Wiring in air shaft

I've heard that running AC wires through a heating/cooling duct is not allowed by code. I figured there are a number of reasons for this.

My house has some ducting for the central heating and air, and there are also a number of air shafts that just use the space in the wall between studs (no metal ducts). While inspecting the wiring in my own house (which has had 4 previous owners), I see some wire that goes into one of the air shafts. It doesn't go into any ducting, but it does go up the same stud-section that is used for the air shaft.

So I wanted to know, what are the specifics about running through air shafts or ducts? My guess is it isn't allowed in either case, but I wanted to find more info on it.

Thanks.

NEC 300.22 applies, and is moderately complex. You can access sections of the Code online for free at http://www.nfpa.org/freecodes/free_access_agreement.asp?id=7008SB. You should also check with your local building official, since local codes often differ from the NEC, and local codes are governing.

For ducts or plenums "specially fabricated to transport environmental air" only certain types of cable and/or conduit are allowed. You say no wires are in ducts, so you're OK here.

For stud cavities, it doesn't apply "where the wiring passes through such spaces perpendicular to the long dimension of such spaces."

From your post, it sounds like the wires go the long way, so your guess is probably correct.