[infogulch]

I wonder if there's a difference in harm caused between human-generated particulates vs particulates from, say, a forest. I assume at some point as air quality improves, natural pollen, bacteria, etc, would become a significant factor in simplified PM-based AQI, and my question is, is that still a problem?

[rubatuga]

Something like aerosolized salt from a humidifier using tap water is essentially harmless. But it still counts as PM2.5

[MichaelZuo]

Certain types of fine particulates are certainly more harmful than even forest fire smoke. For example asbestos, brake dust, leaded gasoline in dust, etc.

But the vast majority would probably be roughly the same.