Health effects of particulates are approximately linear. So the more particulates the more mortality. There's no threshold below which people are healthy and above which they're sick.

It's actually slightly sub-linear, meaning that an increase for one group and an equal decrease for another group produces slightly less total mortality. See https://pmc.ncbi.nlm.nih.gov/articles/PMC9109601/#gh2329-bib... for some numerically fitted curves (search for GEMM).