Small micron particulate matter, commonly called PM2.5, needs an electron microscope to be visible to you but once real smog, PM10, declined by the 1990s, air pollution activists began to tout this new killer. Since it is 1/4th the size of real pollution air quality maps could often be red, or at least yellow, and that is good business for trial lawyers. The problem quickly became that no one ever died from it, humans would've been extinct 50,000 years ago if that were even possible, so they pivoted from deaths to hidden effects that can be claimed using statistics.