Worth noting that 50-60% of passenger cars in France are diesel, but Paris have been gradually banning older higher emission diesels (Crit’air 3, 4, 5) from Paris. Banning cars outright also works, of course, but I suspect a lot of the reduction can be attributed to getting particularly bad diesel cars out vs. the limited areas where cars are entirely restricted.
Also worth noting that with modern emissions standards (and transition to EVs), over 50% of handful particulates come from tires, brakes, road surface wear, and resuspended dust: https://www.oecd.org/en/publications/non-exhaust-particulate... I don't think there's any healthy level of private cars coexisting with humans in a city, without even considering the more immediate harms from crashes, etc.
For example, iron from brakes is heavy but ecologically pretty harmless. OTOH NO₂ weighs almost nothing, but is toxic. You can eat 30mg of iron per day to stay healthy (just don't lick it off the asphalt directly), but a similar amount of NO₂ would be lethal.
Heavy particles don't stay in the air for long, and don't get easily absorbed into organisms. OTOH gaseous emissions and small particulates from combustion can linger in the air, and can get absorbed into the lungs and the bloodsteam.