How thin can I make the wall of a pipe? For a science project, I'm 3D-printing some custom pipes and tubes to regulate the flow of gas (a combination of ethyl alcohol and water vapor) through an apparatus. They need to be pretty small, as the entire experiment is designed on a small scale. I'd also like the use a little filament as possible.

How thin can I make the walls of these pipes and tubes before either they collapse or gas leaks out? I know that's possibly an engineering issue, but I'd also need to take resolution into consideration. Ideally, the pipes would be about two centimeters in diameter, possibly a little larger or a little smaller in some parts. I'd most likely use ABS, but PLA is my backup in case there's some unforeseen reaction between the gas and the pipes.

The printer I'm using is an FDM printer, a version of the MakerBot Replicator.
The thinnest wall your printer can print is determined by its nozzle size, and will be a little thicker than that nozzle size.

A great challenge when dealing with thin, hollow cylinders is that the cross-section has very little surface area and it can delaminate easily, especially if the tube is long.

You could try printing the tube with a very thick extrusion with, and using only a single perimeter. That would give better gas-tightness and layer adhesion than two, thinner perimeters, but it may turn out too fragile for your application. In that case, you'll need to print additional perimeters. Sticking to thicker extrusion widths would still be beneficial.

At a two centimeter diameter I'd say the single perimeter has a decent chance of working if you handle them gingerly.