Exactly what it says on the tin: what are some conditions under which people would use self-contained breathing apparatuses as opposed to evacuating or using lesser equipment?
A self contained breathing apparatus is basically a self-contained atmosphere, intended for use when the outside atmosphere does not like you and is attempting to asphyxiate/irradiate/suffocate/poison/etc. you. It can be either open-circuit, and vent your exhalations to the atmosphere around you, or it can be closed-circuit, and be a rebreather that reprocesses the user's exhalations. Either way, the idea is that you don't have to rely on the air around you, because you're carrying your own.
Now, normally, you generally use such a thing when the environment around you is inimical to life as you know it; for instance, underwater, inside a burning building, in a vacuum, in a collapsed mineshaft, or other such environments. However, these are all environments that a human, generally speaking, will attempt to leave eventually, or will only be subject to for a brief period of time before leaving; after all, their supplies of breathing gas don't last forever.
Conversely, there are some rather messed-up environments in which humans will stay for long periods of time - for instance, smog-choked cities, or the inside of a maximum-capacity COVID-19 treatment and isolation ward. However, nobody there is using SCUBA gear, since a facemask or filtered respirator does the job without relying on an closed or semi-closed system; why carry around your own air when you can just filter that around you?
What I'm looking for, however, are conditions that fall under the following:
Hazardous enough to human respiratory or overall health that a self-contained breathing apparatus must be worn, as opposed to lesser personal protective equipment such as a respirator or facemask.
Wide-ranging, ubiquitous enough, or generally inescapable enough that humans won't try to leave, as opposed to conditions that self-contained breathing apparatuses are usually used under - i.e. ones which people will generally try not to stick around in.
Not temporary - i.e. they won't just go away in a few days, they'll hang around for years or into perpetuity.
Localized - i.e. it doesn't effect the entire planet.
What are some of these conditions?
Good answers will cite atmospheric conditions that can happen on Earth.
Better answers will cite man-made atmospheric conditions that can happen on Earth.
The best answers will cite a man-made atmospheric condition that could happen on Earth, as well as a specific mechanism of action - for instance, "[X] portion by percentage of [Y] chemical in the atmosphere caused by [Z] type of industrial pollution".
How is this related to worldbuilding? Well, the paradigm shift of having to carefully ration your oxygen lets you write some interesting stuff. For instance, a foot chase scene might involve both parties walking after one another rather than running, since doing so is more air-efficient than running; alternatively, stopping to stock up on air supplies is a good reason to have to stop on a post-apocalyptic cross-country journey when you'd normally never stop driving because of the mutant zombie cannibals or whatever. Essentially, it enables different types of plots than normal.