ANY crack, opening, seam in the projector case, even the input connectors, is place for dust to get inside. Every projector has at least 2 fans, some have as many as 4. So that's a lot of air movement and the fans will draw air from wherever they can. So...is this normal? Yes. Should you be concerned? ONLY if the dust interferes with the image, which it usually doesn't if it's on the main lens. Why? Because the dust is not in the plane of focus and therefore, not visible in the image. If you see any "dust bunnies" or similar anomalies in your image, the dust is inside the projector on the INTERNAL optics, which means you have to open the projector to clean it. So if you don't see it in the image, ignore it.