The A/C and Cabin pressuration systems are interconnected In the engine( compressor section) the are a number of pick ups called p1, p2 etc,since the compressor is made in stages(each compressor turbine ring)the P1 stage would be the pick up from the 1st or the lowest/coldest section. As you go back , the compressed air gets hotter. it then passes into the ignition chamber of the engine.
Theses are called engine BLEEDS, one (I know there's more that one) feeds two AIR CONDITIONING PACKS
APU or engine P5 and if necessary P9 stage bleed air (hot) is pre-cooled by fan air before entering the pack. Inside the pack, bleed air is cooled by ram air through heat exchangers and an air cycle machine. A water separator collects water condensed by the cooling process to avoid icing.
Cabin Pressuration Panel B-373
In flight: If one pack fails with the pack switches in AUTO, the other will regulate to high flow (unless flaps are down).
If you dispatch with one pack inoperative, then max altitude is FL250. If a pack fails when above this level, then you may continue at the higher level.
This photo shows the location of the two air conditioning packs underneath the aircraft. Notice that the two ram air inlets have their deflector doors retracted because the aircraft is in-flight (see logic table below). The packs are accessed through the two large access panels between the deflector doors and the wheel well, these panels are hinged inboard. Aft of the access panels are the ram air exit louvres which give you a nice warm blast on your legs when you are stood in the wheel well doing a walk-around on a cold winter day!
As you can see the air (pressure) comming from the engines is never introduced in to the cabbin, it just heats the PACKS and is dumped overboard. So you can't have any outside "BAD AIR" in the cabin !
When the pilot is out of the cockpit it is required that the Co-pilot don the oxegin mask If a loss of pressuration was to occur at least one of the crew would be fuctioning As soon as the bells and whistles go off in a DECOM, the first thing done is the crew dons thier masks retards the powerlevers and desends.!
I find it very hard to belive that a simple LOSS of pressuration caused this Crash
Here are the Schematice of the A/C system
My 2 Cents!
Robe