You can use a computer fan as a way to suck in fresh air, for example if you cut a hole in the side of your growbox and mount a fan. I believe that is called a "passive intake."

But that will not be sufficient. You'll want something like a 6" desk fan in there for good air circulation.
Backpacker420 Reviewed by Backpacker420 on . ???? ON FANS i have an 8 inch portable fan I bought at HOMEDEPOT that is pointed directly at the plant and/or pot and it has 3 power levels would it be bad to have it on full go through the whole grow process???? I was thinking lvl 1 at veg stage then bump up to lvl 3 during flower stage any thoughts????;) Rating: 5