When the air quality is better, your air passageways are in more comfort and stay naturally moist, more oxygen reaches your brain, and less toxins need to be filtered out.  Also your eyes will be more moist and more comfortable.

Looking forward to making the cab as healthy as possible, these ideas came to mind:

Humidifier - when I use a humidifier at home and especially during sleep, I feel much more alert, clean, and awake.  I used to use one with aromatherapy oil when I'd sleep, and I would wake up much sooner than usual, feeling much more rested than usual.
But even in the awake hours, it also helps.  Your nose and throat and lungs will breathe easier and be more open.    There are some units that plug into the cigarette lighter outlet, and others that are corded.

Air Purifier- like an ion generator, or a small air filtering unit, these can help especailly if you're in a heavy metro area, or just to clean the exhaust from any heavy traffic highways.
(note - if you smell an odor from the ionizer, it will smell like seashore air, then unplug it.  You don't want to be breathing in ions  A small filtering (HEPA, activated carbon, etc) unit could be easier since you can just leave it on and it would circulate the air better.

Plants- has anyone tried this?  Plants add healthy humidity as well as oxygen.  Of course they need to be able to handle any temperature changes inside the cab, and handle whatever amount of sunlight that will reach them, and be in healthy soil, free of molds or bugs.  
Plus plants can add a nice living atmosphere and connection with nature. A few flowers ina  vase is easy enough, and a small potted plant could be fairly simply to take care of also.

Does anyone have any experience with these or any other ideas?

wow, these are all helpful; thank you!