Ever wonder how commercial airlines keep the air in the passenger cabin fresh and circulating? They wouldn't just recycle the air, would they? Well, it looks like a lot of them do. An intrepid study in the August issue of Consumer Reports -- 40 volunteers used hand-held instruments to measure air quality on 158 flights by 20 airlines -- found that almost a quarter of commercial flights don't meet a basic standard for air freshness. The problem, airline employee unions and researchers say, is that newer-model planes recycle as much as half of their ventilation air, instead of providing 100 percent fresh air, as older models did.