I'm not an aircraft engineer or professional pilot so apologies if this question has an obvious answer. From the side of engine fumes etc. (leaving aside airborn diseases from other passengers within the cabin), am I right in thinking that air enters the aircraft through the engies in most aircraft. The risks of this are fairly obvious. It doesn't take much for engine fumes to get to the air supply so why not change the point at which air enters the aircraft? Lord knows the manufacturers have had long enough to come up with a new way of pressurising the aircraft, even if it is less convenient.