This quote from from a B&H article on lenses:
Some shooters claim that because you have to change lenses more often with fixed aperture lenses, there’s a better chance of getting dust on your camera sensor. While this argument holds water in theory, the reality is that many zooms, especially push-pull zooms, create a vacuum effect that sucks dust and moisture into the lens barrel (and ultimately the camera body) every time you zoom from wide to tele or vice versa. This is especially true for less expensive zooms that have fewer weatherproofing seals (if any) than their pricier counterparts, though be advised that dust has been known to find its way into the barrels of the priciest optics.
This is why I vacuum out my camera once a week. Yes I use an old house vacuum to do this. My zoom lens is a real dust sucker.
I recommend you read the whole article. There is some really good information about lenses.
The complete article is here: Purchasing a New Lens