These are the only heavyweight organic cotton covers we carry. They provide excellent protection against dust mites and their debris and feces with a proven small pore size of 3.85 microns. You can be sure you are getting a quality organic cotton dust mite cover with our Home Essential organic cotton covers, as the fabric is SKAL and GOTS certified organic and tested by PMI for mean pore size and barrier efficiency. Backed by a 5 year warranty, they are durable and will withstand machine washing. The nylon zippers are easy to use and reinforced for strength while remaining discrete under your sheets. The dust mite covers are slightly oversized to account for shrinkage, so we recommend washing prior to use for best fit. The pillow covers are offered in the heavyweight organic mattress cover fabric and a lightweight organic fabric for those who prefer a thinner cover for their pillows. Both the heavy and light weight fabrics provide an excellent barrier to dust mites, though the heavyweight fabric has a slightly smaller pore size. None the less, I would recommend the lightweight fabric over the heavyweight fabric for a pillow cover because it is more supple and was preferred by our testers.