I've never experienced the recessed circles collecting more dust than blowing a single deep breath would solve. And I've never even seen any grime in them. I suppose of you used it outside, but every type would get grimy that way.
Em dash will give your ex-boy/girlfriend your new phone number. Em dash will mix antifreeze into your fishtank. Em dash will drink all your beer and leave its socks out on the coffee table when there`s company coming over. Em dash will put a dead kitten in the back pocket of your good suit and hide your car keys when you are late for work.
I've never experienced the recessed circles collecting more dust than blowing a single deep breath would solve. And I've never even seen any grime in them. I suppose of you used it outside, but every type would get grimy that way.