Jim Weir wrote:

There's a hell of a good question -- why did the OUTFLOW valve on the 727
(located on the starboard side of the aircraft back in the aft airstairs
compartment) have a FILTER on it?

Obviously to protect the people on the ground from the effects of secondary smoke.

George Patterson
Great discoveries are not announced with "Eureka!". What's usually said is
"Hummmmm... That's interesting...."