Although the bigger ash particles that people find coating their cars and patio furniture during wildfires is the most noticeable part of the smoke, it is the sub-micron particulates in smoke that pose a particular respiratory problem since they are so small that they can elude most of the body's normal mechanisms to capture particles in the air before they reach the deepest regions of the lungs. A dust mask won't filter down to that size level.

The superfine particulates from burning wood are also a primary reason why there's a trend to ban wood burning fireplaces in some areas, on certain winter days, when particular weather conditions would trap the smoke close to the ground, where people would inhale it.

If you're interested in reading more about the topic, this document ("Wildfire Smoke--A Guide for Public Health Officials") seems to be a standard. It was a collaborative effort by staff from different public agencies across several states that often face wildfires and you can find the document on the websites of many governmenet agencies. Looking for a link to this doc this morning, I notice that it has just been updated a few months ago. Excellent! The previous version I had was last updated back in 2001.