In a picture on this article, we see a kohen who has enclosed himself in a plastic bag so as to avoid becoming impure as a result of flying over a cemetery. Why is it that a pressurized fuselage does not protect against this impurity, but a plastic bag with a hole in it does? What factors specify what barriers appropriately protect against this impurity?
EDIT - In light of some answers I consider incomplete, I'd like to make something clear: a proper answer should explain the logic behind why metal is an ineffective barrier but plastic is not. Also, I thought of something else relevant: if plastic is an effective barrier, then would this plastic bag not be necessary in a 787, which is made of carbon-fiber reinforced plastic?