That's why I go with positive pressure and intake filters on my main system. However when building for others they often forget about cleaning them and it gets ugly
I did a build for one of my friends in and Antec Twelve Hundred with no filters and lots of exhaust and it was a mess after a few years without cleaning, he had to break out the air compressor to clean it.
Reminds me of another customer a few years ago that used a 20HP air compressor set at 175 PSI to clean out the dust from inside his gaming computer. He spun all six of the fans so fast it literally destroyed the bearings in all of them . . . including the ones on his ~$700 video card. A-n-d, this is *-after-* I had warned him to use Popsicle sticks, straws or similar to keep the fans from spinning.