If your air is laden with lots of airborne bugs, the air itself that gets drawn back in could potentially contain organisms that could cause issues. I don't know of anyone who's talked about HEPA filtering their airlocks though... S bend bubblers tend to draw liquid less than 3 piece.

+1

That happened to me when I had a better bottle and tried to move it, the star-san got sucked into the beer when the sides flexed. Now I use glass, and if I have to move a carboy I cover the top with foil that has been dipped in sanitizer.

I've switched to these after the bulk of the fermentation is done.

http://morebeer.com/view_product/10937/ ... arboy_Hood

They're a little more than an airlock, but I have no more problems with drawing in air or airlock liquid. I think I'm retaining hop aroma a bit better too when I dry hop in the primary.

I used to fill my airlocks with vodka but then I got tired of cleaning trub off a textured ceiling after a great fermentation and switched to a blowoff tube. When I do use a three piece I usually fill with star san water. I don't have enough vodka.

Like JZ and Palmer always say. "Bacteria doesn't fall up". Why use air locks, I have only used tin foil for the last 2 years.

What do you do after fermentation has completed? I understand there is residual CO2 coming out of solution, but what about if you cold crash a beer? Couldn't that method lead to oxidation?

Cold crashing in any unsealed vessel can lead to oxygen uptake, whether it be with airlocks or foil. Once fermentation has stopped, oxygen may diffuse in under the foil depending on how long it sits.