Airtites (and most plastic cases) would have a Petri effect that would prevent dust/particulate and most contaminates from coming into contact with the coin (contaminates that require convection to move around). What they won't protect against is any contaminates that moves by osmosis. This would include moisture and chemical vapors. Some chemical vapors can even migrate through the plastic case (although I would doubt any of us are storing our coins in such an environment). Having an unspent dessicant packet in a relatively sealed/stable environment is usually sufficient. Emphasis here on stable. Rising/Falling Temperatures cause air to expand/contract. When the air contracts following an expansion, your airtite "sealed" holder literally sucks air into the holder bringing any airborne contaminants with it, not just moisture.
You are correct for rigid holders. However, soft holders (ziplocks, etc.) will merely flex for the conditions.
We're usually 5-10 degrees warmer than Vegas. Our record was in 2001 with 132. With that kind of heat and high humidity, I've often worried about my storage as well. De-humidifiers fail at those extremes, and ac can only remove so much moisture. Granted, I've never noticed a problem yet, and it may be due to the fact that they're in my safe where I have gel pouches to protect my guns from forming rust spots. Guy~
