It's widely thought but not conclusively found that there are certain constituents that are more toxic than others. As you say, the problem lies in the ability

It's widely thought but not conclusively found that there are certain constituents that are more toxic than others. As you say, the problem lies in the ability to measure these constituents more widely. However, there is a hypothesis [1] that maybe there is something about the collective mass as negative association with health effects have been found all over the world where the composition changes drastically. A more recent hypothesis [e.g., 2] is that many health problems from particles stem from persistent reactive oxygen species that cause oxidative stress, and these reactive species form from many different mechanisms so likely scales with the collective mass, which has been serving as a surrogate metric. [1] https://doi.org/10.1016/S0048-9697(99)00513-6 https://doi.org/10.1016/S0048-9697(99)00513-6 [2] https://doi.org/10.1038/srep32916 https://doi.org/10.1038/srep32916