Cause and Risk Factors

The causes of asthma are numerous and not fully understood1
The fundamental causes of asthma are not completely understood. For example, urbanization has been associated with an increase in asthma, but the exact nature of this relationship is unclear.

Risk and triggers
The strongest risk factors for developing asthma are a combination of genetic predisposition with environmental exposure to inhaled substances and particles that may provoke allergic reactions or irritate the airways, such as:
Indoor allergens (e.g. house dust mites in bedding, carpets and stuffed furniture; pollution and pet dander)2
-Outdoor allergens (e.g. pollens and moulds)1
-Tobacco smoke1
-Chemical irritants in the workplace1
-Air pollution1
There are also other triggers (i.e. stimuli that irritate and inflame the airways) for asthma including:
-Cold air
-Extreme emotions such as anger or fear
-Physical exercise
-Certain medications (e.g. aspirin and other non-steroid anti-inflammatory drugs, or beta-blockers which are used to treat high blood pressure, heart conditions and migraine).