When you force air out of a darkroom, air must also enter it. Consider forcing filtered air into the darkroom to control dust. Perhaps there will be enough leaks for the outflow. If not, an exhaust port with a light trap can be installed in one of the windows, preferably far from the fan for cross ventilation. Having the fan exhaust air from around the developing trays would be healthier, but a dust free darkroom saves a lot of spotting. Anyone who can build a darkroom can improvise an efficient ventilation system cheaper and perhaps better than buying one.