a one piece disposable bacteria / virus filter is suited for all lung tests . the 31 / 2 &# 34 ; outside diameter embodiment has a low 70 cc dead space and a low 0 . 60 mm h 2 o / liter / second airflow at 12 liters per second . the 4 &# 34 ; outside diameter embodiment has a low 70 cc dead space and a low 0 . 45 cm h 2 o / liter / second airflow at 12 liters per second . each embodiment has two plastic body halves ; each having fins or pins or crosshairs to support the central area of the filtrete ® internal filter mounted therebetween .