Biological safety cabinets offer protection to the user and environment against airborne particulates generated within the cabinet by a combination of laminar airflow and HEPA filtration. Class 2 cabinets also offer product protection against particles present in the room air.
A High Efficiency Particulate Air, or HEPA filter is far more than just a sophisticated sieving device. In fact, several actions are taking place simultaneously during each cycle, beginning when the upstream airflow meets the pleated folds of the filter media.
Interception
Upstream velocity, combined with the pleated nature of the filter media, causes the airflow to divide into numerous smaller streams which are then forced through the pores. Particles too large to pass between the fibres become trapped against them; a process known as sieving. While this primarily takes place at the surface of the filter, it can also occur at any point within the matrix.
Impaction
The primary filtration process for particles more than 1.0 microns in size, impaction occurs because the folds of the filter contain a web of random fibres. While the air flow can change direction to traverse this web, larger particles tend to continue in a straight trajectory, colliding with the fibres and becoming entrapped while the air continues to flow round them.
Diffusion
Diffusion is the primary collection method for particulates of 1.0 microns or less in size, relying on the fact that smaller aerosolised particles behave in a similar way to gases, which migrate from areas of high to low concentration. A similar thing happens in a HEPA filter, where small particles travel from areas of high airflow to lower airflow at the surface of the fibre, where they settle alongside other particles already trapped there
Electrostatics also aid filtration, as negatively charged particles will be attracted to positively charged fibre surfaces. Even gravity can play a part. The combination of all these mechanisms makes HEPA filtration extremely efficient in removing particles from air flowing through a microbiological safety cabinet.
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