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. L

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?