I was initially skeptical of this claim because I’d previously learned that to cross the blood-brain barrier particles need to be ~200nm (PM2.5 = 2500nm). However, PM2.5 does seem to be an important category of particles for brain damage: somehow these particles can access the brain [1]. Obviously, yes, it depends on exactly the particle whether it will be “neurotoxic,” but generally “unnatural” particles in the brain are not going to do good things. (I am not an expert in particulates) it seems like things larger than this don’t penetrate the blood-brain barrier, so they can’t be neurotoxic. So PM2.5 is probably at an intersection of large enough to be unhealthy but small enough that the blood brain barrier doesn’t help (probably some evolutionary argument to be made here).

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC9491465/#:~:text=PM...