That's what happens with zoom lenses as you zoom in and out they suck in air/ blow air out. It seems all the major dust settles on the front element. Until such a time that it shows up in your shots then don't worry about it. When it does show then the front element is a lot easier to take out and clean than the Rear element. I did my 18-200 vr II Nikon front element in about 15 mins after a quote for £100 . I would not touch a rear element though as there are a lot more electronics in that end.
