Atmosphere that makes people wear masks for breathing

(I'm new here, so I apologize in advance if I do something wrong)

TL;DR of what I'm looking for:

A substance (natural or artificial) that...

• can be suspended in an atmosphere
• is harmful to breathe, though not fatal (edit)in small doses at all, just uncomfortable/inconvenient
• can be filtered out of the air with a relatively simple mask
• would stay in the air, all the time, for many years
• is not caused by fossil fuels
• is preferably nonfictional, though a fictional substance which sounds realistic would be fine

A longer explanation:

I made a foolish mistake of working on aesthetics before thinking about explanations, so this is where I'm at: I am working on a world in which people across the globe all wear masks the bear a resemblance to what is used in heavily polluted cities of our world. They are also similar to pollution masks in that removing a mask shouldn't be instantly fatal to a person, just dangerous if left off for long periods of time. Life should be able to go on fairly normally for the people of this world in every aspect besides breathing.

However, regular pollution doesn't work as an explanation in my world, since fossil fuels haven't come into common usage yet. Technological progress varies a lot from the timeline that has occurred in real life, so there is room to explain various man-made chemicals, just not smog from fossil fuels. Totally natural explanations are fine as well.

The substance could have been there from the beginning of the world's existence, or could have been put there at some point. It just has to have been in the atmosphere constantly for at least a couple decades.

So, anything you guys can think of?

posted over 5 years ago

Ramo'sBored‭

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