I have read that the quality of air in airline cabins is recirculated, full of bacteria and with a low oxygen level.
I have been avoiding Greyhound buses and public transportation buses because their air seems stale too. All windows are closed.
Most of these are publically owned and publically regulated. Three questions.
What info does anyone have about the air quality in these buses?
What is the reason that all the windows are keep closed?
Is riding in these buses a health hazards?
Everything is a health hazard. Ride the bus.
Longer answer…
Airline cabins are pretty much sealed for the flight, and have circulation systems that recycle the air over and over again. They can’t really let a lot of fresh air in and maintain pressurization It can get pretty nasty.
City buses are not sealed. They can get fresh air whenever they want. Most city buses have opening windows and/or top vents. I’ve never heard of one that didn’t. Furthermore, air is exchanged every time the doors open and close. I’d imagine they also have vent systems much like regular cars do.
I know Greyhound and other long distance buses do have some sort of vent system in place. But they have no reason to recycle air, so I’d imagine they use fresh air. The staleness comes from cooling it. Air fresh from a car air conditioner smells the same way.
The world is full of germs. But chances are your kitchen sink is the most bacteria infesting thing you’ll come in contact with in the course of a day. Luckly our body is equipt with an immune system, which allows us to do things like ride buses without grave effects. Get on the bus and just be thankful you were born in a time when we have running water, soap and antibiotics if anything goes wrong.
“City buses are not sealed.”
That has to be the understatement of the century. 
I don’t know how many times I’ve gotten on a bus where the air conditioning was working OK but passengers had opened windows anyway.