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Asthma: What We Know

last updated in 2004

Asthma is serious and widespread. Asthma is a serious chronic disorder of the lungs that causes recurrent attacks of breathlessness and wheezing, and can be life-threatening.

In 2002, an estimated 30.8 million people in the United States had asthma diagnosed at some point in their life, including some 8.9 million children. Asthma is estimated to affect as many as 300 million people worldwide. Asthma is the leading cause of hospitalization in children, the most prevalent chronic disease in childhood, and the most common cause of school absenteeism due to chronic disease. It is more common and more severe in African-Americans, and in people with a low income living in urban areas. In 2001, 4,269 people died of asthma in the U.S.; over 65% of these deaths occurred in women. Asthma accounts for 1 of every 250 deaths worldwide.

Asthma is increasing, especially in young children, both in the number of people affected and in severity. Asthma is increasing in the United States (figure to right) and most developed countries, in many developing countries (e.g., China, India), and indeed, worldwide.

The number of children and the total number of people with asthma in the U.S. has more than doubled since 1980. The number of women in the U.S. with asthma is increasing faster than the number of men with asthma. Comparing children hospitalized for asthma in California in 1993 and 1986, children in 1993 were twice as likely to experience an adverse outcome (such as having a breathing tube inserted into the windpipe, having normal breathing and heart functions stop, and death) compared with children hospitalized there seven years earlier. Experts agree that the increases in asthma are real and not just an increase in reporting due to increased awareness.

What Causes Asthma?

When thinking about asthma, it is important to distinguish between the factors that trigger asthma attacks and the causes of the overall increase in asthma. Much more is known about asthma triggers (see below) than asthma causes.

This distinction is important because “trigger” vs. “cause” may involve very different mechanisms and agents. For example, at least in the US, some outdoor air pollutants that trigger asthma attacks are at lower levels today than they were 3 decades ago. And there is no reason to think that some of the classic indoor allergens that trigger asthma, like cockroach feces and exoskeleton particles, or dust mites or pet dander, are more common today. Thus while asthma triggers aren’t any more common now than they were decades ago, asthma clearly has increased in frequency and severity.

What appears to be happening is that people have become more sensitized to the factors that trigger asthma attacks. By this interpretation, the cause of the increase in frequency is an increase in sensitivity to triggers, not an increase in triggers. With a large increase in sensitivity, the frequency of asthma could go up even if the triggers themselves were decreasing.

Why Is Asthma Increasing?

Genetic factors affect asthma risk. For example, children whose mothers have asthma are more likely to have asthma compared with children whose mothers don’t. But genes alone can’t explain the dramatic increase in frequency and severity of asthma:

  • More and more people without a family history of asthma are getting the disease.
  • The increase in frequency of asthma has been too fast and too large to be explained by inheritance of genetic susceptibility. Changes in gene frequency occur slowly over many generations, except under highly unusual circumstances that would leave many tell-tale signs.

If not inheritance, what environmental factors could be involved? There is no scientific consensus. Most research focuses on factors affecting the development of the immune system in ways that would increase sensitivity of the respiratory system to asthma triggers.

Household allergens are unlikely to be responsible for the increase in asthma:

  • There has not been any significant increase in indoor allergen concentrations during the last few decades to account for the doubling of asthma rates during that time.
  • There are no differences in asthma rates in dry, cool regions with low levels of house-dust mites and fungus compared to warm, humid areas where levels are high.
  • Studies have not shown that children with less exposure to these allergens are any less likely to develop asthma.

Some hypothesize that kids are growing up “too clean” … the so-called “hygiene hypothesis.” Under this interpretation, in normal childhood development kids are routinely exposed to a range of childhood diseases, domestic animals, and bacteria. When kids are protecting against these challenges because of better health care and better personal hygiene, their immune systems become over-reactive, hyper-sensitized, to asthma allergens. While this hypothesis gets a lot of attention in the press, it is not consistently supported by the scientific evidence. It fails to explain the higher risk faced by African-American children, by urban children compared to suburban children, and the increases in asthma seen in developing countries like China and India.

If neither exposure to household allergens nor the hygiene hypothesis explains asthma’s increase, what does? The answer is not known, but recent research suggests a few possibilities:

  • Exposures early in life (before birth and during infancy) can be important in setting the stage for later development of asthma. For example, a recent study found that infants exposed to herbicides and pesticides before age 1 were much more likely to develop early persistent asthma.
  • Immune system development during infancy and childhood involves changes in the predominance of different immune system cells, ones called T-helper cells. Two types of T-helper cells exist. If a baby has too high a proportion of one of these types, they are much more likely to develop asthma symptoms. Hence asthma scientists actively looking for factors that affect the proportions of these cells.
  • Breastfed infants are less likely to develop asthma and allergies compared to those fed infant formula. Breast-feeding enhances immune function.
  • Indoor air quality has changed dramatically over the last 3 decades as new chemical products have been introduced into household goods and building materials, increasing exposures to many different volatile organic compounds. Very few have been tested for effects on the developing immune system, even though it is clear that exposure in the womb can alter immune function later in life. Some types of exposure weaken immune responses while others make the immune system over-react.
  • While some types of outdoor air pollution are decreasing, ozone and fine particle pollution from diesel engine exhaust are ongoing or increasing problems, both in the US and in other countries and regions where asthma is on the rise. Kids who engage frequently sports activity in areas of high ozone have an increased risk of asthma. Several lines of evidence suggest that diesel exhaust may cause asthma. Diesel exhaust, for example, appears to change some immune cells to a type that is linked to the development of asthma. And kids growing up along streets with heavy truck traffic are more likely to have asthma-related respiratory symptoms.
  • Exposure to second-hand smoke before and after birth is linked to asthma risk. But if this were a large factor, asthma rates should be declining because fewer people smoke.
  • Interactions between different exposures may be important in the development of asthma. For example, one recent study found that asthma symptoms in children ill with a respiratory virus are likely to be more severe if they are exposed to the air pollutant nitrogen dioxide, even at levels of nitrogen dioxide below current air standards. In another study, combining exposure to low levels of pollen with exposure to levels of pollutants commonly found in urban air dramatically worsened asthma symptoms.

What Triggers an Attack?

Many things trigger asthma attacks and they vary from person to person. Some people are sensitive to only a small number of triggers while others respond to a wide array.

Common indoor triggers include second-hand smoke, dust mites, cockroach feces and exoskeleton dust, mold and pets. Outdoor triggers include ozone, nitrogen oxides, diesel exhaust and pollen. Many studies demonstrate that different constituents of air pollution can trigger asthma attacks.

In addition, infections, occupational exposures, some drugs (such as aspirin), and sulfites (food additives used in dried fruit, wine, dehydrated potato products, shrimp, etc.) can trigger asthma attacks.

Occupational exposures linked to asthma attacks include wood dust, grain dust, bacterial toxins, and chemicals such as isocyanates (used in polyurethane foams, plastics, paints, and varnishes), methacrylates (used in orthopedic surgery and dentistry as a bonding cement), some metals in a dust or aerosol form, and some pesticides. As much as 8% to 21% of asthma attacks may be explained by workplace exposures.


In sum, asthma is increasing in frequency and severity in all age groups and in most developed countries. In the U.S., it is most common in African-American children living in urban areas. Although genetic factors are involved, environmental factors are almost certainly responsible for the increases.

A growing body of research points toward changes in the immune system forced by exposures in the womb or shortly after birth as the cause of heightened sensitivity to allergens, and thus the cause of asthma's rise.

The challenge now is to identify which exposures are responsible, and then to begin implementing protective measures. Data are already sufficient to target diesel pollution, both because of its ability to trigger attacks as well its likely involvement in causing the disease in the first place.



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