Hey Jeff,
I think its not so much the fabrics, but the cushions themselves. Dust mites would live in the cushions, and the cover would have to be the barrier to the micro-varmits themselves. That's why they are not going to get through the leather, its entirely closed cell, but all fabric is woven there is openings in all fabric. Though leather has vent panels on the bottom, they will not migrate through the vent panels and come on top of the cover (protected or unprotected leather will make no difference).
If they were really concerned about dust mites in one of your sofas, then cover the cushion cores in a plastic barrier before the fabric goes on. There are covers that will not be stiff or make noise such as these. I have no idea if they can make some up for sofa cushions, but they may.....
http://www.allergybuyersclubshopping.com/dusen2.html
However, your customer really needs to be concerned more about their bedding and particularly their pillows. Dust mites prefer warm, moist environments which makes bedding and pillows their primary habitat. Mostly pillows! Its pretty gross to study up on these things, and they're not harmful in the manner of a tick, etc. , but they can cause allergys and asthma for some. That's a large part of the reason I carry Royal_Pedic beds, they don't attract dust mites with all cotton and latex interiors. But feather pillows....lookout!
More on dust mites:
http://en.wikipedia.org/wiki/Dust_mite
Duane Collie
Straight answers from thirty-six years in the business.
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