Every house or indoor environment, no matter how clean it appears, has some amount of dust in it. Within this dust are tiny creatures called "dust mites."
Source: American Academy of Allergy, Asthma and Immunology
Dust mites are probably the most common indoor asthma trigger and are found just about everywhere in the home.
Dust is made up of debris, including small bits of plant and animal material. Dust mites, tiny creatures that are too small to be seen with the naked eye, eat this plant and animal material in the dust. The mites release droppings, and these droppings are the true reason why dust is such a common asthma trigger.
Dust mites can be found wherever there is dust. They thrive most, however, in places where the humidity is high and where human dead skin flakes can be found, such as bed linens, carpet and upholstered furniture. Another reason dust can be such a powerful asthma trigger is that it often harbors other allergens, such as pollen and animal dander.
If you are allergic to dust mites, then your asthma symptoms are likely to be active year round. The reason for this is that dust is always around, so it's difficult to escape contact with it completely. There are some, however, actions you can take to avoid dust mites.