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I don't think it's possible to find a case that will prevent dust. As long as there's a gap, no matter how small, there will be dust. The skins wouldn't have dust if you clean the phone off correctly because there's no gap between the skin and the phone.
 
Like others have said, there is really no chance dust won't get in with the exception being the Lifeproof case, I would think. If that is waterproof then it has to be dust proof, right?
 
I just leave the plastic film that came on the back of the iphone on and then put the case over that. No dust and you don't see the film anyway!
 
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