It is common knowledge that to have a clean environment inside a biosafety cabinet or clean bench, you have to have HEPA-filtered air. Presumably, the filter removes all contaminants, but does it really? How does a HEPA filter work? Are all HEPA filters created equal?
A High Efficiency Particulate Air (HEPA) filter is a “throwaway, extended-media, dry type filter with a rigid casing enclosing the full depth of the pleats. The filter shall exhibit a minimum efficiency of 99.97%, when tested at an aerosol of 0.3 μm diameter.” (DOE-STD-3020-2015). In order to be called a HEPA filter, it has to meet or exceed these specific criteria. The filter medium is made up of densely packed fibers through which air flows. This medium is folded into a series of pleats to increase the surface area and channel the air in a uniform direction. The pleats are held together with separators that can be made of various materials, commonly aluminum or glue. This is then packed into a frame to keep it rigid. The frame is sealed to prevent air leaks, so only air that has passed through the fibrous medium gets through the filter.
HEPA filters work by trapping particles in the fibrous medium. The particles stick to the fibers and are removed from the airstream. This can happen by three different entrapment mechanisms. The first is impaction. This is when large particles hit a fiber and stick to it. The second mechanism is interception. This is when a smaller particle tries to flow around the fiber and gets stuck. The third mechanism of entrapment is diffusion. This is for the smallest particles that move through Brownian motion and get stuck to the fiber.
Depending on particle size, a different method of entrapment may be used, but regardless, it will be caught in the HEPA filter. The definition above states that a HEPA filter is required to have at least 99.97% efficiency for 0.3 μm aerosol particles. Depending on its quality, a HEPA filter can trap from 9,997 to 9,999 of every 10,000 particulates of a diameter greater than and less than 0.3 μm. For most industrial uses, the 99.97% performance is acceptable, however, according to NSF 49, pharmacy and laboratory applications using clean benches and biosafety cabinets should have a minimum filtration efficiency of 99.99% against airborne particles 0.3 microns in size. Filtration efficiency will be greater than 99.99% on particles that are larger and smaller than 0.3 microns. The HEPA filters in nearly all Baker products utilize the Institute of Environmental Science and Technology (IEST) Type C or better (99.99% level) filters.
As HEPA filters load with more and more particles, their efficiency increases and they work better. Filter loading creates a greater surface area for new particles to hit as they are trying to pass through the HEPA media. Note, HEPA filters do not remove vapors or gases, and there is a limit to the filter load benefit (the motor/blower that forces the air through this loaded media will have to work harder). This is why it is important to have your HEPA filters checked annually by a certifier and changed periodically. Check with your manufacturer and certifier for their recommendation on frequency of replacement.
Baker’s exclusive StediFLOW™ motor/blower technology helps lengthen HEPA filter life by compensating for filter loading. See this infographic to learn how you can save up to 80% on HEPA filter changes when you choose Baker over a minimum-standard biosafety cabinet.