When the allergen particles come to rest in the linings of the eyes, nose, or airway of a susceptible person, an allergic reaction can occur. When the immune system has been previously sensitized to a specific invader, it overreacts to the invader; this overreaction to a harmless substance is known as a hypersensitivity or allergic reaction.



This reaction sets in motion a series of responses that culminates in release of chemicals called mediators. Histamine is an example of a mediator. It is the effects of the mediators on cells and tissues that cause allergic symptoms. Dust mites are common indoor allergens. They can be found in most homes, usually in beds and bedding, upholstered furniture, or any cloth material.

Often, when people believe they are sensitive to dust, they are actually sensitive to the dust mites and their waste particles and fragments of dust mites that have died that can be found in household dust. For some people, pets trigger allergic reactions. They simply cannot be around animals such as dogs, cats, gerbils, hamsters, and other fur-bearing animals without developing uncomfortable symptoms such as sneezing, stuffy nose, itchy eyes, and asthma.

Contrary to what many people believe, an allergic reaction to an animal is not caused by the animal's hair. An allergic reaction is actually caused by substances in the animal's saliva, urine, and especially dander. Dander is dead skin flakes (like dandruff) that become loosened from the animal's skin.

The allergens become crusted on the animal's hair or skin from urination or the animal licking or scratching itself; once dry, the allergens are released into the air, where they join the other components of house dust. Many different small animals that are popular as house pets trigger allergic reactions. Animals such as cats and dogs are most likely to cause allergic reactions.

Birds may also cause reactions, although less often than other animals. Animals such as fish, reptiles, and amphibians rarely cause allergic reactions. An allergic reaction may be triggered by any of the following directly touching the animal, being in an indoor area with the animal, being in an indoor area with furniture, carpets, bedding, drapes, clothing, animal beds or cages, even countertops and walls on which animal allergen has landed.

Also, being in an indoor area with another person whose clothes carry the allergen, cleaning animal beds, cages, or litter boxes and touching toys, bedding, towels, or other items that the animal has touched.

Molds are a type of fungus that has no stems, leaves, or roots. Molds generally live outdoors but can normally be found in almost any indoor environment. They can trigger hay fever and asthma symptoms and reproduce by releasing spores into the air, which float around until they find a hospitable environment. They easily pass through open doors and windows and settle indoors, especially where there is excess heat and humidity. They grow and produce mildew. Many of us have seen mildew growing in a shower.

Mold is not always visible, however; it can grow in unseen areas of the house, such as under flooring materials and behind walls. To grow, mold requires water; this can be either liquid water, as from a leaky pipe or roof or a puddle, or condensation on windows. It also requires something to grow on, and mold is not picky, although it is most likely to grow on wood, sheet rock, or fabric.

As it grows, mold releases more spores, many of which become part of house dust. While molds can trigger allergic symptoms, like any other allergen, they rarely cause serious health problems, except in people who are seriously immunocompromised or on chemotherapy.

Most of us don't want to think about insects in our home, especially cockroaches, but they are a fact of life. If you live in a crowded urban area, an older multifamily dwelling, or a warm climate such as the southern part of the United States, you almost surely have cockroaches living in your home, even if you don't see them. Cockroaches like moist places where food is available.

Although the kitchen is their favorite room, they can be found just about anywhere in the house. When they die, usually not in plain sight, their bodies become dried and break apart. These body pieces, as well as their dried waste, become part of house dust.