It has dust filters in front and bottom, but no filters for the optional side panel fans. Which means the side panel could be a way for dust to get into the case.
One way to prevent this would obviously be not to use fans for intake in your sidepanel and to go for "positive air pressure" which means you push more air into the case (from your front fans) than you pull out of it (from your back/top fans). This would keep dust out, too.