Boeing's website says this about cabin air:
"All Boeing jetliners, from the earliest to the latest, have been designed to deliver approximately the same cubic volume of air per minute per passenger. The principal difference is that on newer aircraft, the air is a mixture of about 50 percent outside air and 50 percent filtered/recirculated air. Among the benefits of this design is an increased humidity level compared to the much dryer cabin environment in older jetliners"