Architects use a target of 18 air changes per hour for public buildings. The figure of 10 to 15 changes per hour is therefore superior and suggests that airliners are not especially unhealthy places to be, at least compared to other crowded venues. Many places don't achieve anything like the 18 norm - our draughtproofed homes for a start. The recirculated air in the cabin is filtered before being reintroduced into the cabin so in theory it is pretty clean. Airbus claim that their air is cleaner than the stuff you get in an operating theatre, and they are probably right when the aircraft and filters are new, and assuming they are talking about a UK NHS operating theatre. Of course, if the airline scrimps on servicing the air filters the situation will deteriorate. Personally I improve the air quality on board with a brandy and ginger. It doesn't make the air any better but it stops you worrying about it.