Dust Mites 

The dust mite is an arachnid that reaches no more than 0.3 mm (0.01 in) in size. This tiny arthropod lives in human homes, where it feeds on the dust produced by human and animal skin. Dust mites are not harmful in themselves, but their droppings, which contain left-over digestive enzymes, are a significant cause of asthma and other allergic diseases.

The average waste particle of bed bugs is approximately 9 microns, therefore the pore size of a allergy encasement must be less then 9. Typically, any non membrane fabric with a pore size less then 6 or 7 will block all allergens including pet dander.