Six out of 10 people in the United States come in contact with cats or dogs. The total pet population is more than 100 million, or about four pets for every 10 people.

Allergies to pets with fur or feathers are common, especially among people who have other allergies or asthma. From 15 percent to 30 percent of people with allergies have allergic reactions to cats and dogs.

People with dog allergies may be allergic to all dogs or to only some breeds. Cat allergies are about twice as common as dog allergies.

The job of immune system cells is to find foreign substances such as viruses and bacteria and get rid of them. Normally, this response protects us from dangerous diseases. People with pet allergies have supersensitive immune systems that react to harmless proteins in the pet's dander (dead skin that is shed), saliva or urine. These proteins are called allergens.

Dogs and cats secrete fluids and shed dander that contain the allergens. They collect on fur and other surfaces. The allergens will not lose their strength for a long time, sometimes for several months. They appear to be sticky and adhere to walls, clothing and other surfaces.

Pet hair is not an allergen. It can collect dander, though. It also harbors other allergens like dust and pollen.

Cat and dog allergens are everywhere. Pet dander is even in homes never occupied by these animals because it is carried on people's clothing. The allergens get in the air with petting, grooming or stirring the air where the allergens have settled. Once airborne, the particles can stay suspended in the air for long periods of time.

Reactions to cat and dog allergens that land on the membranes that line eyes and nose include swelling and itching of the membranes, stuffy nose and inflamed eyes. A pet scratch or lick can cause the skin area to become red.

If allergen levels are low or sensitivity is minor, symptoms may not appear until after several days of contact with the pet.

Many airborne particles are small enough to get into the lungs. When inhaled, the allergens combine with antibodies. This can cause severe breathing problems-coughing, wheezing and shortness of breath-in highly sensitive people within 15 to 30 minutes. Sometimes highly sensitive people also get an intense rash on the face, neck and upper chest.

For about 20 percent to 30 percent of people with asthma, cat contact can trigger a severe asthma attack. Cat allergies also can lead to chronic asthma.

If a pet allergy is suspected, the doctor may diagnose it by taking a medical history and testing the blood of the patient. Some people are so attached to their pets that they will deny the pets could cause their symptoms. In these cases, the patient is removed from the animal's environment to see if symptoms go away. It does not help to remove the dog or cat. Allergens still in the area can cause symptoms months after the animal is gone.

To diagnose cat-induced asthma, the patient must have both of the following:

• Asthma symptoms when exposed to cat or cat allergen
• An allergic reaction to a skin test or to a blood test called RAST (radioallergosorbent test). To make sure the diagnosis is correct, the doctor will watch what happens when a cat is added then removed from the patient's environment several times.

The best treatment is to avoid contact with cats or dogs or their dander. Keep the pets out of the house, and avoid visiting people with pets. Avoiding cats and dogs may give you enough relief that you will not need medication.

Keeping the pet outdoors will help, but will not rid the house of pet allergens. Another option is to have pets that do not have fur or feathers. Fish, snakes or turtles are some choices.

To test the effect of household pets on your quality of life, remove them from your home for at least two months and clean thoroughly every week. After two months, if you still want pets, bring a pet into the house. Measure the change in your symptoms, then decide if the change in your symptoms is worth keeping the pet.

If you decide to keep a pet, bar it from the bedroom. You spend from one-third to one-half of your time there. Keep the bedroom door closed and clean the bedroom aggressively:

• Because animal allergens are sticky, you must remove the animal's favorite furniture, remove wall-to-wall carpet and scrub the walls and woodwork. Keep surfaces throughout the home clean and uncluttered. Bare floors and walls are best.
• If you must have carpet, select ones with a low pile and steam clean them frequently. Better yet, use throw rugs that can be washed in hot water.
• Wear a dust mask to vacuum. Vacuum cleaners stir up allergens that have settled on carpet and make allergies worse. Use a vacuum with a HEPA (high efficiency particulate air) filter if possible.
• Forced-air heating and air-conditioning can spread allergens through the house. Cover bedroom vents with dense filtering material like cheesecloth.
• Adding an air cleaner with a HEPA filter to central heating and air conditioning can help remove pet allergens from the air. The air cleaner should be used at least four hours per day. Another type of air cleaner that has an electrostatic filter will remove particles the size of animal allergens from the air. No air cleaner or filter will remove allergens stuck to surfaces, though. (See the Asthma and Allergy Answer article on, "Air Filters.")
• Washing the pet every week may reduce airborne allergens, but is of questionable value in reducing a person's symptoms.
• Have someone without a pet allergy brush the pet outside to remove dander as well as clean the litter box or cage.