How's this for simplicity? ANYWHERE air gets into an LCD projector you have the chance for both dust blobs and LCD panels picking up dust specks. Its part of the technologies DNA. In order to cool the LCD Panels you need air flow. Keep the filters clean and if you are handy build a Pre filter and attach it to the projectors air intakes. Some projectors are more prone to collecting dust in the optical path.
BTW The down side to a DLP projector is the color wheel failing or the DLP chip white spot failure.
-Pick your poison.