Low airflow alarm, 100 f/m face velocity, Self securing hinged door panels, brushless and sparkless centrifugal fan, electronic elapse run time Counter, inter-filter test port for filter saturation detection, clear side panels for 360 viewing, FILTRETE electrostatic pre-filter with “Clean-Change” system, Filter ID window MAIN FILTER SELECTION: (PURAIR 10XT & 15XT require ONE filter, PURAIR 25XT requires TWO)