actually... i worked in the wafer test area of a semiconductor plant over the past 2 summers... the air in there is just filtered to remove dust. during the fab process the dust still is a huge problem, but you have to consider how little dust there is in the atmosphere in the clean room...
in the wafer sort, where i worked, the dust isn't as big a problem because the wafer is already fabbed and finished... but dust is still a problem... it's still a clean room, just not nearly as clean as the fab... no full bunny suits, but we still had to wear gloves, smocks, booties, and head and face coverings. and dust definitely can ruin ICs on a wafer, it's just not as significant. but wafers that have seen more time out in the open tend to have higher failure rates due in large part to dust, and if you actually took one out into non-filtered air... forget about getting a whole lot of good, working parts off it :wink:
and if someone touches it... you can forget about it. watching wafers being tested, you can always identify fingerprints by the fact that all the die that were touched are invariably failing... which makes it even more amusing when you get a whole handprint or something!