From what I've read apparently pretty much all PM2.5 encountered by most people has neurotoxic effects.

It looks like there are a couple reasons for this.

1. There are a lot of substances that are neurotoxic. Most things that create PM2.5 pollution will involve some of them.

2. PM2.5 is good at getting to places where the body really doesn't like foreign objects and so the mere presence of PM2.5 particles can trigger responses, such as inflammation, that can cause neurological damage even if the particle itself is made of a normally non-toxic substance.