Air purification system (HEPA Filtration)
Introduction
HEPA (from High Efficiency Particulate Air)[1] is an efficiency standard for air filters.[2].
Function
HEPA filters are made up of a mesh of randomly arranged fibers. The fibers are typically composed of glass fiber and have diameters between 0.5 and 2.0 μm. The most important factors to take into account in a HEPA filter are the diameter of the fibers, the thickness of the filter and the speed of the particles. The space between the fibers is much larger than 0.3 μm, but that does not mean that particles with a smaller diameter can pass through. Unlike membrane filters, HEPA filters are prepared to retain contaminants and much smaller particles.
These particles are trapped (adhered to a fiber) through a combination of these mechanisms:
Diffusion predominates in particles smaller than 0.1 μm in diameter. Interception and impact predominate in particles larger than 0.4 μm. For particles with an intermediate size, 0.3 μm is the most penetrating particle size; diffusion and interception are quite inefficient. HEPA filter specifications use the retention of these intermediate particles to define the type of filter.
Biomedical applications
HEPA filters prevent the spread of bacteria and viruses through the air and are therefore very important in preventing infections. HEPA filtration systems for medical purposes typically incorporate high-energy ultraviolet light to eliminate any live bacteria and viruses trapped by the physical filter. Some of the top-rated HEPA units have an efficiency of 99.995% which ensures a high level of protection against airborne diseases.
Use on airplanes
Modern airplanes use HEPA filters to reduce the spread of pathogens through recirculated air. A US Department of Transportation study of 92 random flights showed that levels of fungi and bacteria found in cabins are the same or lower than those in homes. These very low levels of microbes are due to the complete change of air inside the cabin carried out between 10 and 15 times per hour and the high filtering capacity of the air recirculation systems. These filters used in common airplanes are similar to the HEPA filters used in industrial clean rooms and critical hospital rooms such as the transplant and burn units. Compared to these systems, filtration systems used in normal buildings are not capable of removing bacteria and viruses from the air.[3].