HEPA Filters

The function of a biosafety cabinet is dependent upon the High Efficiency Particulate Air (HEPA) filter (See Figure to Right) within the plenum of the unit. The overall basis for HEPA filter function relies on the combination of multiple physical forces: inertial impaction (direct blocking of the particulates via the filter medium), diffusion (Brownian motion), Interception (“straining”) and electrostatic forces. Larger particulates are typically more efficiently blocked by interception and impaction; whereas smaller particles tend to be more efficiently retained by diffusion or electrostatic forces. Together, these forces combine to enable these HEPA filters to remove a minimum of 99.97% of particles 0.3 μm in size. The overall efficiency of the HEPA filter is greater for removal of particles that are larger or smaller than 0.3 μm (see Figure below). Aerosolized bacteria, spores and viruses are removed from the air by these filters to ensure sterile air in medical, biomedical and industrial settings. However, HEPA filters will not filter out gases or vapors. HEPA filters have a limited life span and are relatively fragile; they can be easily dislodged or damaged when biosafety cabinets are moved or repaired. For this reason, biosafety cabinet function must be certified at least annually or when moved or repaired.

                                                   HEPA Filter Efficiency Relative to Particle Size.

The HEPA filter medium is a composed of a single sheet of borosilicate fibers treated with a wet-strength water-repellant binder. The filter medium is pleated to increase the overall surface area inside the filter frames and the pleats are often divided by corrugated aluminum separators (Figure, Bottom-Left). The separators prevent the pleats from collapsing in the air stream and provide a path for airflow. Alternate designs providing substitutions for the aluminum separators may also be used. The filter is glued into a wood, metal or plastic frame (Figure, Bottom-Right). Careless handling of the filter (e.g., improper storage or dropping) can damage the medium at the glue joint and cause tears or shifting of the filter resulting in leaks in the medium. This is the primary reason why filter integrity must be tested when a biosafety cabinet is initially installed, annually thereafter and each time it is relocated or repaired.