Peter - quite large volumes of air are recycled by, you guessed it, recirculation fans, on many RPT type aircraft. All in the name of fuel saving and econony. While the air is passed through HEPA type filters, who knows what does and doesn't get back into the cabin from the TB carrier in 66A or the 100's of other exotics from around the world.
speeling edit to avoid the peacekeepers wrath