The Harvard Medical School Guide to Taking Control of Asthma: Chapter 1
Chapter 1: What is Asthma?
WHAT ASTHMA FEELS LIKE
I can't swim across the pool underwater and it took me three tries to blow out all of my birthday candles.”
— Daniel, age 6
“Even if you talk about it with people, they can’t grasp the depth of asthma. Most people are preconditioned: ‘Oh asthma is all in your head; oh, you can do this and you can do that.’ They don’t realize that this is your lung involved, this is your heart, everything you’ve got is involved in asthma trying to breathe. People don’t understand unless they have it or are directly affected by it."
— Hazel, age 56
“It feels like someone has their hands around your lungs and is squeezing them very tightly and you are trying to break that grip. You can’t breathe, and you can’t get away from that fierce set of hands around your lungs.”
— Margaret, age 60
The experience of asthma varies greatly among different people with asthma, mainly because of its variable severity. For some people it is a minor annoyance, a tickle of a cough felt high in the throat after exercise. For others it is the cause of restless nights with frequent awakenings due to cough and labored breathing. For still others asthma manifests as severe attacks, causing a suffocating sensation and the sense that the next breath may be the last. Some people with asthma are Olympic athletes, able to compete at the highest levels of physical strength and endurance. Others find themselves frequently in and out of the local emergency room, unable to plan routine daily activities because of unpredictable episodes of difficulty breathing.
The loved ones of people with asthma often share much of the emotional burden – if not the direct experience – of asthma. It can be exceedingly difficult to watch someone struggle with his or her breathing and feel unable to help. No one is likely to feel this distress more deeply than the parent of a child with asthma, especially a small child still too young to verbalize what he or she is going through. How can you keep your child safe, you wonder, through this respiratory tract infection, through the day at school, the soccer game, or the summer away at camp? You try to find the right balance between safety and restrictive limitations. Many children and parents express complaints about the unpredictable nature of asthma. They say that never knowing when they will have an “episode” and if that episode will be “severe” makes having asthma so stressful. This is reflected in asthma surveys that show many families suffer disrupted plans and activities because of the disease.
Whatever your experience with asthma, you are not alone. In a recent national survey, an estimated 15 million Americans reported asthma symptoms during the preceding 12 months, including approximately 3.5 million children under the age of 15 years.
Nearly 27 million Americans have at one time in their lives been diagnosed with asthma by a physician. People of all ages have asthma; it occurs in all countries and among all populations around the world. Most striking is the observation that over the past 20 years, asthma has become increasingly common in many parts of the industrialized world. We are in the midst, some physicians would say, of an “asthma epidemic.”
Asthma is an ancient disease. Greek physicians like Hippocrates in 460 B.C. wrote of asthma. They used the word, "asqma", meaning to pant or breathe hard. A description by the Greek physician Aretaeus from the second half of the first century A.D. vividly captures the struggle to breathe characteristic of a severe asthmatic attack:
“They go into the open air, since no house suffices for their respiration; they breathe standing, as if desiring to draw in all the air they can possibly inhale; and, in their want of air, they also open the mouth as if best to enjoy more of it. Pale in countenance, except the cheeks, which are ruddy; sweat about the forehead and clavicles; cough incessant and laborious; expectoration small, thin, cold resembling the efflorescence of foam; neck swells with the inflation of the breath.”
For most people, difficulty breathing is the most overwhelming symptom of asthma, but not the only one. Shortness of breath is often accompanied by a sensation of tightness in the chest that can make you feel as if you had a wide rubber band bound tightly around your torso. Wheezes—musical whistling sounds heard especially with breathing out—are a trademark symptom of asthma. Cough, often worse at night, is also common.
To give you just a few specific examples of the variable nature of asthma, one patient in our practices, a 15-year-old girl who competes in high school athletics, experiences brief bouts of coughing and shortness of breath usually only after a demanding track meet or basketball game. Another one of our patients, a man in his forties, is aware of a slight wheeze most of the time and has been hospitalized many times because of his asthma. He has multiple allergies that cause his asthma to flare up, including allergy to furry pets and sensitivities to cigarette smoke and strong fragrances, and finds himself out of breath merely climbing up a single flight of stairs.
The majority of our patients fall somewhere in between these extremes. But wherever you are on the continuum, you share with these patients—and with everyone who has asthma—a tendency for your breathing passageways, the bronchial tubes, to narrow abnormally in response to certain stimuli.
As shown in the pictures below, your lungs are built to bring oxygen into your body and to release a gas called carbon dioxide out of your body. Oxygen is essential to supply energy to your cells, and CO2 is a waste product made by your cells.
Figure 1: What Happens As You Breathe
As you breathe in, your diaphragm (a muscle at the bottom of your lungs) contracts, causing your rib cage and the lungs inside it to expand. As your chest expands, it fills with air, and your belly protrudes outward as the diaphragm pushes downward. When you exhale, your diaphragm relaxes, and the rib cage and lungs — elastic structures that have been stretched during inspiration — recoil to their original shape and position. Gas rushes out of the lungs, carrying with it carbon dioxide. You can watch your belly fall back to its initial position as the diaphragm moves upward again.
Figure 2: How the Inside of the Lungs Work
Healthy lungs are like balloons that inflate and deflate, taking in oxygen-rich air and expelling carbon dioxide (CO2) waste. But unlike balloons, the lungs aren’t hollow. Instead, they’re filled with millions of microscopic sacs, called alveoli, in a configuration that resembles bunches of grapes. The air we inhale passes through the trachea, which branches into the two bronchi that feed the right and left lungs. These main bronchi divide into smaller and smaller branches throughout both lungs, ultimately narrowing into small stems (called bronchioles) that terminate in alveoli.
When we inhale, the airpassageways dilate and the alveoli expand to admit air, providing oxygen to networks of tiny blood vessels in the alveoli walls. These capillaries also transport CO2 back to the alveoli. When we exhale, the alveoli shrink, forcing CO2 into the bronchioles, back through the bronchi and trachea and out of the body. Asthma inbolbes the bronchi and bronchioles throughout both lungs.
Narrowing of the bronchial tubes of your lungs leads to the symptoms of asthma. Difficulty breathing results from the extra work involved in having to move air through these narrowed tubes. Wheezes are the sounds made by air rushing through the narrowed tubes. Cough results from irritation of nerves in the walls of the breathing tubes, and in some cases it’s the body’s attempt to remove excess secretions.
Figure 3: Normal airways, and airways narrowed by asthma.
In a healthy airway (left), the passageway is unrestricted and airflow is normal. During a mild asthma attack (center), the airway walls thicken with inflammation and muscles in the walls contract, narrowing the airways. The airways also may begin to fill with mucus, which further interferes with the flow of air. Wheezing and coughing develop as airflow deteriorates. During a severe asthma attack (right), the combination of muscle spasms, wall thickening and increased mucus in the airways can block airflow severely.
Having asthma does not mean that your airways are always constricted. It means that you always have the potential for your airways to narrow abnormally. Much of the time your airways are fully open, and at those times you can breathe normally. At other times your breathing may be very labored, as though you were trying to breathe through a straw with a great weight on your chest. This variability in the diameter of your breathing passages is characteristic of asthma and distinguishes it from many other lung diseases, such as emphysema and chronic bronchitis that result from cigarette smoking. With these diseases, breathing limitation tends to be permanent and unchanging, day in and day out. (For more on emphysema and chronic bronchitis, see chapters 2, 14 and 15.) This variable narrowing of the breathing tubes that can come and go over minutes to hours or days is called reversible airway narrowing and is one of the hallmarks of asthma.
Triggers and Twitchy Airways
The concept of reversible airway narrowing is good to keep in mind because it is so central to our understanding of asthma. Another, less medical-sounding term that you will want to have in your asthma vocabulary is “twitchy,” which is used to describe the hypersensitive airways of people with asthma. Having twitchy airways doesn’t mean that they feel twitchy—they don’t. Twitchiness in the airways refers not to a sensation but to an underlying tendency for the airways to react by narrowing when they encounter certain stimuli—such as air pollutants or cold air. The very same stimuli will have no such effect on your partner, friend, or sister who does not have the twitchy airways of asthma. Medical terms used to describe this fundamental property of asthmatic airways are “hyperresponsiveness” and “hyperreactivity.”
If you have asthma, the word “trigger”—in its medical sense—may already be in your asthma vocabulary. Triggers are those stimuli that set off airway narrowing in people with asthma, causing their symptoms of asthma to flare up. You may have found that if you go for a jog on a cold winter day, you begin to cough and breathe heavily afterwards. Perhaps if you are around cigarette smoke or strong perfumes, you experience tightening in your chest and need to use your asthma medications. If a simple head cold settles in your chest, you may begin to wheeze and become seriously short of breath. You may even find that your asthma is sensitive to weather changes, particularly changes in humidity and barometric pressure.
Common, non-allergic triggers
Exercise, particularly in cold air
Common allergic triggers
Certain medications can also be asthma triggers. Aspirin is an asthma trigger for about 1 in 20 adults. (The effect of aspirin and related medications on asthma is discussed in greater depth in Chapter 11.) Everyone with asthma should avoid the group of medicines called beta-blockers, which are used to treat heart disease, high blood pressure, glaucoma, migraine headaches, and some forms of thyroid disease. One of the side effects of these medications, unique to people with asthma, is narrowing of the airways.
Food allergy can cause severe and potentially life-threatening reactions, typically involving the lips, throat, and tongue, and sometimes precipitating a very severe, generalized reaction called anaphylaxis (low blood pressure due to dilation of blood vessels throughout the body). However, food is very rarely the cause of an isolated allergic reaction involving the bronchial tubes (that is, an asthmatic reaction). (Still, if you notice a connection between eating certain foods and worsening of your asthma, by all means avoid those foods.) There is one exception: we know that the sulfites used to preserve certain foods and beverages can cause some people’s asthma to flare up. Sulfite preservatives are found in processed potatoes and shrimp, and in many dried fruits, beers, and wines. (For more on food allergies, see Chapters 4 and 15.)
Our overview of asthma triggers wouldn’t be complete without a brief word about the roles of stress and strong emotions. There was a time when asthma was thought to be caused by your emotions. Ulcers, colitis, and back pain were also at one time or another blamed on the psyche. We now know much more about the biology of asthma (and of these other illnesses) and can say with certainty that your strong emotions do not cause you to have asthma. They may be one of your asthma triggers and may cause some (generally mild) airway narrowing if you have asthmatic airways. They do not, however, cause you to have asthma in the first place. If you have asthma, it’s not your fault!
Having lived with asthma, you can probably identify most of the things that make your asthma worse. Knowing as best you can what these things are is important, because you can then often avoid these “triggers.” Sometimes avoidance is relatively easy, like staying away from the cat at your neighbor’s house. At other times it involves hard work, like removing mildew from the bathroom or reducing dust accumulation in the bedroom. The payoff is better breathing and fewer attacks of asthma. (See Chapter 4 for guidance on minimizing your exposure to your asthma triggers.)
Now that you know about triggers and twitchy, or hyperresponsive, airways, you can understand why asthma is different in different people. For one thing, you may react to a specific allergic trigger, whereas another person with asthma, who does not have that particular allergic sensitivity, will not. For example, if you are allergic to cats, you will likely develop symptoms of your asthma when sitting on the cat’s favorite chair, but your friend with asthma who does not share your sensitivity to cat dander can sit in the very same chair without any adverse effect on his or her asthma. Furthermore, the extent of your reaction to an asthma trigger will depend on the degree of twitchiness of your airways. Two people with asthma who are sensitive to the same asthmatic trigger may react differently to it, depending on how sensitive their bronchial tubes are. A person with very sensitive airways may develop a severe asthmatic attack when entering a smoke-filled room. A person with only mildly sensitive airways will likely have a more minor reaction under the same circumstances.
Your particular asthma triggers and the degree of sensitivity of your airways also explain why your asthma gets better or worse at different times. The twitchiness of your airways can vary over time. It can worsen, for example, during the late summer and early fall in New England, if you are sensitive to ragweed pollen, and then lessen after the first frost. It may be the case that during your allergy season, exercise that previously caused no symptoms now precipitates cough and wheezing. Similarly, being around paint fumes now leaves you breathless and feeling constricted across your chest. The reason is that once ragweed pollen has irritated your sensitive airways, they become extra sensitive to all your asthma triggers (not just to ragweed allergen). When the pollen exposure is over, your airways gradually become less sensitive again and your asthma quiets down.
Why Do the Breathing Tubes Narrow?
One cause of airway narrowing is contraction of the muscles that form a ring around our breathing tubes. These muscles are "involuntary" muscles, like the muscles that constrict the pupils of our eyes or move food through our intestines. When exposed to an asthma trigger, they can contract quickly—in less than one minute—squeezing the bronchial tubes and causing them to become narrow. These muscles can likewise relax relatively quickly (in just a few minutes), either on their own or in response to medication. The terms used to describe these processes are bronchoconstriction or bronchospasm (for narrowing of the tubes) and bronchodilation (for their opening wider).
Inflammation is the other cause of airway narrowing in asthma. Inflammation is a medical term used to describe how our bodies react to various injuries, irritations, or infections. Inflammation generally involves some swelling (leakage of fluid from the blood vessels) and an influx from the blood of cells not normally present in that part of the body.
In asthma, inflammation involves special inflammatory cells that come out of the blood and take up residence in the walls of the bronchial tubes. These inflammatory cells contain powerful chemicals that can cause swelling of the walls of the bronchial tubes and can also stimulate the production of extra mucus. Mucus is also called phlegm. In medical dictionaries mucus that is coughed up is called sputum, although most people use the phlegm and sputum interchangeably. The mucus can plug the tubes, leaving little room for air to move in and out. Anyone who has had a head cold (all of us!) knows the effect that swelling of the nasal passageways and increased nasal mucus can have on efforts to breathe through one’s nose. Similarly, difficulty breathing in asthma can be the result of swelling and excess mucus in the bronchial tubes. Unlike contraction of the muscles of the bronchial tubes, the swelling of the walls of the bronchial tubes and excess secretions within them take many hours or even days to resolve.
Remembering these two different causes for narrowing of the breathing tubes is important. Medications used to treat asthma focus on these two mechanisms. They are designed to relax the muscles surrounding the airways, to reduce the inflammation of the breathing tubes, or both. Also, the speed with which asthma medications take effect depends on their site of action: bronchodilators (see Chapter 5), which relax the muscles surrounding the airways, can have a rapid effect; anti-inflammatory medications (see Chapters 6, 7 and 8) act more slowly.
What Causes the Bronchial Tubes to Be “Twitchy”?
By now you may be wondering what causes the bronchial tubes to become twitchy (hypersensitive) or prone to abnormal narrowing in the first place. In other words, what causes asthma? No one is certain of the answer to this question, and there may be not one, but many different causes.
An important medical discovery made several years ago was that some inflammation is present in the bronchial tubes of people with asthma even when they feel well and when their breathing is normal. Medical scientists performed experiments in which small pieces of the walls of the bronchial tubes were sampled by biopsy. The samples were taken at times when the subjects were feeling well, free of symptoms of asthma. In those samples, scientists found evidence for persistent inflammation of the bronchial tubes.
Airway inflammation in asthma is chronic, at least to some degree. The inflammation may be so mild that it does not cause narrowing of the bronchial tubes. But the persistent presence of this inflammation is probably a major reason that the bronchial tubes are twitchy, or capable of narrowing abnormally. Medically speaking, persistent airway inflammation is an important (and treatable) cause of airway hyperresponsiveness.
Does Asthma Ever Just Go Away?
It is well known that children often “outgrow” their asthma. Particularly around early adolescence, asthma symptoms often go away, at least temporarily. However, up to two-thirds of children with asthma continue to suffer from the disorder through puberty and adulthood.
Exactly what happens biologically to cause this change is uncertain. Part of the equation is probably simply growing bigger: as we grow, our lungs enlarge, and so do the bronchial tubes. To some extent, this makes us less prone to critical narrowing of the breathing tubes and to asthma. Another part of the equation may have to do with changes in our immune systems, particularly as they come under the influence of the sex hormones (estrogen and testosterone), although this remains speculation.
If you had asthma as a small child and now as a young adult you are free of symptoms, you may not be entirely out of the woods. Recurrence of asthmatic symptoms in early adulthood is a common experience. The property of twitchy airways, characteristic of asthma, may stay with you, although in a milder form. Then, under the right conditions (for example, you get a pet cat or move to a new city), the sensitivity of your airways again increases and you once again experience the consequences: cough, tightness in your chest, and intermittent wheezing.
If you are an adult with asthma, the prospect that you will yet outgrow your asthma is slim. Its severity may wax and wane, but only infrequently (less than 10% of the time) does asthma simply disappear for good. Better to plan to live well with asthma than simply to wish it away ...at least for now, until an asthma cure is found.
What Do Allergies Have to Do with Asthma?
As you know from our discussion of asthma triggers, and perhaps also from your own experience, not all asthma is allergic. People with no allergic sensitivities can have asthma triggered by exercise, respiratory infections, air pollutants, and other non-allergic stimuli of asthma. Nonetheless for many people (especially children), asthma involves an allergic sensitization of the breathing tubes, and, consequently, allergic reactions in these tubes.
The idea of an allergic reaction in the bronchial tubes may seem strange. When we think of someone’s allergies, we’re likely to think of their sneezing and a runny or stuffy nose; their red, itchy, watery eyes; or perhaps their dry, red, itchy skin. In fact, allergic rhinitis (allergies of the nose), allergic conjunctivitis (allergies of the lining of the eyes), and allergic dermatitis (allergies of the skin, also called eczema) frequently occur together in various combinations both in individuals and in families, and they often occur together with asthma.
The reason is that these conditions share a common mechanism; they result from a specific type of allergic reaction. The tendency to make this specific type of allergic reaction is called atopy, and people who have this tendency are said to be “atopic.” Asthma accompanied by allergies is often referred to as atopic asthma or extrinsic asthma. When no tendency toward these allergic reactions can be found, asthma is said to non-atopic or intrinsic. Approximately 75% of all children and perhaps half of all adults have atopic asthma.
In asthma, allergens are inhaled substances that cause a characteristic (allergic) reaction in the bronchial tubes of susceptible people. Common allergens include animal dander; house dust; pollens of grasses, trees, and common weeds; and spores of various molds. Some people encounter allergens in the workplace, such as the baker who becomes allergic to flour dust and develops so-called "baker’s asthma." Other substances that we commonly breathe in, including air pollutants, appear to be the wrong size and shape to function as allergens. Although they may further irritate already sensitive airways and make your asthma flare up, they do not cause true allergic reactions.
The Allergic Immune Response
Although we all breathe in allergens such as house dust, not everyone’s bronchial tubes have the allergic tendency to recognize the particular allergen and react to it. If you have asthma and house dust allergy, you make a special kind of protein, called an antibody, that is precisely shaped to recognize and attach itself firmly to house dust allergen. This antibody belongs to the family of immune defenders called immunoglobulins. Those that are specifically designed to recognize allergens are referred to as immunoglobulin E, or IgE (pronounced eye-gee-ee) for short.
If your genes program your body to make IgE antibodies when you are exposed to house dust, you typically make IgE antibodies to this allergen within your first few years of life. Thereafter, whenever you breathe in house dust, these antibodies are waiting and ready to grab onto the house dust particles inhaled onto your breathing tubes. You may also make other IgE antibodies, shaped slightly differently, that recognize cat allergen, or seasonal pollens, or any of scores of different allergens to which the human body produces allergic reactions. We don’t know why some people make these antibodies and others do not. However, the tendency (atopy) appears to be passed on in the genes from one generation to the next.
The IgE antibodies are not roaming freely around in your breathing tubes but are firmly attached to immune system cells called mast cells. Mast cells are located mainly in parts of the body that regularly encounter substances from the outside world: our skin, intestinal tracts, the lining of our eyes (conjunctivae), our noses, and our breathing tubes. In people with allergies, the surfaces of their mast cells are coated with firmly attached IgE antibodies. All remains quiet until the IgE antibodies recognize and attach themselves to an allergen. Then, within seconds, an explosive reaction takes place. The mast cells are stimulated to release a barrage of chemicals that carry out the inflammatory reaction. Those chemicals include histamine—one that you may be aware of if you are familiar with anti-histamines for treating allergies—other chemicals called leukotrienes (about which we will talk more later), and many others. Together, these inflammatory chemicals cause blood vessels to leak fluid, producing swelling in the breathing tubes. They stimulate the walls of the breathing tubes to secrete mucus. They also make the muscles surrounding the breathing tubes contract, causing narrowing of these passageways.
As if that weren’t enough, mast cells also call in reinforcements, other cells involved in allergic inflammation that travel from the blood to the site of the allergic reaction and make things worse. The most important of these reinforcements are blood cells called eosinophils. People with asthma characteristically have eosinophils in the walls of their breathing tubes, where these cells linger, even when asthma is quiet. When asthma flares, more eosinophils accumulate. Like mast cells, they also release chemicals that cause airways to narrow.
Figure 4: Asthma—An Inflammatory Response
- Immune system cells called B lymphocytes make IgE antibodies.
- One end of the IgE antibodies attaches to several types of cells, particularly mast cells and eosinophils. Mast cells live in the lining of the breathing tubes (and other parts of the body). Eosinophils circulate in the blood, and can leave the blood to enter the lining of the airways.
- The other end of the IgE antibodies lies waiting for a particular allergen (for example, cat dander) to come into the lungs. The shape of that end of the IgE antibodies is such that it can attach to that particular allergen.
- The allergen enters the lung. The ends of the IgE
antibodies catch the allergen. When this happens, a
signal is sent to the inside of the mast cells. That
signal causes the mast cells suddenly to release
chemicals that cause narrowing of the airways:
- The muscles that ring the airway contract, causing the airway to narrow.
- Excess mucus flows into the airways, making it even harder for air to move through the airway.
- Fluid leaks from the blood to the walls of the airways, causing them to swell.
- Eosinophils are attracted to leave the blood and enter the walls of the breathing tubes. There they release additional inflammatory chemicals.
Imagine This Scenario
You shuffle across the not-so-recently vacuumed bedroom carpet and lie down on the old mattress you’ve been meaning to replace, releasing house dust allergens into the air. You breathe the house dust allergens into your lungs, and there, waiting on the surface of mast cells in your bronchial tubes, are antibodies that recognize this allergen and attach themselves to it. The mast cells then release an explosion of inflammatory chemicals that cause the muscles surrounding your bronchial tubes to contract. These chemicals attract other inflammatory cells out of the bloodstream and into the bronchial tubes; they cause fluid to leak out of the blood vessels and into the bronchial walls, making them swell; and they stimulate the mucous glands to release mucus into the bronchial tubes. You can see that the stage is set for an asthma attack.
Knowledge is a powerful weapon as we seek to protect ourselves against the allergic responses our bodies make in response to the outside world. Medical scientists are learning more and more about the many molecular reactions and interactions involved in the allergic process, and pharmaceutical companies are using this knowledge to develop new drugs that can block various steps in this allergic response. In addition, there is a lot that you can do on your own to reduce your exposure to allergens to which you are sensitive, stopping an allergic reaction before it starts. In chapter 5, we’ll explore a number of ways you can achieve this.
What Causes Asthma?
As we’ve noted, no one knows exactly the cause or causes of asthma, and we can’t even say for sure if it is one disease or a group of diseases with very similar manifestations. (A wise lung doctor once said, “Asthma is like love. Everyone knows what it is, but no one can agree on its definition.”)
We have, however, come a long way in our understanding about asthma since the days when asthma was commonly—and incorrectly—believed to have psychological origins. (Years ago, the disease was referred to as “asthma nervosa.”) As is the case with many illnesses, strong emotions and stress can make asthma worse—which is to say they can trigger asthma—but they do not cause it. Our understanding of the abnormal airway narrowing in asthma has firmly established asthma as a disease of the lungs, not the mind.
Still, much has been learned about the root causes of asthma? We know that asthma probably results from a combination of genetic susceptibility (for more on genes and asthma, see Chapter 8) and exposure to things in the environment. We inherit a tendency to develop asthma from one or both of our parents. However, even if we are born with the genes for asthma, we may never go on to develop the disease. We know this from studies of identical twins, who share all the same genetic information. When one twin has asthma, the chance that the other twin will also develop asthma is only about one in three. Clearly, something in the environment must also be responsible for asthma.
Genes linked to asthma have recently been discovered. It’s almost certain that not just one gene but a complex combination of genes accounts for the predisposition to asthma. Within the next few years we will likely learn which genes these are. We can then study how the proteins made under their direction interact to cause asthma and allergies, and perhaps explain how their interaction with stimuli in the environment has come to cause, in recent years, more frequent asthma.
We do not know exactly what in our environment contributes to the development of asthma in people with a genetic predisposition. It may involve breathing particles they are allergic to, cigarette smoke, air pollution, viruses or other germs, or some combinations of these and possibly other factors.
Strange as this may seem, not being exposed to enough of certain stimuli such as bacteria and toxins early in life may make a person more likely to develop allergic diseases like asthma (see the “hygiene hypothesis,” discussed in the next section). When considering the environmental causes of asthma, it is worth remembering that most asthma begins in early childhood. Compared to older children and adults, young children naturally tend to spend more time closer to the ground. It is that environment, rich in both allergens and bacteria, that researchers must turn their attention in their search for the exposures that cause asthma.
Why Are Increasing Numbers of People Suffering From Asthma?
You may have noticed that in recent years more people are carrying inhalers filled with asthma medications, and that more children miss school and more colleagues miss work because of asthma. If so, your observations are borne out by scientific studies showing that asthma has become increasingly common. Community surveys conducted in the 1960s and then repeated in the same communities 25 years later found that significantly more people had asthma and allergies at the time of the second survey. In the United States the annual prevalence of asthma increased from 3.1% in 1980 to 5.5% in 1996, a 74% increase.
In the United States and other Westernized countries, such as Western European nations, Australia, and New Zealand, between 5 and 7 of every 100 people are now estimated to have asthma. In rural Africa and Asia, on the other hand, less than one percent of the population has the disease. The higher prevalence of asthma in urban versus rural environments, and in Westernized versus. developing countries, has made researchers wonder if something about our westernized lifestyles is contributing to the modern epidemic of allergies and asthma.
Air pollution would seem an obvious culprit, 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 after the fall of the Soviet Union, 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 more than air pollution must be responsible.
Two other intriguing theories, both having to do with early childhood, are worth considering. According to one theory, modern urban life is exposing us at an early age to more common household allergens and making us more susceptible to allergy-related asthma. As a rule, we’re much more likely than our ancestors to live in the city, which means we’re indoors more of the time, in close quarters with dust mites, cockroaches, and animal hair (both pet’s and pest’s). Add to this the fact that in recent decades, television, computers, and video games have seduced us and our children into staying inside even more, and you can see that we spend a lot more of our time in the company of indoor allergens than our grandparents did. Also, in response to the energy crisis of the 1970s, home insulation was upgraded and our houses and apartments tended to be more tightly sealed. The result was far less circulation of fresh air, and consequently, exposure to higher concentrations of allergens.
The second theory–called the hygiene hypothesis–has to do with how our immune system is challenged in the early years of life. According to this hypothesis, the immune system of children who are exposed to lots of important infections and toxins early in life tends to ignore less serious challenges – such as from allergens like pollen and dog dander. On the other hand, when children are exposed to relatively fewer serious infections, as has been the case in the developed nations over the past 50 years, the immune system has more “free time” to direct its attention against harmless allergens. Stated another way, the good news of the last 50 years is that serious infections such as tuberculosis and whooping cough are much less common. The bad news, according to this theory, is that our immune systems are more likely to react in a way that causes allergic diseases, including asthma.
There is growing evidence for the hygiene hypothesis: several major studies supporting it have been published in recent years. For example, one recent large study found that infants who go to day-care and/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 were more frequently exposed to other children, the infants who went to day-care and had siblings were naturally exposed to more germs. It may be that their developing immune systems learned to focus on germs and to ignore allergens, thereby decreasing the likelihood of allergies and asthma later in childhood. Another study conducted in rural communities in Europe found that higher amounts of certain bacterial toxins found in children’s mattresses were protective against the development of asthma and allergies. If your immune system is exposed to these non-living parts of germs at a young age, it seems to become tolerant to other aspects of the natural environment, including pollens and animal dander. In parts of the world where children grow up in close proximity to farm animals and their droppings, the prevalence of asthma tends to be low.
Yet another theory about the increase in asthma has to do not with our immune system but with our weight. We all know that obesity is a common medical problem in the United States and that it has been linked to heart disease and diabetes, among other diseases. Findings from several studies suggest that we can add asthma to the list of obesity-related illnesses. In one study, researchers looked at more than 7000 children, age 4 to 17, and found that those who were overweight were almost twice as likely to develop asthma as normal-weight children. The exact connection between asthma and obesity is unknown, but it may be that the pressure of excess weight on the chest can contribute to constriction of the airways. We have some supportive evidence for these various theories, but not enough to cononsider any of them fact.
One thing we are sure of, though, is that the burden of asthma is not shared equally throughout this country. African-Americans in the United States are hospitalized at least three times more often for asthma than white people, and the gap has widened over the last decade or two. The highest rates of asthma hospitalizations occur among Hispanics and African Americans living in our inner cities and, more specifically, within the poorer neighborhoods of our inner cities. The racial differences in asthma severity probably relate mostly to poverty. Being poor and living in the inner city typically mean worse housing conditions, greater exposure to air pollutants and indoor allergens such as debris from cockroaches, less access to medications and preventive medical care, and more medical, social, and psychological conditions that interfere with good asthma care. One of the greatest challenges facing modern asthma care is achieving an equitable distribution of the available asthma therapies to people of all racial and ethnic backgrounds, regardless of socioeconomic status. (For more on inner-city asthma, see Chapter 11.)
Can Asthma and Allergies Be Prevented?
Our inquiries about what causes asthma and why it is becoming more prevalent suggest the obvious follow-up question: Can asthma and allergies be prevented? The answer is “not yet, not with any certainty.”
Here are some things that you can do. Children born to mothers who smoke cigarettes are more likely to develop asthma. Teenagers and adults who smoke cigarettes are themselves more likely to develop asthma. For these and innumerable other reasons, you can improve your own health and that of your children by not smoking.
Children born of atopic parents more often develop asthma if they grow up in homes with high levels of dust mite or cockroach allergens than children with less allergic exposure during early childhood. If you have asthma and allergies and you worry about your child developing asthma, keeping a clean home makes good sense. We cannot make the same claim about a pet-free house. Recent evidence does not support the idea that having a cat at home will predispose your child to developing asthma and allergies later in life. In fact, just the opposite may be true under certain circumstances.
Finally, we hear a lot about breast-feeding and immune protection, which raises the question of whether or not breast-fed babies are less likely to develop asthma. Although we can say for sure is that breast milk is the best nutrition for babies, it is hard to know whether breast feeding helps to protect against development of asthma. Breast milk is full of nutrients, and maternal antibodies and hormones that help protect infants from infections. As to whether or not breast milk protects against asthma, the research findings are inconsistent. One recent large study suggested that breast-feeding an infant for at least four months after birth protects against childhood asthma, whereas another study found a potential connection between breast feeding and increased childhood asthma, but only if the mother had asthma.
More definitive recommendations about the primary prevention of asthma will need to await better understanding of the cause(s) of asthma. Studies are under way to test anti-inflammatory medications as a means of preventing the development of asthma in children as young as 2-4 years old who were judged to be at increased risk. We have to await the results of these studies. In the meantime, it is probably best not to rearrange your life or lifestyle based on speculation and theory (with the notable exception: don’t smoke cigarettes!).
Although the possibility of preventing asthma is still pretty “iffy,” the probability of controlling it and preventing disabling episodes is excellent, with careful attention to avoiding the things that cause flare ups, and close adherence to a medication plan tailored to your particular needs. We began this book with the observation that if you have asthma, you have plenty of company. That company includes plenty of people who lead, or have led, active, successful lives. Among them are John F. Kennedy, the 35th president of the United States, and Theodore Roosevelt, the 26th president. Other famous people said to have asthma are the actresses Liza Minnelli and Elizabeth Taylor, the singer Judy Collins and the saxaphonist Kenny G. A number of Olympic athletes have asthma, including swimmers Tom Dolan, Amy Van Dyken, and Nancy Hogshead; and track-and-field star Jackie Joyner-Kersee.
Asthma needn’t prevent you from leading an active life, including participation in intense physical activities. If you work closely with your doctor to control your symptoms, you can probably be symptom-free and fully active most of the time. Our purpose throughout this book is to help you achieve this goal. Don’t settle for anything less.