Reevaluation of the biological certification test for the class II biological safety cabinets

The biological collectors for the biological challenge tests of the class II biological safety cabinets (BSC) described in the National Sanitation Foundation Standard No. 49 (NSF 49) were reevaluated. These tests are designed to evaluate personnel and product protection, and cross contamination control capabilities of BSC's. In the personnel protection test, the sampling efficiency of the impingers was approximately 1/10 of that the slit-to-agar samplers located at the same position. In the personnel protection test, the collection efficiency of the top pair impingers was 90% less than that of the middle pair of impingers (see text for placement location). Depending on the construction of the cabinet, the ratio of spores collected in the middle and the lower pair of impingers, and in the slit samplers varied considerably, from 98:7 to 3:40. In the product protection and the cross contamination tests, the number of colonies collected in a column of agar plates decreased by factors of 1/10 to 1/30 by deviating one column from the outlet of the impinger. In the cross contamination test, the nebulizer located at the division point distributed spores approximately 100 mm further from the side wall than that located at the midpoint of the internal side wall. For the class II BSC which has the air intake between the side wall and the work plate, the Collison 6-jet nebulizer with an outlet diameter of 10 mm (discharge velocity at 0.7 kg/cm2 = 1.8 m/s) was the choice instead of that of 14 mm (0.8 m/s), since the discharge velocity of aerosol from the nebulizer strongly affected the result of the cross contamination test.