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In Serenity (2005), the crew finds on planet Miranda a video recorded by a woman. That woman explains how the Pax was added to the air system and the effects it had on the inhabitants eventually.
How did that woman herself survive when all other people either stopped breathing or became Reavers?
The woman who made the recording arrived after the Pax was released on Miranda. She was most likely part of the research team itself that released the Pax. In this Youtube video of the monologue, the way she talks about it as well as her reference to "they have killed most of us" also indicate that the group she was with was something separate from the population of Miranda.
Now, once the crew of the Serenity are on Miranda, they are attempting to locate the source of a beacon. Their search leads them to this crashed ship...
...they enter the ship, and this is on the wall...
Research and Rescue. Within the ship, Wash says "This is the source?" meaning the beacon they were looking for, and Jayne says "This ship's banged up all to hell." confirming for the audience that they are NOT in a building, but on a ship. They then play the recording.
So, the woman who made the recording was part of a Research and Rescue team who at the very least knew about the Pax beforehand, and crashed on Miranda in an attempt to discover what happened after it was inflicted on the population of the planet. They weren't in a sealed environment on the planet watching it all unfold. Exactly when that ship crashed isn't explained (immediately on releasing the Pax, or long after) but since the details in the recording indicate that the Pax had time to cause everyone to die or change into Reavers, it is likely that the ship was forced down by Reavers in orbit.
The fact that none of the crew of Serenity is affected by the Pax also suggests that the ship crashed on Miranda at some point in time after the Pax had dissipated, so that the woman who made the recording wasn't affected at all.
As far as I can make out, the video message was recorded by one of the original researchers who were studying the effects of Pax on the planet's populace. The woman certainly seems to have all the information to hand, statistically and scientifically. This suggests that the researchers, like any 'Earthbound' equivalent, conducted their studies in a sealed environment, with air purifiers and other filters.
This would explain why she was still alive, albeit just long enough to make the recording before the Reavers attacked and massacred the team; the woman's on-screen death being a final footnote in the whole sorry state of affairs.
