People who have positive allergy skin prick or blood tests to one or more of a range of common airborne allergens are more likely to develop clinical asthma.  It is therefore clear that allergy is very commonly a key element in the early stages of the asthmatic disease process. 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 immunoglubulin E 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 using allergy skin or blood test have shown that a hidden kind of food allergy may play an important part in some cases of asthma. 

When asthma is under-treated, the airways remain twitchy and they can be readily triggered by a wide variety of airborne particles including dusts, spores, pollens and other biological materials together with chemical irritants and microbes, acting either alone or in combination. 

When asthma occurs only occasionally, the cause can often be readily identified. For example, a patient with seasonal allergic asthma wheezes when the pollen count is high  Such a patient will show sensitisation to pollen (positive skin prick test or allergy blood test), and may well be symptom-free for the rest of the year.  Seasonal allergic asthma may be accompanied by summer hay fever, although it can also occur alone.  

When asthma is persistent, allergy may also be an important factor.  For example, asthmatics may be sensitised to the house dust mite, moulds and fungi, or animals such as cat and dog.  However, such patients usually also show non-specific bronchial hyper- responsiveness (i.e. they have twitchy airways), and will be liable to wheeze after exposure to non-specific triggers, such as smoke and fumes, and may become wheezy for a week or two after a viral infection such as the common cold. It is important that patients with asthma are aware which non-allergic asthma triggers are likely to affect them.

Airborne allergens

Identification of allergic triggers is an important aspect of asthma care and leads to improved management and better quality of life.  The significance of a a particular allergic trigger in a particular patient may be suspected by taking a careful allergy history.   Dust mite allergy is an important trigger in chronic asthma and should be considered if symptoms occur or get worse at night, when exposed to vacuuming or bed making, or when asthma gets worse in dusty or damp premises (dust mites thrive in damp conditions).  Asthmatic symptoms that occur in relationship to animals are usually more easily identified.  However in severe cases, symptoms can be provoked when the sufferer gets wheezy when they are exposed to the clothing of someone who handles horses or who has recently ridden a horse. 

Exposure to an allergen or to a chemical fume present in the sufferers  workplace must always be considered.  This can be surprisingly difficult to detect and it there is more than a suspicion that this is an overlooked cause of chronic asthma.  Often the only clue may be when symptoms clear up with a prolonged holiday, only to recur within a few hours of starting work again.  Keeping a regular record of the peak flow rate may be the best way to establish a relationship between going to work and the degree of airway narrowing.

Potential airborne allergic triggers of asthma occur in the following groups:

House Dust Mite

The house dust mite is the most important single known allergen involved in the causation of asthma in the UK.  Both the incidence and the severity of asthma are known to be linked to the level of dust mite exposure; older homes, older carpets and mattresses, damp and condensation are all associated with higher levels of mite allergen.  

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Pets and animals

Dust particles arising from cats are another important trigger for asthma.  Although in the UK they are not as quite as important as the dust mite, in some countries the cat is the most predominant single asthma trigger. However, for asthmatics who know they are allergic to animals, it makes sense to try to reduce exposure.

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Moulds and fungi

Inhaled fungal spores are a well-known cause of allergy and asthma. Indeed in some places the rate and severity of asthma in the population have been linked to airborne levels of the mould spores alternaria and cladosporium. Allergy to these fungal spores is an important cause of severe seasonal asthma in the late summer and autumn.  

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Asthma and food