How did the Rotating Service Structure achieve environment control?

Question

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?

Answer 1

The Payload Changeout Room (PCR) ...

...is an environmentally controlled clean-room-type enclosure. It has a set of outer airlock doors with inflatable environmental seals so that the payload may be transferred from the payload canister into the PCR without exposure to the external environment.

From https://commons.erau.edu/cgi/viewcontent.cgi?article=3378&context=space-congress-proceedings

The PCR (Payload Changeout Room) on the RSS (Rotating Service Structure) at launch complex 39 is equivalent to a class 100,000 clean room.

From https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19830014027.pdf

The Payload Changeout Room (PCR) is a controlled environment room 50 feet wide by 45 feet deep by 130 feet high. The temperature, humidity, cleanliness and pressurization are controlled by circulating 52,000 cfm of air through HEPA filters mounted on the ceiling with the return air passing though a pre-filter bank and chilled water cooling coils located above the fan room. The room is maintained at 0.5 inch W.G. positive pressure to prevent outside air from entering the room.

From https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19850008628.pdf

This picture shows one of the inflatable seals. It's taken from outside the PCR near the starboard aft of a stacked orbiter.

(personal photo)