I was looking around at some different blogs and forums and noticed people talking about what fabric is best for making dust mite covers. Several mentioned Pristine as being the best while others said the ones with a membrane were better. One person even suggested saving money and wrapping the mattress in plastic garbage bags.
Personally I prefer the ones made with Cotton. To me they are lighter, cooler and more comfortable. The dust mite covers we have on our mattress and pillows are 100% cotton, chemical free and naturally soft. I guess they are not completely chemical free since they have been bleached whit. The covers made from organic cotton were too expensive for my budget.
Some will say the pore size of cotton is too big to stop dust mite allergens. That is just a bunch of bull. In most cases the cotton sheets you have on your bed stop the allergens. Dust mite allergens are somewhere between 8-10 micron. The AllergyCare cotton covers sold by the Allergy Store have a pore size of 4.8 microns so no problem.
If you have children the cotton may not make the best mattress cover. In that case maybe the ones with the membrane would do better. Besides protecting against dust mites they are also waterproof and a bit less expensive.
BTW taking care of the covers is not a big deal. We wash our mattress cover a couple times a year and the pillow covers about every 4-6 weeks. Simply wash in warm water and dry on medium heat.