Why Is Asthma So Common - What Causes Asthma: Asthma

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Why is asthma so common?

Asthma used to be on the increase in the United States (see "Asthma in America"), but its prevalence has stabilized. Despite this good news, official estimates of asthma prevalence may not truly reflect how many people really have the disease, because studies have shown that there are many people with undiagnosed asthma.

According to the Global Initiative on Asthma, more than 10% of the people in North America, Australia, and much of South America have asthma, compared with fewer than 2.5% in the former Soviet Union and much of Asia. The higher prevalence of asthma in urban versus rural environments, and in westernized versus developing countries, has made researchers wonder if some lifestyle factor has contributed to the dramatic increase of allergies and asthma in the industrialized world. Several hypotheses have been developed, although there remains no consensus about what may have caused the increase.

Air pollution. This would seem an obvious culprit for the increase in asthma in industrialized nations, since air pollution can trigger an asthma attack, but this popular explanation for the increase in asthma does not hold up under close examination. When Germany became a unified country again in 1990, researchers compared asthma prevalence in the former East Germany and West Germany. The expectation was that asthma would be more common among people living in the highly polluted East German cities. In fact, just the opposite was true: Asthma turned out to be more common in West Germany, indicating that something other than air pollution must be responsible.

Increased exposure to allergens. Another theory holds that modern urban life is exposing young children more to common household allergens and making them more susceptible to allergy-related asthma. As a rule, people are much more likely than their parents and grandparents to live in the city and spend much of the time indoors, in close quarters with dust mites, cockroaches, and animal hair (from either pets or pests). Television, computers, and video games have seduced people into staying inside even more. To further complicate matters, home insulation was upgraded in response to the energy crisis of the 1970s, so that houses and apartments today tend to be more tightly sealed. The result is far less circulation of fresh air and greater exposure to higher concentrations of allergens.

The hygiene theory. According to this hypothesis, the immune systems of children who are exposed to lots of important infections and toxins early in life tend to ignore less serious challenges, such as those from allergens like pollen and dog dander. But when children grow up in a germ-free environment, they are exposed to relatively fewer serious infections, and the immune system has more "free time" to direct its attention against harmless allergens. So the good news is that serious infections such as tuberculosis and whooping cough are much less common than they used to be. The bad news, according to the hygiene theory, is that children's immune systems are more likely to react in a way that causes allergic diseases, including asthma.

Several major medical studies have supported the hygiene hypothesis. For instance, infants who go to day care or have older siblings are less likely to develop asthma later in childhood than those who don't attend day care or have siblings. Because they are more frequently exposed to other children, the infants who go to day care and have older siblings are naturally exposed to more germs. Similarly, children living in farming communities with close contact with farm animals are less likely than others to develop asthma (and their risk decreases the more they are exposed to toxins from animal droppings). It may be that their developing immune systems learn to focus on germs and toxins and to ignore allergens, thereby decreasing the likelihood of allergies and asthma later in childhood.

Obesity. Yet another theory about the increase in asthma is that it is linked to being overweight. Obesity is common in the United States and has increased since the 1970s. Being overweight raises the risk for heart disease, diabetes, and other diseases. Findings from several studies suggest that asthma can also be added to the list of obesity-related illnesses. In one study, researchers looked at more than 7,000 children, ages 4 to 17, and found that those who were overweight were almost twice as likely to develop asthma as normal-weight children. Another study found that women who are overweight have an increased risk of asthma, wheezing, and allergy. The exact connection between asthma and weight is unknown, but one simple explanation is that excess weight places pressure on the chest, which contributes to constriction of the airways.

Last updated:	September 27, 2007