Most manufacturers include the mean pore size (also called average pore size) of their fabric in the specifications of their dust mite covers. Typically, mean pore size ranges from 5 microns (1 micron = 1/1000th millimeter) down to 2.7 microns. From advertising it appears, the smaller the better.
Do all types of fabric have a mean pore size?
First of all, mean pore size only applies to dust mite covers made out of woven materials. The pores are the small openings between the woven threads. So for dust mite covers made out of non-woven material, or cotton with a membrane bound to it, you typically don’t find a mean pore size.
What does mean pore size actually mean and is it relevant?
When it comes to woven material, the pores don’t all have the same size. So the mean pore size is really the average pore size of all pores measured, either by the manufacturer himself, or a testing laboratory. For example, a fabric with a mean pore size of 5 microns could have pores with diameters ranging from less than 1 micron up to 20 micron.
In 1999, the University of Virginia tested a wide range of different fabrics for their allergen blocking ability. The experimenters concluded that woven fabric with an average pore size of 6 microns or less effectively filters common indoor allergens, including allergens from cat droppings which are even smaller than dust mite allergens .Their conclusion was of course based on the materials they tested. In the experiment, a vacuum pump was used to try to suck the allergens obtained from the dust in a vacuum cleaner bag through the fabric.1
Even though materials with a mean pore size of 10 microns were effective against the house dust mite allergen, the University of Virginia experimenters suggested going for 6 microns, since they also found that pore size increased with repeated washing for some of the materials they tested.
Independent testing of the real allergen blocking ability and breathability of a dust mite cover: In addition to providing information on the mean pore size, some manufacturers send their fabric to an independent laboratory. The laboratory performs a test similar to the experiment by the University of Virginia and issues a fabric-specific report confirming that dust mite allergens (and in some cases even the smaller cat allergens) are effectively filtered out. Some manufacturers allow you to download this report. These manufacturers also tend to provide information on the airflow through the material. Airflow (in liters per min) is a number giving you information about the breathability of the material. The higher the number, the more breathable a material is.
Vaughan, J. W., T. E. McLaughlin, M. S. Perzanowski, and T. A. E. Platts-Mills. “Evaluation of Materials Used for Bedding Encasement: Effect of Pore Size in Blocking Cat and Dust Mite Allergen.” Journal of Allergy and Clinical Immunology 103, no. 2 (1999): 227–231.