Inflammation makes the airways sensitive to stimuli such as allergens, chemical irritants, tobacco smoke, cold air, or exercise. When exposed to these stimuli, the airways may become swollen, constricted, filled with mucus and over sensitive. The resulting airflow restriction is reversible, completely in some patients, but incompletely in others. This reversibility can be spontaneous or it may be due to treatment. With adequate asthma treatment, the inflammation can be reduced, symptoms controlled, and most asthma-related problems prevented.

There is a strong link between asthma, hay fever and eczema, these three diseases occur together far more commonly than by chance. And we also think of these three diseases as allergic. There is absolutely no doubt that allergy is associated with all three. However, convincing evidence that allergy causes asthma is still accumulating. 

Does allergy cause asthma?

The risk of asthma is increased in people who have a positive dust mite allergy test and it increases in line with the number of positive allergy tests to other airborne allergens. For example, seasonal increases in levels of grass pollen in June and July and of the mould spore Alternaria in August or September are associated with an increase in the number of asthma attacks. 

So there has been a renewed interest in the idea that if any stage in the allergic reaction could be blocked, then asthma attacks might be prevented. Moreover, if such a treatment worked, it would be strong evidence that allergy causes asthma. This is not a new idea. Antihistamines were the first of this type of anti-allergy treatment to be developed. They were introduced with the aim of blocking the effects of histamine, the main chemical that causes the sneezing, itchy eyes and runny nose of hay fever and the itchy skin in eczema. They remain the most popular and effective treatments for the control of hay fever and the itch and swelling that accompanies certain persistent skin rashes. 

A recent ambition of medical scientists has been to block the effects of  immunoglobulin-E (IgE); the class of proteins that contains the important antibodies commonly present in most allergic diseases. They argued that if allergy was causing asthma, then this drug should help asthma sufferers. Moreover if the treatment worked, it would provide good evidence of allergy as the cause. A successful treatment known as anti-IgE now exists and early studies on children and adults with severe asthma show that it works. The number of attacks of asthma, and the amount of other asthma treatments required, are both significantly reduced for patients on this treatment.

However, recent population studies suggest that other  environmental factors are also involved. It has been known for a long time that a proportion of people who possess antibodies to airborne allergens (dust mites, pollens, animals, fungal spores etc.) suffer no symptoms when exposed to those allergens. If studies of different populations found this proportion to be constant, a genetic (inherited) factor could explain the difference. Studies that have compared Albania with the UK and Nigeria with Australia have now shown that this proportion is far from constant. In each study the percentage with positive allergy tests was similar, whilst those with asthma were about three time commoner in the UK and Australia than in Albania and Nigeria respectively. This difference is unlikely to be genetic, and more likely to be due to other environmental influences such as diet and lifestyle.

So we need to consider the known risk factors that increase both the likelihood of asthma and the likelihood of its persistence.

Risk factors

When asthma first develops in childhood, in less than half of the cases does asthma continue to be a problem until adulthood. in many cases children who have had nothing more than one or two wheezy colds are wrongly labelled as asthmatic. This condition used to be called wheezy bronchitis, a useful description although the term is not often used nowadays. The truly asthmatic child often has evidence of being allergic to airborne allergens and is also likely to have close relatives with asthma or hay fever. 

These are the risk factors that increase both the likelihood of asthma and the likelihood of its persistence:

Stress and hormonal influences (including menstruation, pregnancy, following childbirth, the menopause and HRT) are also known to increase the likelihood of asthma.  Heart burn with associated acid reflux is known to be an aggravating factor and even silent acid reflux (this needs a special test to diagnose it) is known to play a part.  The condition known as rhinitis (hay fever and conditions like it) has been shown to aggravate asthma, and effective asthma management is known to benefit greatly from the proper assessment and treatment of this problem.

So, it can be seen that the full and proper assessment of a patient with asthma involves the identification of the likely asthma triggers, which in turn includes an assessment of the environment.  A strategy needs to be put in place to both prevent acute attacks and in turn prevent the progression of the disease to a more persistent chronic state.  Asthma can no longer be treated as a single and uniform disease, especially at a time when the patients themselves wish to know more about their condition and understand what started it and what is keeping it from getting better.

Asthma with allergy

Standard skin prick tests will show up that an allergy is present in over half of all cases of asthma. These tests will show up allergy to various airborne allergens such as cats, dogs, house dust mites, pollens, feathers and sometimes mould spores. 

When taken into account, the hobbies and occupations of asthma patients will extend the possible sources of troublesome allergy.  Allergy skin tests and blood tests may show up the causes of occupational asthma and there is evidence that this kind of asthma, in some causes caused by the patient's job, is underestimated.

However, when all the skin tests undertaken are negative, it does not necessarily mean that no external trigger is involved in the remainder.  Standard allergy skin and blood tests may underestimate the role of external triggers, especially when the common type of allergy (the type that involves IgE antibodies) is not the explanation. For example food allergy is generally thought of as a rare cause of asthma.  However, studies that have looked for this by using food elimination and challenge rather than by relying on allergy skin or blood tests have suggested that a hidden kind of food allergy may play an important part in some cases of asthma, and this may include some of those who suffer quite severe and unpredictable attacks (see 'asthma and food').

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Asthma without allergy

In about a quarter of asthmatics, no obvious allergic triggers can be identified by skin prick or allergy blood tests.  This kind of asthma is sometimes called intrinsic asthma, a condition that commonly starts in later life.

When such cases are studied in greater depth, non-allergic environmental triggers of asthma may often be suspected. For example, the patient may be in no doubt that exposure to a particular substance (perhaps a particular fragrance, perhaps a particular chemical odour) invariably triggers an asthma attack and yet the standard tests fail to confirm it.

Some of these late-onset asthmatics suffer from a considerable degree of accompanying rhinitis, sinusitis and nasal polyps. Such asthmatics are sometimes made considerably worse by aspirin, ibuprofen and similar non-steroidal pain killers. In a small proportion of such patients, the inadvertent use of drugs such as the ones mentioned above can be dangerous. This is not a true allergy, the problem arises because these drugs are known to directly interfere with immune mechanisms involved in triggering airway inflammation. The disorder is due to a disturbance in certain chemical substances produced by the body's normal immune metabolism (leukotrienes).  The condition can be difficult to treat, such patients may have severe asthma, and often require regular treatment with steroids. It has also been suggested that a small proportion of these patients may suffer a less severe kind of sensitivity to certain food dyes and naturally occurring food compounds called salicylates. However, when a group of aspirin-sensitive asthmatics was studied, the avoidance of high-salicylate containing foods did not help the majority.  

In another group of patients, foods may still be triggering asthma attacks even though the allergy skin and blood tests to these foods are negative.  In these cases, the supervised trial of an elimination diet and, if successful, the testing of avoided foods by reintroduction, can occasionally lead to the identification of foods that require to be avoided to improve asthma control. It is not recommended that patients should undertake this kind of testing except under medical advice and with dietetic supervision.

Asthma triggers

The following asthma triggers are not caused by allergy and they may affect any asthmatic: