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?