Pollen allergy, commonly called hay fever, is one of the most common chronic diseases in the United States. Worldwide, airborne dust causes the most problems for people with allergies. The respiratory symptoms of asthma, which affects approximately 15 million Americans, are often provoked by airborne allergens (substances that cause an allergic reaction).

Overall, allergic diseases are among the major causes of illness and disability in the United States, affecting as many as 40 to 50 million Americans. The National Institute of Allergy and Infectious Diseases, a component of the National Institutes of Health, conducts and supports research on allergic diseases. The goals of this research are to provide a better understanding of the causes of allergy, to improve the methods for diagnosing and treating allergic reactions, and eventually to prevent allergies. This booklet summarizes what is known about the causes and symptoms of allergic reactions to airborne allergens, how these reactions are diagnosed and treated, and what medical researchers are doing to help people who suffer from these allergies.

What is an allergy?
An allergy is a specific immunologic reaction to a normally harmless substance, one that does not bother most people. People who have allergies often are sensitive to more than one substance. Types of allergens that cause allergic reactions include pollens, dust particles, mold spores, food, latex rubber, insect venom, or medicines.

Symptoms of Allergies to Airborne Substances

The signs and symptoms are familiar to many:

• Sneezing often accompanied by a runny or clogged nose
• Coughing and post nasal drip
• Itching eyes, nose, and throat
• Allergic shiners (dark circles under the eyes caused by increased blood flow near the sinuses)
• The "allergic salute" (in a child, persistent upward rubbing of the nose that causes a crease mark on the nose)
• Watering eyes
• Conjunctivitis (an inflammation of the membrane that lines the eyelids, causing red-rimmed, swollen eyes, and crusting of the eyelids)

In people who are not allergic, the mucus in the nasal passages simply moves foreign particles to the throat, where they are swallowed or coughed out. But something different happens to a person who is sensitive to airborne allergens.

As soon as the allergen lands on the mucous membranes lining the inside of the nose, a chain reaction occurs that leads the mast cells in these tissues to release histamine and other chemicals. These powerful chemicals contract certain cells that line some small blood vessels in the nose. This allows fluids to escape, which causes the nasal passages to swell, resulting in nasal congestion.

Pollen Allergy
Each spring, summer, and fall, tiny particles are released from trees, weeds, and grasses. These particles, known as pollen, hitch rides on currents of air. Although their mission is to fertilize parts of other plants, many never reach their targets. Instead, they enter human noses and throats, triggering a type of seasonal allergic rhinitis called pollen allergy, which many people know as hay fever or rose fever (depending on the season in which the symptoms occur). Of all the things that can cause an allergy, pollen is one of the most widespread. Many of the foods, drugs, or animals that cause allergies can be avoided to a great extent; even insects and household dust are escapable. Short of staying indoors when the pollen count is high -- and even that may not help -- there is no easy way to evade wind born pollen.

People with pollen allergies often develop sensitivities to other troublemakers that are present all year, such as dust mites. For these allergy sufferers, the "sneezin' season" has no limit. Year-round airborne allergens cause perennial allergic rhinitis, as distinguished from seasonal allergic rhinitis.

Mold Allergy
Along with pollens from trees, grasses, and weeds, molds are an important cause of seasonal allergic rhinitis. People allergic to molds may have symptoms from spring to late fall. The mold season often peaks from July to late summer. Unlike pollens, molds may persist after the first killing frost. Some can grow at sub-freezing temperatures, but most become dormant. Snow cover lowers the outdoor mold count dramatically but does not kill molds. After the spring thaw, molds thrive on the vegetation that has been killed by the winter cold.

In the warmest areas of the United States, however, molds thrive all year and can cause year-round (perennial) allergic problems. In addition, molds growing indoors can cause perennial allergic rhinitis even in the coldest climates.

Where do molds grow?
Molds can be found wherever there is moisture, oxygen, and a source of the few other chemicals they need. In the fall they grow on rotting logs and fallen leaves, especially in moist, shady areas. In gardens, they can be found in compost piles and on certain grasses and weeds. Some molds attach to grains such as wheat, oats, barley, and corn, making farms, grain bins, and silos likely places to find mold.

Bakeries, breweries, barns, dairies, and greenhouses are favorite places for molds to grow. Loggers, mill workers, carpenters, furniture repairmen, and upholsterers often work in moldy environments.

Dust Mite Allergy
Dust mite allergy is an allergy to a microscopic organism that lives in the dust that is found in all dwellings and work places. Dust mites are perhaps the most common cause of perennial allergic rhinitis. Dust mite allergy usually produces symptoms similar to pollen allergy and also can produce symptoms of asthma.

What is house dust?
Rather than a single substance, so-called house dust is a varied mixture of potentially allergenic materials. It may contain fibers from different types of fabrics; cotton lint, feathers, and other stuffing materials; dander from cats, dogs, and other animals; bacteria; mold and fungus spores (especially in damp areas); food particles; bits of plants and insects; and other allergens peculiar to an individual home.

House dust also contains microscopic mites. These mites, which live in bedding, upholstered furniture, and carpets, thrive in summer and die in winter. In a warm, humid house, however, they continue to thrive even in the coldest months. The particles seen floating in a shaft of sunlight include dead dust mites and their waste-products. These waste-products, which are proteins, actually provoke the allergic reaction.

Waste products of cockroaches are also an important cause of allergy symptoms from household allergens, particularly in some urban areas of the United States.

Animal Allergy
Household pets are the most common source of allergic reactions to animals. Many people think that pet allergy is provoked by the fur of cats and dogs. But researchers have found that the major allergens are proteins secreted by oil glands in the animals' skin and shed in dander as well as proteins in the saliva, which sticks to the fur when the animal licks itself. Urine is also a source of allergy-causing proteins. When the substance carrying the proteins dries, the proteins can then float into the air. Cats may be more likely than dogs to cause allergic reactions because they lick themselves more and may be held more and spend more time in the house, close to humans.

Some rodents, such as guinea pigs and gerbils, have become increasingly popular as household pets. They, too, can cause allergic reactions in some people, as can mice and rats. Urine is the major source of allergens from these animals.

Allergies to animals can take two years or more to develop and may not subside until six months or more after ending contact with the animal. Carpet and furniture are a reservoir for pet allergens, and the allergens can remain in them for four to six weeks. In addition, these allergens can stay in household air for months after the animal has been removed. Therefore, it is wise for people with an animal allergy to check with the landlord or previous owner to find out if furry pets had lived previously on the premises.

Chemical Sensitivity
Some people report that they react to chemicals in their environment and that these allergy-like reactions appear to result from exposure to a wide variety of synthetic and natural substances, such as those found in paints, carpeting, plastics, perfumes, cigarette smoke, and plants. Although the symptoms may resemble some of the manifestations of allergies, sensitivity to chemicals does not represent a true allergic reaction involving IgE and the release of histamine or other chemicals.

Diagnosing Allergic Diseases
People with allergy symptoms, such as the runny nose of allergic rhinitis, may at first suspect they have a cold -- but the "cold" lingers on. It is important to see a doctor about any respiratory illness that lasts longer than a week or two. When it appears that the symptoms are caused by an allergy, the patient should see a physician who understands the diagnosis and treatment of allergies. If the patient's medical history indicates that the symptoms recur at the same time each year, the physician will work under the theory that a seasonal allergen (like pollen) is involved. Properly trained specialists recognize the patterns of potential allergens common during local seasons and the association between these patterns and symptoms. The medical history suggests which allergens are the likely culprits. The doctor also will examine the mucous membranes, which often appear swollen and pale or bluish in persons with allergic conditions.

Air Conditioners and Filters
When possible, an allergic person should use air conditioners inside the home or in a car to help prevent pollen and mold allergens from entering. Various types of air-filtering devices made with fiber glass or electrically charged plates may help reduce allergens produced in the home. These can be added to the heating and cooling systems. In addition, portable devices that can be used in individual rooms are especially helpful in reducing animal allergens.

An allergy specialist can suggest which kind of filter is best for the home of a particular patient. The airflow should be sufficient to exchange the air in the room five or six times per hour; therefore, the size and efficiency of the filtering device should be determined in part by the size of the room.

Very small air cleaners cannot remove dust and pollen. Buyers of electrostatic precipitators should compare the machine's ozone output with Federal standards. Ozone can irritate the nose and airways of persons with allergies, especially those with asthma, and can increase the allergy symptoms. Other kinds of air filters such as HEPA filters do not release ozone into the air. HEPA filters require adequate air flow to force air through them.