High Efficiency Particulate Air (HEPA) Filtration
Laminar Flow Hoods/Clean Benches and Biological Safety Cabinets rely on HEPA filtration. HEPA filters are 97.97% effective at removing particles smaller than 0.3µm in size.
As the contaminated air stream enters the filter from the BSC or LFH, it encounters the filter fibers (made of glass or polymers) causing the airflow to change direction. The harmful particles contained within the air stream interact with the filter fibers in different ways (depending on their size).
The largest particles (.5-5µm) having the most inertia are collected by actually impacting into the fibers. Smaller particles (0.1-1µm) are intercepted, by filter fibers as the air stream passes around them.
The filter fibers are small enough for diffusion from Brownian motion to seize the smallest particles (≤0.1µm). Brownian motion pertains to extremely small particles that appear to jump or zigzag as they travel; their motion is affected as they are struck by surrounding molecules.
All three capturing methods increase as filter density increases. The more the filter is folded the more tumultuous path the air stream must take to pass through it and more particles are trapped in the HEPA filter. Even though HEPA filters are believed to be able to capture nanoparticles, little is known and more study is warranted. Figure 7 provides an illustration of HEPA filter efficiency.