I know this is an old thread but...
I go to burning man, an environment that is SATURATED with ultra fine dust (playa dust). It is highly alkaline, and very corrosive. However, I need my phone out there because of my job there.
I use the otter box defender, and it works like a charm. I'm out there for two weeks in extremely hostile conditions, 40 mph dust storms with near zero visibility, and my phone comes though clean.
Otter box defender.
Seriously.