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Causes of Asthma

Although research has revealed much about asthma over the last decades, we still don't really understand what causes the disease. We have more and more pieces of intriguing information about asthma, but we don't know how those pieces will end up fitting together. Some pieces will be causes, some will be contributing factors, some will be dead ends.

Through ongoing research, we continue to draw closer to the day when all the pieces are in place. When we know what causes asthma, we will be much better equipped to prevent it and possibly to cure it.

Until then, we have some interesting puzzle pieces to work with.


One risk factor for asthma is a condition called atopy, which is essentially a predisposition to be allergic to things. (Atopy is believed to be present in around 20% to 40% of humans.) The evidence is quite clear that heredity is partially responsible for a person having atopy. In fact, progress has been made in recent years in identifying where on the chromosomes some of the genes responsible for atopy might be located.

So we know that genes contribute to atopy, which is a risk factor for asthma. But are there specific asthma genes? The answer is still uncertain. The evidence is convincing that asthma is not a "single-gene" disease. Involvement of several different genes, though, in such asthma components as airway hyperresponsiveness and non-atopy-related inflammation seems likely but has not been proven.


A large percentage (probably about 1/3) of babies and young children will experience wheezing at some point, usually during a viral respiratory infection. Some, but not all, of these children go on to have chronic asthma. It is unclear just what the role of the viral infection is, but it is possible that it is part of the cause of asthma. The viruses that most commonly cause wheezing in infants are RSV (respiratory syncytial virus), parainfluenza virus, and, in slightly older children, rhinovirus (the virus responsible for the common cold).

Air Quality

The evidence is clear that air pollution can have a negative effect on the breathing of people who already have asthma. Gases such as sulfur dioxide, nitric oxide, and ozone, are all known to have the potential to worsen lung function in asthmatics, particularly at high concentrations. Particulate matter (tiny particles of liquids and solids) in the air has also been shown to have adverse effects on breathing in general, particularly (but not exclusively) in people who already have respiratory problems. It is unfortunately very difficult, though, to do research that can tell us whether or not air pollution actually causes asthma in people who don't already have the disease.

The case against cigarette smoke is even stronger than the case against air pollution. It is thought that exposure to cigarette smoke in childhood contributes both directly and indirectly to the development of asthma. In study after study, children exposed to tobacco smoke in the home have been shown to have higher rates of hospitalizations for respiratory illness. Serious respiratory illness before the age of two is an important risk factor for asthma. It is believed that cigarette smoke also has a more direct role in causing asthma, either through effects on the immune system, on the formation of the airways, or both. Exposure to cigarette smoke in utero (through the mother smoking during pregnancy) appears to be at least as harmful as exposure in the home during babyhood and early childhood. The risks also rise as the amount of cigarette smoke exposure rises (an effect called "dose response," usually seen as evidence of a causal link.)

First World / Third World

Another observation that may hold clues to the causes of asthma is that asthma is much more common in industrialized nations (such as the United States, Australia, and Western European countries) than in the developing world. We still do not know why this is so. It is unlikely to be related to the genetic makeup of the populations, since descendants of immigrants from developing nations to industrialized ones have similar asthma rates to those in their adoptive country.

Several explanations have been suggested to explain the higher prevalence of asthma in industrialized nations. Some researchers theorize that the higher rates of different infectious diseases in the developing world (from measles to malaria, bacteria to parasitic worms) may somehow affect people's immune systems in a way that is protective against asthma. Others propose that the difference is related to how much more time children in industrialized nations spend indoors compared to their Third World counterparts, suggesting that exposure to various indoor allergens or pollutants is the key difference. Still others believe that it is the disparity in body weight that accounts for the discrepancy-children in developing nations are thinner than Western children (in the U.S., particularly, we have noted increasing rates of obesity during the last decades), and there is some evidence that higher body mass index may be a risk factor for asthma.

The real explanation of the difference may lie somewhere in these proposed reasons, or it may be due to another factor entirely. (One well-known researcher teasingly recommends at scientific conferences that everyone keep pigs at home, since research in the Third World has shown lower rates of asthma in pig-harboring households.)

How Might Causes of Asthma Fit Together?

One theory for explaining how asthma is caused involves a combination of underlying susceptibility with environmental exposures (often referred to as "hits"). Under such a model, it is believed that a person who is not born with a genetic susceptibility to asthma will never get the disease, no matter what the environment. A person who is born genetically vulnerable to asthma may also escape the disease if he or she is not exposed to enough "hits" in the environment to bring it out. Asthma occurs, by this theory, only when a person both is susceptible (through heredity) and experiences those factors (still unidentified) that cause the disease to manifest itself.

This is all rather complex, but the picture may in fact be more complicated still. Some scientists believe that what we call "asthma" is actually several different diseases, caused in different ways. It is possible, for instance, that asthma that begins in infancy or childhood could be something else entirely from asthma that comes on in adulthood. Or perhaps asthma in people with allergies could be a different entity from asthma in people who are not allergic. These are some of the questions that asthma researchers are currently attempting to address.

What Does Not Cause Asthma?

In the first half of the twentieth century, it was fashionable to attribute all kinds of ailments to psychological causes. Conditions ranging from autism to cancer (which we now know to have biological causes) were believed to be due to poor parenting and neuroses. Asthma was one of the conditions that theorists supposed was psychological in origin. Unfortunately, this belief caused confusion and suffering. Parents of asthmatic offspring often felt guilty, thinking they had failed their children in some way.

Although it has been a long time since medicine supported this psychoanalytic theory of asthma, the idea has still not disappeared entirely. A few people continue, mistakenly, to believe that, for instance, a parent being too protective can cause a child to have asthma, or that a person with asthma could just "snap out of it" if he or she really wanted to. These beliefs are entirely unfounded, and it is a shame that they still sometimes cause pain to people with asthma and their families.