The main purpose of the Rotating Service Structure on the Space Shuttle launchpads is to have a clean environment to transfer payloads into the orbiter payload bay:
The major feature of the rotating service structure is the Payload Changeout Room, an enclosed, environmentally controlled area that supports payload delivery and servicing at the pad and mates to the orbiter cargo bay for vertical payload installation. Clean-air purges help ensure that the payloads being transferred from the payload canister into the Payload Changeout Room are not exposed to the open air.
I am wondering how this clean environment was achieved. Looking at the photo below it seems that the RSS would envelope most of the orbiter, but the fit is nowhere near tight enough to keep water and dirt out:
(Space Shuttle Endeavour on Launch Pad 39A - source)
Here you can see some mats (?) around the edge where the RSS and the payload bay meet, but this seems hardly enough to keep cleanroom-like cleanliness. They seem more for protection from people walking than to seal:
(Inside the Payload Changeout Room - source)
How was the Payload Changeout Room environment kept clean?