How is asthma triggered by dust?

Allergies: Dust and dust mites can trigger allergies which in turn trigger asthma.

Different Ways: Also thick particulates in the air can trigger asthma without allergies, such as a dust cloud. And see dr windisch's answer.

Allergen...: In many asthmatics, dust is an allergen which means the inflammatory cascade in the airways is triggered by the dust resulting in bronchospasm and inflammation, the hallmarks of an asthma exacerbation. Treatment with bronchodilator and steroid inhalers are usually effective in controlling symptoms.