HEPA Filters: Introduction
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.” (DOP-STD-3020-97). In order to be called a HEPA, it has to meet or exceed these specific criteria. The efficiency of your biosafety cabinet HEPA filter should be provided at purchase. The filter medium is made up of densely packed fibres 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 aluminium 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.
How Does a HEPA Filter Work?
HEPA filters work by trapping particles in the fibrous medium. The particles stick to the fibres 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 fibre and stick to it. The second mechanism is interception. This is when a smaller particle tries to flow around the fibre 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 fibre.
Biosafety Cabinet HEPA Filter Efficiency
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 the filter is required to have at least 99.97% efficiency for 0.3 μm aerosol particles. What this means is that HEPA filters are the LEAST efficient at catching 0.3μm particles. The efficiency actually increases with particles larger AND smaller than 0.3 μm, as seen on the diagram below. Of course, there is a cut-off with decreasing particle size, since gasses can flow freely through a HEPA, but that turning point on the graph is currently unknown.
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. Again, there is a limit to this benefit, as 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.