You are right - I am building a mental model along these lines:

a) compounds which induce a chemical reaction inside cells and as a result cause damage. Benzene, PFAS, phtalates, BPA etc are in this category. Size is less relevant but usually these are smaller and more reactive molecules.

b) small particulates, which are still probably much larger than those in group a). Thinking here microplastics, asbestos, PM10. These particulates are not necessarily super reactive with our cells, but they can cause problems through physical accumulation(?). So the question is: does the size of microplastic particles matter? Ie if I get 1K of 100 micron microplastic particles vs. 1K of 1 micron - does that alter my health risk?