Asthma causes and triggers can be divided into two major groups, namely the primary and the secondary causes, triggers or risk factors.

PRIMARY RISK FACTORS
If a child's mother smoked during pregnancy, or if a child is exposed to household tobacco smoke in infancy, the child is at increased risk of developing asthma. Smoking during pregnancy may also cause abnormal fetal lung development.

Breastfeeding for 6 months in babies with a family history of asthma may reduce the risk of developing asthma.

Childhood obesity may increase the risk of developing asthma.

SECONDARY RISK FACTORS
Acute exposure to specific substances in sensitized individuals may trigger attacks of asthma and continued exposure can lead to persistent asthma. Therefore, identification and avoidance of trigger factors (secondary prevention) are important components of the care of asthmatic patients. Common trigger factors are discussed below.

Inhaled Allergens
Inhaled allergens include household animal danders, dust (mite), fungal allergens (mould), and tree, grass, and weed pollens.

There seems to be a link between indoor allergen exposure (eg dust mite or cockroach allergen) and asthma. Current evidence suggests that exposure to dust mite allergen can be minimized by:
(a) encasing mattresses, pillows and blankets/doonas in dustproof covers
(b) polished floors (or linoleum) replacing carpet
(c) washing bedclothes in hot water (>55C) weekly
(d) not using sheepskin bedding

Although chemicals, such as aciracides, can temporarily reduce dust mite populations, there is little evidence of any benefit to asthma control.

There is some evidence to suggest that house dust mites may accumulate faster in synthetic pillows and quilts than in those filled wit feathers.

Exposure to animal allergens (eg fur, hair, secretions) should be minimized in sensitized individuals.

Fungal allergens may trigger asthma. Levels can be minimized by:
(a) good ventilation
(b) eradicating indoor dampness
(c) avoidance of evaporative coolers
(d) avoidance of indoor plants, rotting vegetation, mulches and composts, particularly in hot, humid weather

Outdoor allergens, such as Alternaria (outdoor seasonal fungus), grass and tree pollens (rather than flowers) may trigger allergic symptoms. Remaining indoors with windows and doors closed may be the only practical means of minimizing outdoor allergen exposure.

Tobacco Smoke
Personal smoking and passive smoking can aggravate

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