I have seen good hay in sheds with NO ventalation get dusty in Feb/march.  I belive it was because the hay would warm up in the daytime & then draw moisture out of the night air causing the hay to get "Dusty" inside the bales. This is something that takes weeks to cause. 

 1 time yrs ago I got a new costomer that stated that every yr that in the spring the hay that they had been feeding all winter would go "bad". It was about a 95 degree day out, but when I walked into his hay shed it was about 120-130 degrees. I told him there was NO WAY I was going to put hay in that shed unless he got some air flow. I had him take 2 sheets of tin off the back, & vent the roof.  The hay I took him had gotten baled on a good dew, felt wet. I let it sit for 2 weeks outside BEFOR I put it into the shed. He KNEW that that hay was going to be spoiled befor spring. The next yr when he called for new hay I asked "how was the hay?". He said "we NEVER found a bad bale out of 4000".  & couldn't belive that the "wet" hay had kept that well.

 IMHO Hay needs some air flow & you don't want it getting warmer than the outside air.  Kinda like running the fans on grain to cool grain.

 On my 3 sided 60x95 shed I pulled 2 sheets of tin off the back to keep some air flow going through. On the 2 new 80x125 sheds I'm putting up  I'm going to leave 3 sheets of tin off the back.  I don't want the sheds to be much warmer than the outside air, perferably cooler. I also have 2 open sided sheds.

 But your in a differant area, so what works for me in Montana might not work in your area.  High Humiditys I belive can cause hay to get dusty. Esp if the hay gets WARMER than it needs to during the day. Heat will draw moisture out of the air.