Share thisThe purpose of directional airflow is the protection of people and the environment where there has been an airborne release of an infectious agent. In BSL-3 and ABSL-3 spaces (except for BSL-3Ag spaces) this is referred to as the “secondary barrier” function. It is a secondary function because work with infectious material is always conducted in a primary containment device, such as a biosafety cabinet or sealed centrifuge rotors. Therefore the room is a backup measure in the defense against escape of infectious material, in the event that there is a breach in primary containment. Directional airflow, enhanced by relatively airtight room construction, is the main means of preventing the escape of these hazards outside the room.
The previous column looked at reliability of directional airflow systems in light of the recent BMBL requirement that there must be no reversal of airflow. This is a significant challenge even in well-designed BSL3 laboratories given control response times, the inertia of most air movement systems, and the very dynamic conditions that are produced in complex facilities with a multitude of failure scenarios. Through this discussion, we developed a qualified “simpler is better” philosophy of airflow control that leverages inherent reliability and stability attributes. However, no strategy is right for all applications and certainly more complex containment facility layouts require more complex airflow controls and device configurations. But more moving parts add more challenges in keeping the system running properly in all conditions. The optimum design solution matches the systems complexity to the specific properties of the laboratory configuration and operating protocols.
Further developing on the design considerations of directional airflow, we will look at the quantitative aspect of directional airflow: what volume of air transfer and what differential pressure between rooms or control zones is adequate, optimum, or excessive. What measurements achieve the goals of containment, economy, and sustainability?
Standards and Guidance
As a starting point, let’s review what is published as standard and guidance on this subject from three main sources in the U.S.: the BMBL, the NIH, and the USDA.
The BMBL makes no reference to specific ventilation system values. It is a performance-based document, specifying: “This system must provide sustained directional airflow by drawing air into the laboratory from ‘clean’ areas toward ‘potentially contaminated’ areas.”
The NIH Design Requirements Manual for BSL-3 and ABSL-3 Biocontainment specifies a negative pressure differential of 12.5 Pa (0.05 in. wg). The manual specifies for laboratory ventilation systems in general: “… these systems are designed to maintain 47 L/s (100 cfm) air flow from the corridor into each lab module.” The NIH also states in the NIH document, Biosafety Level 3-Laboratory Certification Requirements: “Ideally, at least - 0.05 in WG (-12.5 Pa) should be maintained from clean areas to more contaminated areas. In no case should the differential be less than -0.03 in. WG (-7.6 Pa) when the door is closed.”
The USDA ARS Facilities Design Standards recommends a 15 percent airflow differential between exhaust and supply, or sufficient exhaust to create a 0.05" water column differential between the containment area and the access area. With either method, it is recommended that the infiltration of air into the containment area be at least 50 CFM per doorway at all times.