (Singulair) is a prescription medicine approved to help control asthma and to help relieve the symptoms of seasonal allergies.

The most appropriate medication depends on the type and severity of symptoms. Specific illnesses that are caused by allergies (such as asthma and eczema) may require other treatments.

Allergy shots (immunotherapy) are occasionally recommended if the allergen cannot be avoided and symptoms are hard to control. Immunotherapy includes regular injections of the allergen, given in increasing doses (each dose is slightly larger than the previous dose) that may help the body adjust to the antigen. As mold, animal, and dust avoidance are more feasible than avoiding outdoor pollens, allergy shots are less commonly prescribed for these types of allergies.

Expectations (prognosis)

Most symptoms of allergies to mold, dander, and dust can be readily treated, and regular treatment can minimize the symptoms.

In some cases (particularly in children), people may outgrow an allergy as the immune system becomes less sensitive to the allergen. However, as a general rule, once a substance causes allergies for an individual, it can continue to affect the person long term.

People who are allergic to mold may also be allergic to penicillin, which is derived from a variety of mold. They may also be allergic to blue cheese, which is an ingredient of some salad dressings.

As noted earlier, the most severe cases of allergic rhinitis from these allergens may require immunotherapy (allergy shots) or removal of tissue in the nose (e.g., Nasal polyps) or sinuses.


  • frequent ear infections (otitis media) in children  
  • drowsiness and other side effects of antihistamines  
  • side effects of other medications (see the specific medication)  
  • sinusitis and/or Nasal polyps  
  • hives or other skin rashes  
  • anaphylaxis (a rare but severe allergic reaction)  
  • disruption of lifestyle  
  • children may breathe through the mouth instead of the nose, with resultant facial changes

Calling your health care provider
Call your health care provider if severe symptoms of allergy occur, if previously successful treatment has become ineffective, or if symptoms do not respond to treatment.


There is no known way to prevent development of allergies, but recent research has given us clues.

Breastfeeding can be effective at preventing and decreasing allergies. There is also evidence that exposures to certain allergens in the first year of life may prevent some allergies. This is called the “hygiene hypothesis” and sprang from observations that infants on farms tend to have fewer allergies than those in environments that are more sterile. Once allergies have developed, avoiding known allergens may decrease symptoms and prevent other allergies.

Mold spores are everywhere. Minimize exposure by keeping rooms dry, using a dehumidifier if neccessary. Discard moldy or mildewed articles (books, toys, shoes, and so on). Use synthetic fabrics for clothing and household furnishings whenever possible. Disinfect bathrooms, basement walls, and furniture with diluted bleach or other appropriate disinfectant solution.

Clean frequently to minimize dust. Vacuum frequently, preferably using a small-pore filter to capture dust mites. Damp-mop and dust often. Eliminate as many “dust catchers” as possible, including rugs, bed ruffles or canopies, and curtains. Bedding and mattresses harbor dust mites.

You can buy special covers for mattresses, box springs, and pillows to reduce dust mite allergens. Wash rugs, bedding, and furniture coverings weekly if possible. Central heating and air-conditioning systems may be helpful, particularly if they include special filters to capture dust.

People who are allergic to animals may need to avoid keeping pets. Frequent bathing and grooming of the pet (preferably by someone who is not allergic to the animal) may help. Allergy to animals may also include wool, which may contain tiny amounts of dander (skin).

Avoid other irritants whenever possible.

Johns Hopkins patient information

Last revised: December 3, 2012
by Levon Ter-Markosyan, D.M.D.

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