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Why Are My Allergies Worse Indoors? Symptoms, Treatment & Test Kits

Indoor allergen facts

Indoor allergiesYour living environment may contain a variety of animal and plant life, most of which can become a source for allergens, the triggers of allergic reactions.

  • House dust is a mixture of components that can cause allergies.
  • Particles and debris from dust mites are common causes of allergies from house dust.
  • Dust mites thrive in warm, humid places.
  • Cockroach allergy can be a major factor in serious asthma and nasal allergy.
  • Symptoms of hay fever (allergic rhinitis) and asthma can be caused by the inhalation of airborne mold spores.
  • The "dander," or skin shedding of an animal, is more potent in causing allergic reactions than the animal's fur or hair.
  • About 6% of the U.S. population is allergic to cats.
  • Indoor plants, especially those that are kept in damp wicker baskets, are a source of molds.

What are allergens?

Allergens are substances that are foreign to the body and can cause an allergic reaction in certain people. Allergens are everywhere in the world around us. Examples of common allergens are pollen, foods, and mold.

Your living environment may contain a variety of animal and plant life, most of which can become a source for allergens, the triggers of allergic reactions. Pollens are the main cause of seasonal allergic rhinitis. However, if your nasal stuffiness, sneezing, watery eyes, and constant postnasal drip bother you year-round, then you most likely have nonseasonal hay fever, sometimes referred to as perennial allergic rhinitis. This condition is typically caused by indoor allergens such as dust mites, cockroach parts, mold, and animal dander.

Understanding the nature and location of common and indoor sources of allergy is fundamental. The key to managing allergies that are caused by indoor allergens involves reducing your level of exposure to them or avoiding them entirely, if possible.

This article describes some of the most common sources of allergens found inside the home environment.

What are symptoms and signs of reactions to indoor allergens?

Indoor allergens can cause symptoms in susceptible people. Seasonal allergic rhinitis can be caused by indoor allergens and manifests as nasal congestion, runny nose, watering of the eyes, sneezing, and postnasal drip. Symptoms of asthma, including wheezing or difficulty breathing, can also occur in susceptible individuals.

Allergic reaction to indoor allergens can also cause conjunctivitis, an inflammation of the lining membranes of the eyes. Symptoms of this condition include red, watery, and itching eyes.

Allergic symptoms involving the skin include eczema, or allergic dermatitis, characterized by a skin rash associated with itching. This usually results from direct contact with the allergen.

Allergy Symptoms and Signs

Allergies are exaggerated immune responses to environmental triggers known as allergens. Allergies are very common, and about 50 million people in North America suffer from allergies. One of the most common forms of allergy is allergic rhinitis (“hay fever”), which produces symptoms like:

  • nasal congestion,
  • itchy and watery eyes,
  • sneezing,
  • stuffy or runny nose,
  • scratchy or sore throat,
  • throat clearing,
  • cough from postnasal drip.

Learn more about allergy symptoms and signs »

Indoor allergens list

A number of different substances, ranging from contaminants to pets, can provoke allergic reactions in susceptible people. The following is a list of some of the most common indoor allergens:

  • House dust
  • Mold spores
  • Pollen
  • Fabrics
  • Dust mites
  • Cockroaches
  • Cat dander
  • Dog dander
  • Pet rodents
  • Horsehair
  • Houseplants

What actually is house dust?

House dust is a mixture of diverse substances that can cause allergies. House dust is composed of a number of natural substances, including dried food particles, mold spores, pollen, fabric fibers, animal dander, and insect parts, especially those of dust mites and cockroaches. However, particles and debris from dust mites are the major source of allergens in dust. As is the case with other allergens, these particles contain proteins that are small enough to become airborne and inhaled.

What are dust mites?

Dust mites, which were discovered in 1964, are microscopic arachnids (resembling tiny spiders). They are about 1/3 mm in length and are not visible to the naked eye. They have eight legs, are blind, and naturally live indoors. Their presence does not indicate that the house is dirty. This is because usual cleaning procedures, such as vacuuming and dusting, do not eliminate them.

Dust mites have "sticky" pads at the ends of their legs that help them to firmly attach to fibers, which allow them to live deep within carpeting, upholstery, and mattresses. Most of the mites found in houses are from the Dermatophagoides family, with pteronyssinus and farinae being the most common species. (Dermatophagoides comes from Latin and means skin-eating. Pteronyssinus comes from Latin and means feather-loving. Farinae is Latin for flour.) Fortunately, dust mites do not bite, spread disease, or actually live on humans.

What conditions are most favorable to dust mites?

Dust mites thrive in warm, humid places. The ideal temperature for dust mites ranges from 65 F-80 F, a common range for indoor room temperatures. Since dust mites have no means of drinking, they are totally dependent on the humidity surrounding them for water. They live best at a relative humidity above 55%. However, it is important to be aware that monitoring the humidity really isn&'t sufficient to control dust mites. As the humidity falls, dust mites will retreat from the surface. However, even in very dry conditions, it can take quite a few months to eliminate the dust-mite population.

Dust mites derive their food supply from human secretions and the skin cells we shed. (The average human sheds up to 1.5 grams of skin particles per day. A gram is about the weight of a paper clip.) The mites thrive in articles such as feather pillows, upholstered furniture, and stuffed animals that collect sloughed skin cells and secretions. The most favorable conditions for dust mite growth are found in the bedroom. Their favorite breeding grounds are mattresses, pillows, and box springs, as well as blankets, curtains, carpeting, and other fabric items in the room.

The perfect climatic conditions for dust mites occur in North America in the mid- and southern Atlantic coast, Gulf Coast, central Midwest, Pacific Northwest, and British Columbia. Dust mites are rarely found in dry climates where the elevation is over 5,000 feet above sea level.


Indoor Allergens
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How do dust mites cause allergic symptoms?

The digestive enzymes that are discharged into the mite feces are the most bothersome of the dust-mite allergens. Less potent allergens are found in the mite bodies. The mite's tiny fecal pellets disintegrate to form a very fine powder that can easily float into the air when disturbed. This commonly occurs during vacuuming, making the bed, turning in bed while sleeping, or walking on the carpet. When an allergic person inhales these particles, asthma or nasal allergy symptoms may occur. There is also evidence that allergic eczema can be aggravated by this exposure.

Allergy facts

  • Each dust mite lives for approximately 30 days and produces about 20 fecal pellets per day. During that time, females may have added 30 new dust mites to the population.
  • About 10% of the population is allergic to dust mites. About 80% of asthmatic children are allergic to dust mites.

Can cockroaches cause allergic symptoms?

Over the past decades, cockroaches have become recognized as a powerful indoor allergen. Cockroach allergy can be a major factor in serious asthma and nasal allergy. Cockroaches tend to be very troublesome in inner-city areas, multifamily dwellings, and around areas of food preparation.

Cockroaches are among the oldest of all living species (about 350 million years old). The three species of cockroaches that are commonly found in the United States are Blatella germania (German), Periploneta americana (American), and Blatella orientalis (Oriental). (The genus name for the American cockroach, periploneta, is derived from the Greek word, planetes, which means wanderer.)

Cockroaches are hardy, adaptable creatures that thrive in areas where food and water supplies are plentiful. They may be found around dripping faucets and kitchen areas. They do stray, however, to other areas and can commonly be found in children's bedrooms where food is often eaten. The major cockroach allergens are found in their digestive enzymes, saliva, and body parts. As is the case with dust-mite allergens, these microscopic particles become airborne when disturbed by motion in the room.

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What about allergies to molds?

Symptoms of hay fever (allergic rhinitis) and asthma can be caused by the inhalation of the spores of molds. Molds, or fungi, tend to grow on wet surfaces, either indoors or outdoors. The body of the mold produces spores or seeds, which are smaller than pollen and less potent as allergens. Large amounts of mold spores can be found in the air, especially in the spring and fall. They are usually most abundant at night, especially when it is damp, foggy, or during the rainy season.

Outdoor molds thrive in the soil, grass, dead leaves, haystacks, barns, and stables. They are often found in greater concentration when a hedge is located against a house, especially in shady areas. Spores from outdoor fungi typically enter the house through opened windows and doors.

Indoor molds can proliferate in bathrooms, especially in the shower stalls, damp cellars, drains, potted plants, closets, attics, house foundations, crawl spaces, and sealed-off rooms. A "musty" smell often radiates from these areas. On indoor plants, it is often the mold overgrowth and not the flowers that cause allergy.

What about allergies to pets?

The "dander," or skin shedding, of an animal is more potent in causing allergic reactions than the animal's fur or hair. In addition to the skin sheddings and fur, allergic reactions can occur to the saliva and/or urine of cats, dogs, horses, and rodents.

The scope of the animal allergy problem is enormous. These allergies are believed to affect up to 20% of North Americans and are directly related to the increasing popularity of pets, particularly cats and dogs. Studies have shown that dog allergens were found in all examined homes in the U.S., even those without family dogs. Likewise, almost all homes were shown to contain cat allergens.


The most well-known indoor allergy is probably due to Felis domesticus, the domesticated cat. The main allergen is a protein that is produced by the sweat glands (sebaceous glands) in the skin and appears in the skin flakes or dander that are shed from cats. The allergen is also found to a lesser degree in the fur, saliva, and urine of cats. Even with a past history of tolerance to cats, it is possible for a person with an allergic tendency to develop a sensitivity to cats after constant exposure.

The cat dander allergen is not only confined to the cat but also clings stubbornly to carpets, walls, and furniture. The protein can linger there for months and can serve as a reservoir from which allergens can become airborne when disturbed. The allergen is also lightweight and can float in the air for hours. People can also carry the cat allergen around on clothes, thereby spreading it to work, school, or a friend's house. Accordingly, for those who are allergic to cats, it does not reduce the risk of allergic reaction to simply isolate the cat in another room of the house. The cat dander is present wherever the cat generally exists, and it is this dander that is the problem.

Allergy facts

  • An estimated 30% of households in North America have at least one cat.
  • About 6% of the U.S. population is allergic to cats.


Domesticated dogs (Canis familiaris) are found in over 40% of homes in North America. About 33% of allergic individuals are sensitive to dog dander (as compared with almost 50% of allergic individuals who are cat-allergic). Since the allergic reaction is prompted by skin shedding and not fur, it makes little difference whether the dog has long or short hair; you can be as allergic to a Chihuahua as you can to a sheepdog. Small dogs can also cause as many allergy symptoms as large dogs. There is certainly no evidence that one species is less allergy-provoking than another one. Clearly, no breed is non-allergenic. Even poodles and wheaten terriers (often thought to be hypoallergenic) will likely induce allergy symptoms in sensitive individuals upon continuous exposure.

If your child has asthma and a known allergy, be especially careful not to allow the child to spend the night at the home of a friend or relative with a pet. Severe allergic reactions and even fatalities have been reported. Do not let this happen to your family or your friends.


Only 10% of allergic individuals have sensitivity to horses. The reason is probably due to less exposure since there is little horsehair in furniture or bedding anymore. Sensitive people, however, must avoid not only horses and stables, but also objects directly related to them, such as bridles, saddles, and riding clothes. Also be aware that horsehair may still be found in antique furniture and old toys. People who have problems with horses may also react to donkeys, mules, and zebras.

Allergy alert

Remember that a trip to the barn not only exposes you to animal dander, but also to mold, pollen, and lots of other irritants as well. If you suffer from asthma, be careful and be prepared.


Allergy to birds is more common among bird breeders where the exposure is highest. People who are sensitive to the feathers of chickens, geese, turkeys, and ducks can still eat the meat or eggs from these animals. They may well react, however, to the feathers in down comforters, pillows, and duvets. You should also remember that dust mites, another common allergen, hide in these bedding accessories.


This family includes hamsters, rabbits, guinea pigs, gerbils, ferrets, mice, and rats. Most allergic reactions are caused by exposures in laboratories, but these animals are also common pets. Mouse urine is an especially potent allergen for personnel that handle laboratory animals. The urine of rats and guinea pigs also contains allergens as do the saliva and fur of rabbits. Rabbit hair can be found in fur coats, glove linings, slippers, foot muffs, pillows, and quilts. The fur of the Angora rabbit is said to be 10 times warmer than that of sheep wool. The soft yarn spun from Angora rabbit fur can be found in hand-knitted trimming, crochet work, gloves, hosiery, and knee pads. Alone or mixed with silk, it is also used in sportswear. And, of course, rabbits often appear in schools as the classroom pet.

Allergy alert

Frequently, parents will report that their child has "won the privilege" of caring for the classroom pet over the weekend or the holiday vacation. This often leads to the onset of a particular animal sensitivity. If you or your child already has allergies or asthma, do not volunteer for the job.

What about allergies to indoor pollens and houseplants?

It is surprising, considering how frequently outdoor pollens cause allergies, that few indoor plants are troublesome. Indoor plants are more leafy than flowering and do not pollinate as much as outdoor plants. Some offending indoor plants are the weeping fig (Ficus benjamina), which can cause eye allergies, and the flowering maple (Alutilon hybridum), which can cause asthma.

Indoor plants, especially those that are kept in damp wicker baskets, are a source of molds. Plant terrariums and large indoor plants are culprits as well. Dried flowers and live Christmas trees should also be avoided, as they are common carriers of molds.

Table 1: Where Animal Allergens Come From Animal Source CatSkin, SalivaDogSkin, SalivaRatUrineGuinea PigUrine, SalivaRabbitUrine, SalivaGerbilSkin, SerumHamsterSkinMouseUrineHorseSkin

What is the treatment for allergic reactions to indoor allergens?

Avoidance of identified indoor allergens can be very effective in controlling allergy symptoms. If such avoidance is not possible or incomplete, antihistamines are a common treatment for reactions to indoor allergens. These are medications that fight the effects of the histamine released during an allergic reaction by blocking the action of the histamine on the tissue.

The so-called first-generation antihistamines include diphenhydramine (Benadryl), chlorpheniramine (Chlortrimaton), dimenhydrinate (Dramamine), brompheniramine (Dimetapp and others), clemastine fumarate (Tavist, Allerhist), and dexbrompheniramine (Drixoral). Dryness of the mouth and sleepiness are well-known side effects of first-generation antihistamines. Newer second-generation antihistamines, sometimes referred to as non-sedating antihistamines, include loratadine (Claritin), fexofenadine (Allegra), cetirizine (Zyrtec), fexofenadine (Allegra), and azelastine (Astelin Nasal Spray). These second-generation antihistamines typically have a slower onset of action, are longer acting, and do not have the sedating effects of the first-generation antihistamines.

Decongestants are drugs that reduce swelling in the nasal passages. These drugs can be helpful in reducing allergy symptoms, but they do not treat allergic reactions. Decongestants are available as pills or tablets or can be taken by nasal spray. Decongestant nasal sprays often cause a so-called "rebound effect" or worsening of symptoms if taken for too long, so they should not be used for more than five days without a doctor's recommendation. They should be used with caution in people with heart disease.

Prescription medications, including corticosteroids, cromolyn, and ipratropium (Atropine-like) nasal sprays, are used to manage some allergy symptoms. Montelukast (Singulair) is an inhibitor of a chemical involved in allergic reactions known as leukotriene. Montelukast is used to treat asthma and has also been approved for treatment of hay fever (allergic rhinitis).

Allergy immunotherapy (allergy shots) is a process of stimulation of the immune system with gradually increasing doses of the substances to which a person is allergic. Immunotherapy is very effective for management of allergies to pollen, mites, cats, and especially stinging insects (for example, bees). This treatment usually requires a series of injections and takes up to one year to become effective. The required length of treatment varies, but a typical course of treatment lasts about three years.

Is it possible to prevent reactions to indoor allergens?

Symptoms and signs of allergic reactions to indoor allergens can only be prevented to the extent that the allergens can be avoided or removed from the environment. Sometimes immunotherapy, as described above, can help prevent or decrease the severity of allergic reactions.

Indoor allergens testing

Diagnostic testing is possible to establish whether someone is allergic to common indoor allergens. Both skin tests and antibody tests may be used. Skin testing using the scratch or skin prick method involves placing a small amount of the test substance onto the skin, then pricking or scratching the skin so that the allergen enters the skin. An allergic reaction typically takes place within 15 to 20 minutes. Another type of skin test that generally shows an immediate reaction is the intradermal skin test, in which a small amount of the substance to be tested is injected beneath the skin surface, as with a TB (tuberculin) skin test.

With patch skin testing, test substances are taped to the skin for 48 hours, and the area is then observed for signs of an allergic reaction.

Skin testing for allergies is the most common way to determine if someone is allergic, but in some cases, allergy blood tests are used. These tests look for specific antibodies. Both the enzyme-linked immunosorbent assay (ELISA, or EIA) and the radioallergosorbent test (RAST) are tests to identify allergen-specific antibodies in a blood sample. Since the introduction of the ELISA test, RAST testing is less common.

Indoor allergens and mold test kit

A number of manufacturers offer indoor allergen and mold test kits that can be used to obtain samples of air or other substances in the home that are sent for analysis to a professional laboratory. A number of providers also offer air quality and allergen testing services.

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