Oh yeah, PM2.5 is so much better.
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Researchers at Harvard University exposed rats with bronchitis either to filtered air or, for 6 hours a day, to the PM2.5s present in the air at Harvard, concentrated 30 fold to simulate a pollution event. Within three days, 37% of the rats breathing PM2.5s were dead, but none of those breathing filtered air.
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The fraction of the PM10's which are thought to be the most poisonous are less than 2.5 micrometres across and are called PM2.5's. The smallest of the particulates are the ultrafine particles which are smaller than 0.1µm across (PM0.1) and often contain less than a million molecules. There are many millions of PM10's suspended in each cubic metre of even clean air. The chemistry of suspended particulate matter is varied and depends upon the source and can contain carbon, nitrates, sulphates, metals, polycyclic aromatic hydrocarbons to name but a few. When coal or wood is burnt, many of the poisonous emissions start as vapour but quickly condense onto surfaces such as the inside surface of chimney stacks or onto the surfaces of the suspended particles. Particles are the main delivery system for many of the inhalable poisons.
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~21% of PM2.5 in US urban areas come from gasoline engines, 2% from diesel sources.
If you are exposed to a concentration of 30ug/m3 of PM2.5 vs none (average city levels are between 15 and 35ug/m3), your relative risk of mortality increases by 1.18. For comparison, if you are a long time smoker your relative risk increases by 2.5.
The new diesel cars sold in the US meet tier 2 bin 5 specs, which calls for 0.01 g per mile of PM. Just the same as bin 2, the most stringent bin, as far as PM is concerned.
All cars pollute. It's not so much what we drive, than how much we drive.