Why the bed is the problem
Dust mites feed on shed skin and need humidity to survive. A mattress and pillow supply both, which is why mite allergen concentrates there rather than in the carpet or the curtains people usually blame.
Mite allergen particles measure roughly 10 to 40 microns. The mites themselves run 200 to 300. A barrier fabric works by having pores smaller than the particle, and nothing else. Porous Measurement International measured the AllergyCare™ woven cotton at a mean flow pore diameter of 4.74 microns, with no membrane and no coating involved.
Tight enough to stop the allergen, open enough to breathe. That is why it does not sleep hot the way a laminated cover can.