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It's not toxic in an acute sense, but it's equivalent to cigarette smoke and chronically terrible for you (I love smoking and diesel, btw). Both are low T impartially combusted complex hydrocarbons that generate a stew of different nanoparticles and reactive species.

It's terrible stuff!

You can argue that it's benign in the middle of nowhere, where the concentration drops to nothing quickly. But not in the city!

"even with the dirtiest diesel engines, is going to get you serious wins over GHG/CO2/VOC emissions from automobiles"

Not for particulates. Even a clean diesel bus is going to be dirtier in particulates vs. gasoline. Its the nature of the fuel (i.e. not the thermo cycle used).

ON the other hand, converting to CNG eliminates particulates completely (you use the same engine, just feed it CH4)

For particulates you can switch to CNG, and you can probably convert the diesel engines to run off that too.

"but unlike most major religions, they discourage retreating from the world and ordinary secular life."

I think that would be a mischaracterization of major religions. For example Christianity has its mystics and ascetics, but they it doesnt preach that all, or even the vast majority should pursue this path. Instead, Christianity teaches that its faithful should take time to retreat and pray, to go back into the world a more faithful servant of God.

Even the Saints and ascetics don't retreat from the world but rather seek silence. St. Benedict, the paradigm of the Catholic monk would leave the monastery every month to meet up with his sister.