What does "aerosol" have to do with spitting in the wind, or winnowing?

Question

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.

Answer 1

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.

Answer 2

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".