The importance of air renewal and characteristics of absolute filtration

We are sure that air renewal is a first-order safety aspect in relation to the contagion of the coronavirus. However, some buildings do not have forced ventilation and natural ventilation through doors and windows may be poor. For those premises where the existing systems cannot guarantee an air renewal equivalent to at least IDA 2, or as an additional method for the reduction of airborne pathogens, there are specific systems for cleaning and sanitizing the air of the air in premises.

Air purification systems with ABSOLUTE filtration
It is an air filtration process that is considered the standard for air purification. Highly efficient, it traps particles and does not release them into the air, it is safe for all respiratory problems. There are autonomous types of equipment with this type of filtration, which have absolute HEPA13 / 14 or ULPA filters tested and with their corresponding individual certificate. This type of filter is used in the hospital sector, ICU, operating rooms, etc.


The Absolute high-efficiency HEPA “High Efficiency Particulate Air” and ULPA “Ultra Low Penetration Air” filters are composed of a randomly arranged mesh of fibres. Fibres are typically made up of fibreglass and with diameters between 0.5 and 2.0 µm. The most important factors to consider in a HEPA filter are the diameter of the fibres, the thickness of the filter and the velocity of the particles. The space between the fibres is much greater than 0.3 μm, but that does not mean that particles with a smaller diameter can pass. Unlike membrane filters, HEPA filters are designed to retain much smaller contaminants and particles.

These particles are trapped (attached to a fibre) through a combination of these mechanisms:

  1. Interception: Where the particles that follow an air flow rub against a fibre and adhere to it.
  1. Impact: Where large particles are not able to avoid the fibres while following the airflow and are forced to impact directly with one of them. This effect increases with decreasing fibre spacing and increasing airflow velocity.
  1. Diffusion: The smallest particles, especially those smaller than 0.1 µm, collide with the gas molecules, which prevents and delays their passage through the filter, therefore, in addition, the high gas filtration system also captures the Ultrafine particle (virus). This behaviour is similar to Brownian motion and increases the probability that a particle will be stopped by one of the two previous mechanisms. It is the most dominant when the airflow is slow.

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 (in English: Most Penetrating Particle Size), diffusion and interception are quite inefficient. The HEPA filter specifications use the retention of these intermediate particles to define the filter type.


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