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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?

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That's a very good question! –  Yehoshua Apr 23 '13 at 9:51
    
Yeah, well I'm having trouble understanding the proposed mechanism of action by which impurity is transferred. Clearly it can't be airborne. And if it travels straight upwards, I'm not sure how we decided that it can penetrate aircraft-grade aluminum, but not polyethylene. My memory may be failing me, but I don't recall a Talmudic discussion on the properties of various plastics. Though despite my facetiousness, there is perhaps something that discusses the properties of various materials, some of which may apply to plastics. –  Avi Apr 23 '13 at 9:56
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Two articles that may be of use: dl.dropboxusercontent.com/u/1448492/735713.pdf and yated.com/… –  LazerA Apr 23 '13 at 11:58
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Start with 'Eiruvin 30b at the bottom. dafYomi.org/index.php?masechta=eruvin&daf=30b&go=Go –  Seth J Apr 23 '13 at 12:22
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Related: judaism.stackexchange.com/questions/27872/… –  Ariel Apr 23 '13 at 19:43

3 Answers 3

up vote 9 down vote accepted

Anything which can contract impurity cannot block impurity from passing through it (Megillah 26b, Shulchan Aruch YD 371:1). A vessel can only contract impurity if it is made from cloth, sackcloth, leather, bone, wood, metal, or earthenware (Rambam Keilim 1:1, see Leviticus 11:32-33 and Numbers 31:22). Plastic therefore cannot contract impurity, so it can block effectively. The metal of the plane very possibly can contract impurity (depending if "metal utensils" means any kind of metal or just the six listed in the verse (gold, silver, copper, iron, tin, and lead)), so it can't block effectively. I note that even a vessel which ordinarily does not contract impurity can sometimes become susceptible if some metal is present and essential to its function, but these laws are quite complicated and beyond the scope of this answer.

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i didn't see your answer while I was typing mine but I think we both arrived at the same basic conclusion –  user2110 Apr 23 '13 at 16:29
    
@nikmasi Except yours deals with points beyond the difference between metal and plastic, and ignores issues surrounding the use of aluminum as a metal and the rishonim who hold a moving box works –  Double AA Apr 23 '13 at 16:31
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To be clear, I understood the question to be asking what the advantage of plastic is over metal, NOT a whole discussion about what ways can and can't allow kohanim to travel above cemeteries. –  Double AA Apr 23 '13 at 17:43
    
That was a good understanding Double AA, but your answer doesn't really answer my question: why can plastic not contract impurity, but metal can? Also, would this mean that a bag wouldn't be required in a 787, which is made out of composite materials and not aluminum? –  Avi Apr 23 '13 at 23:37
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@Avi you mean it does answer it but it isn't sourced well :). I grant you that and will fill in details in a bit –  Double AA Apr 24 '13 at 0:06

Based on this article there are a number of points to consider

  1. Anything that absorbs tumah cannot serve as a separation (Rema to Yoreh Deah 371:1)
  2. Anything that is metal or even plated with metal absorbs tumah (see note 15 in article)
  3. If someone enters a forbidden area in a "box or tower that moves through the air" he becomes tameh because a moving container cannot serve as a sufficient barrier (note 26 l.c.)

So it would seem that a fuselage contracts and thus transmits rising tumah. Thus a Kohen would need to encase himself in a tent or other device (plastic does not contract tumah) to separate himself from the tumah inside the plane.

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The rulings in those notes are not universally agreed upon. –  Double AA Apr 23 '13 at 16:31
    
@DoubleAA I didn't think it was necessary to state that for an actual halachic decision one should CYLOR –  user2110 Apr 23 '13 at 16:33
    
Based on your point 3, even plastic doesn't work. Your final conclusion does not follow from the premises. –  Double AA Apr 23 '13 at 16:49
    
@DoubleAA the container is moving, the container within the container is not (relatively speaking) –  user2110 Apr 23 '13 at 17:33
    
That's a HUMONGOUS chiddush. You could also then just put a second box in the first box and be carried through a cemetery! –  Double AA Apr 23 '13 at 17:37

Even if we were to assume that plastic is not Mekabel Tum'ah according to what the Rambam writes in Hilchos Tum'as Meis 13:3&4 it would not help to block the Tum'ah.

Halacha 3: These substances convey ritual impurity and intervene in the face of it: over sized wooden vessels, keilim made from stone, animal turds, or earth that are over sized, simple leather keilim, a curtain, a sheet, or a reed mat that are made like tents..

Halacha 4: The following convey ritual impurity, but do not intervene in the face of it: a human being, wooden vessels that are not over sized, because they are like all other keilim and convey ritual impurity, simple leather keilim, a curtain, a sheet, or a reed mat that are not made like tents, but merely extended outward, without being on a slant or having walls... All of these convey ritual impurity, but do not intervene in the face of it.

So we see that even something that is not Mekabel Tum'ah doesn't automatically mean that it would block Tum'ah.

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