What are HEPA filters?

They are called HEPA filters for their acronym in English (High Efficiency Particulate Air) whose translation could come to mean something like "high efficiency particulate air".

HEPA filters need to meet a number of conditions to be considered as such. In other words, there is a standard that any filter must meet to be considered a HEPA filter, which means that it must follow specific procedures for its manufacture, pass a series of tests and have certain labels.

Within the HEPA classification there are subclasses, but there is a level that any filter must meet in order to have it, and that minimum establishes that it is capable of capturing at least 99.95% of the particles. If you do not reach that minimum, you will not be able to trade with such a rating.

With greater filtration capacity there are the ULPA filters generally used in rooms where a maximum level of cleanliness in air is required.

There is a big difference between a purifier with a normal filter and one with a much more advanced HEPA filter in terms of efficiency .

Therefore, when an air purification device or any other device that requires a filtration system comes on the market with a HEPA filter, it can be considered that it has passed the most exhaustive controls and that it offers the highest possible particle retention capacity. get.



TYPE FILTER

INTEGRAL VALUE LOCAL VALUE
EFFICIENCY % PENETRATION% EFFICIENCY % PENETRATION%
H13 ≥ 99.95 ≤ 0.05 ≥ 99.75 ≤ 0.25
H14 ≥ 99,995 ≤ 0.005 ≥ 99.975 ≤ 0.025
U15 ≥ 99.9995 ≤ 0.0005 ≥ 99.9975 ≤ 0.0025
U16 ≥ 99.99995 ≤ 0.00005 ≥ 99.99975 ≤ 0.00025
U17 ≥ 99.999995 ≤ 0.000005 ≥ 99.9999 ≤ 0.0001

How do HEPA filters work?

A HEPA filter is a mechanism whose mission is to stop the passage of airborne particles such as pollen, animal dander, mites, ...

Each type of particle has different dimensions and therefore, a specific measure can be used to stop some, but not to achieve it with others.

What a HEPA filter does is place different barrier methods to ensure that the particles that manage to exceed one level are trapped in the next.

A HEPA filter is made up of a mesh of randomly placed fibers . These fibers are usually layers of cellulose, synthetic fiber and / or glass fibers arranged in an accordion shape, which means that their surface area is larger and therefore they have a greater capacity to capture said particles.

The process is carried out gradually:

  1. The coarser particles are trapped by sieving in a first phase, which means that they literally “collide” with the fibers and cannot continue their advance.
  2. Obviously, not all the particles are stopped in this first barrier, but those that continue their trajectory driven by the air are being trapped in successive barriers. Those of medium size are then intercepted by rubbing with the following fibers, remaining adhered to them.
  3. Finally, those that have managed to circumvent the two previous systems, already very small in size and imperceptible to the human being, end up colliding with gas molecules in what is called diffusion.

Flow chart for finding solutions

HEPA filters in school classrooms

  • Outdoor activities are always preferable indoors, including breakfast.
  • If the activity is to be indoors, it is preferable in classrooms with the possibility of natural ventilation, especially cross ventilation (windows and doors on opposite sides).
  • If natural ventilation is not sufficient, sufficient ventilation can generally be achieved by using individual exhaust equipment or impellers with adequate air flow.
  • When centralized forced ventilation systems are available, the outdoor air rate should be increased and recirculation should be reduced.
  • When all of the above is not possible or is not sufficient, the air must be purified with equipment equipped with HEPA filters.
  • The final solution can be a combination of options, for example natural ventilation and purification can be combined.
  • To assess whether a given configuration is sufficient, one of the two methods described in this guide can be used, both based on CO2 measurements.
  • The use of masks, distance maintenance and hygiene measures are still necessary in all solutions.
Flow chart for finding solutions Source: Higher Council for Scientific Research (CSIC)

HEPA filter calculator for schools and large venues

The ventilation necessary to reduce the risk of contagion depends on the volume of the room, the number and age of the occupants, the activity carried out, the incidence of cases in the region and the risk to be assumed. The Harvard guide recommends 5-6 air changes per hour for classrooms of 100 m², with 25 students aged 5-8 years, and establishes this classification:

Renewals per Air hour

Enter the required information and the results will be calculated automatically.

Classroom or enclosure dimensions

Select how you want to enter the room size:


Current ventilation level

This is an approximation of the average rate of air change in many schools, based on research studies.

The value recommended by Harvard University is 5

Minimum CADR required

288.41 ft 3 / min
Cubic feet per minute CADR in cfm, for filters that bring only that unit.
490 m 3 / h
Clean Air Delivery Rate. Filters (individual or multiple, must add this together).