We've decided to use an HEPA certified filter to provide a high level of protection. A vacuum filter was the cheapest solution, as custom-made units would cost at least twice as much.
We ran a series of tests with different fans and studied professional PAPR units. To get a good enough airflow we needed a static pressure of about an inch (25mm) in H20. So we've chosen a fitting blower fan and added three 18650 cells to get about 12V. At this point we used a charge indicator module and were searching for an affordable battery manager/charger.