There has been a steep rise in cases of asthma and allergies in the U.S. over the past few decades. Why?

According to the Centers for Disease Control and Prevention (CDC), about one in 12 people in the U.S. has asthma, or about 25 million people. And the rate appears to be on the rise. From 2001 to 2011, the CDC says the number of Americans with asthma grew by 28 percent.

Asthma affects people of all ages, but it most commonly starts in childhood. According to the CDC, “the greatest rise in asthma rates was among black children (almost a 50 percent increase) from 2001 through 2009.”

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And new research presented at this year’s American College of Allergy, Asthma, and Immunology (ACAAI) annual meeting shows that scientists are seeing a corresponding rise in allergy rates as well.

“From 1976 to 1994, positive allergy skin tests in people with asthma increased significantly,” said Leonard Bielory, M.D., an ACAAI fellow, in a press release. “Not only have we found the number of asthma sufferers allergic to cats has more than doubled, but those with asthma are also 32 percent more likely to be allergic to cats than those without asthma.”

“There has been an increased prevalence of asthma in the U.S. over the past several decades,” explained allergist Clifford W. Bassett, M.D., also an ACAAI fellow. “Although we may not have definitive explanations as to this increase, there are several possible theories.”

These theories include the familiar “hygiene hypothesis” that over-sanitizing a child’s environment can lead to decreased disease resistance. The rise in allergies and asthma may also be due to an increase in airborne pollens, climate changes that trigger a rise in pollen levels, the energy-proofing of indoor home and work spaces, urban air pollution, or the overuse of antibiotics.

Many experts believe environmental factors are a main contributor to asthma and allergies. Allergic asthma, the most common type, affects one in six people with asthma in the U.S. Allergic asthma is triggered by allergens like dust, pet dander, or cockroaches. Non-allergic asthma is usually caused by exercise, stress, smoke, or airway infections.

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As the weather gets colder and more time is spent indoors, indoor asthma and allergy triggers are of growing concern. “We also know that researchers have observed, in some studies, a link of seasonal and indoor allergies to the later development of asthma,” added Dr. Bassett.

Indoor and outdoor trigger include: smoke pollution and strong odors, pet dander, mold, pollen, dust mites, exercise, pests like roaches and mice, colds and flus, certain foods, and changes in the weather.

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There are many ways to make your home more asthma- and allergy-friendly.

1. Avoid smoking around children and inside homes and cars. Chose a “smoking jacket” or other clothing used when smoking that you can remove before coming into contact with children. Also, refrain from using strong chemical-containing cleaning supplies or fragrances.

2. Chose pets without fur or feathers. If you have pets, wash them weekly and avoid having them on furniture, beds, and toys.

3. Dust often with a wet cloth and keep children away from the area as you dust. Wash bedspreads and linens once a week. If possible, buy “dust-mite impermeable” covers for mattresses and pillows.

4. To avoid pests, such as mice and cockroaches, don’t leave food or crumbs laying out. Contact pest control if you have pests in your home.

5. Many foods can cause allergies, rashes, and asthma attacks. These include milk products, eggs, peanuts, peas, beans, nuts, chocolate, shellfish, and food additives, such as those in dried apricots or red wines. If there is a possible food allergy, refrain from eating those foods and contact your doctor right away.

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If you think you have asthma or allergic asthma, be sure to talk to your doctor about your symptoms. You can get tested to see if your asthma is triggered by allergens.

“Do not refrain from exercising and being physically active just because you have asthma,” said Dr. Bassett. “In fact, many Olympic athletes have asthma. See an allergist or asthma specialist for an individual asthma action plan, who can provide you with safe ways of exercising and living a normal life with asthma.”