How is mildew and mold avoided in the pipeline and interior spaces being heated by steam?
You should be more worried about Legionella, but proper steam and hot water systems will stay above 140F (60C) as that temperature not only prevents the bacteria from multiplying but will kill 90% of them within 2 minutes.
But what counteracts this is a loss in humidity caused by the heating of cold air from the radiator itself (cold air warmed up reduces relative humidity), and in my experience, this more than counteracts the little bit of steam hissing out of the radiator valve.
As a side note, I've always wondered if the steam being vented that way is actually coming from the steam plant, or whether there's a heat exchanger somewhere in between that isolates a building's steam network from the distribution one (which is the case for liquid water heating, as far as I understand).
Historically, you open a window.
inside the pipe, can much life survive? is there anything to eat in there? has anything evolved specifically to live in NYC steam pipes?