House dust mites, those microscopic insects we unwittingly sleep with every night, are among the few known causes of asthma and allergic symptoms.
But pesky mites alone can't explain why the prevalence of childhood allergies has risen worldwide over the last 30 years. We haven't changed—or at least our genetic makeup hasn't—so what has?
Allergies, or immune system disorders that cause the body to overreact to substances called "allergens," are more prevalent in industrialized countries.
Exact figures aren't known, but research shows a 100 percent increase in the prevalence of asthma, an allergy-related disease, over the last 30 years in the U.S. and other industrialized countries, said Robert Wood, director of pediatric allergy and immunology at Johns Hopkins University. The prevalence of food allergies, meanwhile, has increased 18 percent between 1997 and 2007 among children under 18 years old, according to a report by the Centers for Disease Control and Prevention. The CDC report also found that children with food allergies are more likely to have asthma or other allergic conditions.
"It's likely that similar mechanisms underlie the dramatic increases in all allergic diseases," said Wood, the author of "Food Allergies for Dummies."
Still, while no one can say exactly what's causing the increase, researchers have plenty of theories. Here are a few of the leading ones:
Suspect: Dust mites, cockroaches, mold
What we know: Housing has changed — our dwellings are built more tightly and more well insulated — and we spend the majority of time indoors, where most of the exposures thought to be associated with allergies occur, said J. David Miller, a chemistry professor at Ottawa's Carlton University. The major allergen in house dust comes from mites, creatures that were rare in the U.S. in 1970 but now populate most homes.
Missing pieces: Scientists have seen increases in asthma in countries where dust mites aren't present in homes. Other proteins, including cockroaches and mold, are associated with asthma but the evidence isn't strong enough to say they actually cause the illness. For indoor fungi, "the allergens are not known," said Miller, the chair in fungal toxins and allergens for Canada's Natural Sciences and Engineering Research Council.
Suspect: Air pollution/cigarette smoke
What we know: Air pollution has been shown to exacerbate — but not cause — asthma. "It is likely that it acts as an adjuvant (speeds up the process), but this is new and under active study," said Miller. "Asthmatics are more sensitive to some components of outdoor air pollution, hence their symptoms are increased during episodes," said Miller. In preschool-age children, there's an association between cigarette smoke and asthma. It's a strong connection, but researchers still can't say tobacco smoke causes allergies.
Missing pieces: Air pollution rates have actually gone down as allergy and asthma prevalence has increased. Modeled estimates of exposure to air pollution are imprecise and incomplete, according to a 2009 study in the journal of Occupational and Environmental Medicine. Still, the same study found that urban air pollution has little or no association with the prevalence of asthma, or other allergic disorders. The link between tobacco smoke and asthma in preschoolers doesn't hold up in school-age children and adults.
What we know: Bacterial and other infections have decreased due to improved hygiene, immunization and antibiotics. Industrialized countries have higher rates of allergies than developing areas. The popular hygiene hypothesis contends that the more we restrict our exposure to microbes, the less the immune system understands how to deal with them, said Wood. Living with pets, older siblings, on a farm or in areas that have bacteria in the drinking water has been associated with lower rates of allergies and asthma. Birth method may also be important: Babies born via cesarean section miss a dose of beneficial bacteria found in the birth canal and may be more prone to asthma and allergies, research shows.
Missing pieces: The hygiene hypothesis falls apart in urban areas, where the rates of asthma have doubled in the U.S. since 1981 even though children are exposed to plenty of microbes. "If dirt were the solution, we would not have any problem with asthma in inner cities, which is emphatically not the case," said Miller. In Africa, children whose families move into cities have experienced increases in infections, wheezing and asthma, studies have shown. Farm living may be associated with reduced allergies because those who go into farming are less likely to be genetically vulnerable to allergies.
What we know: Since 1994, several studies have shown an association between elevated body mass index and asthma. Eating a " Mediterranean diet" (fruits, vegetables and fish) may provide some protection against wheeze and asthma in childhood, according to a study published in the April issue of the respiratory journal, Thorax. Eating a Western-style high-fat diet, meanwhile, can increase inflammation in the airways of asthmatic patients, research shows. Exercise has an anti-inflammatory effect. "Diet, physical activity and obesity are three distinct but strongly interrelated aspects of lifestyle that could be relevant to the prevalence and severity of asthma," Thomas A.E. Platts-Mills wrote in a 2005 study published in PLoS Medicine.
Missing pieces: Many unanswered questions have been raised about the increased use of genetically modified ingredients, which are commonly found in processed foods, the skyrocketing use of soy, pesticides in the food supply and antibiotic use in farm animals. So far, the concerns "don't have very much supporting data," said Dr. Wesley Burkes, chief of pediatric allergy and immunology at Duke University Medical Center.
The Allergy Mystery
Why do so many kids have allergies? Theories abound, but answers are few.
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