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In this answer over here it is explained, that a "question" comes up with aerosol cans, based on a gemora which asks if spitting in the wind counts as winnowing.

Now, the spitting in the wind case makes sense. There is wind, and the wind causes liquid to be "strewn".

However, an aerosol can is normally used indoors, where no wind exists. Even if wind does exist, the answer seems to imply that the presence of wind is not a factor here.

Now, aerosol works in the following way. A compressed liquid is put into a canister; when the nozzle is pressed, the pressure from that compression forces the liquid up the pipe, into the nozzle; and then that tiny hole of a nozzle causes the contents inside to come out in a pressure stream. (Having little space causes more pressure.) Due to the lack of pressure outside of the can, the contents then spread out in a cone shape.

So why is spitting into the wind compared here? Notice the gemora doesn't talk about spitting on shabbos in general, just into the wind.

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I found a document online citing "39 melachos" b'shem Igros Moshe that aerosols are not a problem because of the above. I don't have a 39... and did not see the siman in the IM. Can anyone can find either? –  YDK Dec 26 '11 at 15:49
    
That aerosols are not a problem of winnowing, doesn't answer my question. It's the basis of my question :) –  avi Dec 26 '11 at 19:08
    
I'm saying that you seem to be correct. The question reverts to Rabbi Mansour. H' Gabriel mentioned in the question linked to above that he can ask him about it. H' Gabriel, can you do that? –  YDK Dec 26 '11 at 19:25
    
Didn't see this comment until now. @YDK BN BH he is very busy but I'tll try him on his cell. –  Hacham Gabriel Dec 28 '11 at 19:31
    
@YDK I texted HaRav Mansour. –  Hacham Gabriel Dec 28 '11 at 21:01
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2 Answers

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I read (I don't remember where) that an aerosol does not trangress the melachah of winnowing, but an atomizer does.

An aerosol keeps its content under pressure, and pressing the nozzle simply unblocks an opening and allows it to escape. An atomizer blows air past the liquid which gets picked up by the low pressure generated.

Principle of operation of an atomizer

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I do believe this is the answer I was looking for. Do you have any source that Atomizers are not allowed? –  avi Dec 28 '11 at 19:09
    
@avi, This is discussed in the same part of R' Ribiat's "39 Melachos" that discusses aerosol cans quoted by Will in his answer. –  jake Dec 28 '11 at 22:07
    
@Jake it would make the answer bounty worthy. –  avi Dec 29 '11 at 8:16
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Using an aerosol can is permitted on shabbos.

There was a discussion at one point, if aerosol is like "winnowing" (sorting wheat from chaff with the power of the wind.), which is one of the 39 categories of action prohibited on shabbos.

However, in order to be considered winnowing, the substance must be separated by the wind in the atmosphere - or by extension found in Talmud Yerushalmi, by the "wind" of a person's lungs, via the mouth.

R' Moshe Feinstein notes that, in the case of an aerosol can, the substance is not propelled by either type of wind. Rather, the contents are under high pressure, and pressing the nozzle merely exposes the high-pressure material to an atmosphere of lower pressure, which naturally expels the substance into the lower pressure area (i.e. the room).

More information on air pressure here:

http://www.rcn27.dial.pipex.com/cloudsrus/pressure.html

The source for this R' Moshe Feinstein psak is the book "39 Melochos", p. 377-8;

Also see Minchas Yitzchok 6:26, who comes to the same conclusion.

As a final note, I will point out that the majority of shabbos-keepers already utilize aerosol cans on shabbos, so one need not worry about "al tifrosh min ha tzibur" or "maras ayin".

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But R'Moshe feinstein does not write that Aersol is allowed and not a case of say, Carrying, even though it isn't. So I'm trying to understand why aerosol might be winnowing. –  avi Dec 28 '11 at 19:11
    
Because according to the Talmud Yerushalmi, any indiscriminate scattering of stuff in the atmosphere is a toldah of winnowing. The sources above explain why that view is factually incorrect, but that won't stop others from being overly stringent. –  user1095 Dec 29 '11 at 7:14
    
I can't tell if you are saying that according to the yershalmi it is winnowing, or if you are saying someone might think it is... but lets say I hold by the yershualmi. (I do) Why would I think aersol is winnowing? –  avi Dec 29 '11 at 8:07
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I'm saying someone might think it is. The crux is: is the prohibition scattering stuff IN THE AIR, or BY FORCE OF THE AIR. Some say that putting anything IN THE AIR is a toldah of winnowing. Most say that has to be done BY FORCE OF THE AIR. Although, according to the minority, stringent opinion, farting should also be prohibited on shabbos :o) –  user1095 Dec 29 '11 at 9:24
    
Ahh interesting. Ok, whats the shift from "wind" to "air"? That is, "in the wind" or "by the force of wind", seems like no difference to me. But with the word "air", it is as you say, a hava minah. –  avi Dec 29 '11 at 9:30
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