I just started airbrushing and was concerned about how much spray I must be breathing. Other airbrush artists I have worked with or watched do not wear masks.
It is unhealthy to inhale spray mists.   The best way to avoid ths is to use a spray booth or hood which would draw the overspray through a filter, keeping it entirely out of your breathing zone.  If this is not practical, at a minimum, we recommend a  NIOSH approved disposable dust and mist respirator.  Avoid the cheaper paper masks that are not labeled with a "NIOSH" approval, as they will be more unlikely to fit adequately, or trap particulates as well.   Better would be the use of a NIOSH approved half-mask rubber or silicone respirator, fitted with "P-100" cartridges, which signifies they are engineered to capture the finest dust and mists.   We use silicone 6000 series respirators, made by 3M in our shop, and find them to be quite comfortable.  They also fit much better than disposable paper respirators.  These are available from safety supply houses, such as Northern Safety.  (www.northernsafety.com).  Bear in mind that in OSHA's opinion, you should get medical clearance to wear a respirator.