All aircraft fuel tanks are vented to the atmosphere for two reasons; first, if the tank was not vented you would not be able to get fuel out of the tank (just take a straw, put one end in a fluid [your choice] plug the top of the straw with you finger and lift the straw out of the fluid and nothing will come out until you take your finger off the top end, fuel tanks work the same way); two, when you are fueling the tank you could pressurize the tank if it were not vented and inflate the tank this has happened when the volumetric shut-off system did not open back up, inflated wings are very bad things
!! I've seen pictures of a F-28 that this happened to.
This flow of air into and out of the tank via the vent system also allows for moisture to enter the tank. Surprisingly, a large quantity of water will accumulate in the tank over time, if not drained.