Unlike most other asthma triggers, exposure to some allergen triggers can be reduced or avoided. This can lead to improved control of asthma and a reduced need for medication.

Information on specific allergen avoidance strategies are outlined in this brochure in Management Allergy Avoidance.

Triggers of asthma include:

• upper respiratory infections, particularly viral, as well as bacterial infections, such as sinusitis and bronchitis
• allergens – these are usually small glycoproteins which provoke an immune response in allergic people. They include house dust mites, animal skin and saliva, pollen, moulds and in rare cases, foods
• physical factors such as exercise, cold air, changes in temperature and oesophageal reflux
• irritants such as cigarette smoke, irritant smells (eg paint, cleaning agents, perfumes) and air pollution (eg high levels of car exhaust fumes or fumes from gas heaters)
• medicines such as aspirin, non-steroidal antiflammatory agents, beta blockers
• food additives including some preservatives
• emotion upheaval.

Seasonal (or intermittent) allergies tend to be caused by pollen.

Perennial (or persistent) allergies occur all year round and may be caused by allergens such as house dust mites.

Allergen triggers of asthma

In individuals with asthma, it is well established that exposure to relevant allergens may trigger an asthma attack and/or worsen asthma symptoms.

Evidence of this exists for indoor allergens such as house dust mite, pets and cockroach, as well as outdoor allergens such as pollen and moulds.^{20-29 [LE III-2]}

However, there is not necessarily a direct relationship between allergen exposure and symptoms in some individuals as:

• patients are often sensitised and exposed to a number of allergens
• there are other triggers such as viral infection, passive smoke exposure and medication
• the magnitude of response to the same level of allergen exposure can vary considerably between individuals
• intrinsic airway factors unrelated to allergens may contribute to the level of symptoms
• there can be a late phase response, that is, reactions evolve over hours.

Occupational asthma

Occupational asthma is often allergy related. Examples include:

• laboratory workers allergic to rats and mice
• health care workers allergic to natural rubber latex
• bakers allergic to flour dust.

Over 300 causes of occupational asthma have been described and the majority of these are due to allergens rather than irritants.³⁰