Yes - there are other poisons as well hidden behind those particles, but it's very hard to measure the composition of those particles in real time. Particulate matter can be measured with a $20 sensor from Sparkfun, so it's an often-used proxy.

As for why the particles sizes are differentiated, I think it's because some sizes can cross barriers into the blood - and are therefore concerning regardless of the contents. pm 2.5 can reach the deepest parts of the lungs and just muck up that whole area regardless and cause all kinds of systemic inflammation. pm 10.0 generally doesn't reach that deep but still causes eye/ nose/ throat problems IIRC.

If there is near-zero or undetectable particulate matter, then that would preclude lead particles as well, obviously.