How harmful is an insidious atmosphere?

An insidious atmosphere “requires the use of protective suits or vacc suits” (inherited from corrosive atmospheres) and “will defeat any personal protective measures in 2 to 12 hours.” (CT-TTB p.83, “Atmosphere”)

How harmful to a human is time spent in a corrosive or insidious atmosphere? Is it insta-death once the protective measures fail? Ongoing damage, which would quickly lead to unconsciousness, but perhaps your party can get you back to a safe environment?

Absent any rules-guidance on the matter, what sort of gaseous mixtures would be an “insidious” atmosphere, such that I can do my own RL research?

How do I set up an atmosphere of alienation and horror for a halloween game?

It is Halloween soon and I am planning to run a horror game featuring zombies. Lots of zombies. In fact, hordes of zombies. The setting is more akin to Walking Dead than Shawn of the Dead and is set in the 12th century in France around Carcassonne with (clearly) some magic thrown in. Building more and more the hopelessness and horror of the situation that the characters are in up to the climax of the game — all the zombies coming over the horizon to get to the last stronghold where the characters are. Maybe they can stop it all, maybe not (read: probably not) but certainly at a great cost.

What can I do to set up the gaming room (a living room) to enhance the atmosphere of the game? What music can I use? What about sound effects? What about props? What else did you do that worked?

Note: It is my full intention to scare the living hell of the players as well as their characters. I want their hearts to pump faster and have them leave the lights on that night when they go to sleep. Yes, they are aware I am doing that and are fine with it.

This is linked to this question about props but I am focusing on a theme of horror and alienation around music, sounds, lighting, and props — not just props.