Pot odds to play any two cards (NLHE)?

[Poker Clif]

I played a hand recently where I had A6s and called a minraise because I was getting big pot odds. There was a reraise, and by the time it was back to me I had to call for another $20, now with with $480 in the pot.

It turned out rather well, as the flop was A66, but it got me thinking. With those odds, what hands, if any, are mathematically unplayable? This was early in a micro donakment, by the way.