[Jorchime]

Why? If critical levels of "unhealthy stuff" is exceeded, it's not safe. Your spectrum is on the threshold of "unhealthy stuff"

[rootusrootus]

So there is a magic point, greater than zero, where it simply switches from healthy to unhealthy? That seems improbable.

Edit: Downvotes but no comments. That's a pretty low effort way of admitting your position is faith-based, not scientific.

[Jorchime]

You have a multitude of things to measure in breathing air. For each you can establish thresholds for safe levels through the scientific method with evidence. Likely there is a transition area, but also regions with a very high likelihood of being unhealthy.

If any of these measures is "in the red", your air is unhealthy.

Why not?

[peteradio]

Why? Here is a model that would make sense: your body is capable of clearing x ug/day and anything over that causes inflammation. Well then stay below that level and it is

[now__what]

As with most things, even if there is no "magic point," you need to draw the line somewhere as accurately as you can, or you'll be trapped in pedantic bickering and inaction forever.