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Standardized filter testing using ultrafine particles

Respirators and masks are tested around the world to define how well they protect us against particles that can carry viruses. Perhaps this is the time to think about filter testing and the importance of proper filtration in general.

Many of us know how aerosol particles are generated and that they influence our climate and health. But how well known are the efficiencies of common filters against submicron and ultrafine aerosol particles?

Particles of different sizes are removed due to different phenomena: inertia, interception, diffusion and electrical mobility. Summing up their theoretical influence on filtration gives you a curve with a minimum efficiency for about  0.1 – 0.2 micron particles. For (good) air filters the most penetrating particle size is 0.1 – 0.2 microns, and particles larger and smaller are theoretically easier to remove.

In urban air the most abundant aerosol particles are ultrafine. In many industrial indoor environments ultrafine particles are generated in high numbers. Cooking and cleaning indoors may generate ultrafine particles in our homes.
And in all the above high concentrations of even single digit nano aerosol particles are emitted.

Looking at filters used around us:

  • Filters in the ventilation systems of buildings are often not very efficient in filtrating ultrafine particles. They are designed and tested based on their ability to reduce the number of fine particles. In good certification laboratories they might be tested using particles down to 300 nm. They are often called fine filters, i.e. they are meant to protect against fine, not ultrafine particles.
  • Coarse filters are intended to remove large dust that would quickly block fine filters. They are often used for example as pre-filters in ventilation systems and tested based on their dust (mass) collecting capacity.
  • HEPA and ULPA filters are designed to catch also ultrafine particles. They are tested by first defining the most penetrating particle size (mpps) and then testing for local and overall filtration efficiency for that particular size (efficiencies from 95% to 99,999 995%).
    Still sometimes optical counters that only go down to about 100 nm are used to test HEPA filters. Use of HEPA filters in normal air handling units is not feasible due to their high pressure drop.
  • Generally particles larger than 1 micron are reasonably easy to filter with proper filters.

To improve the filters used and to ensure proper testing, an international group of specialists have written a standard and a technical specification on how to use 3 nm – 500 nm particles. This testing is based on particle number and well-defined challenge aerosol:
ISO/TS 21083-2:2019 Test method to measure the efficiency of air filtration media against spherical nanomaterials — Part 2: Particle size range from 3 nm to 30 nm
ISO 21083-1:2018 Test method to measure the efficiency of air filtration media against spherical nanomaterials — Part 1: Particle size range from 20 to 500 nm

When selecting an air filter—or a mask—it is important to know how the efficiency of the filter has been verified. The filter also needs to be used correctly: any bypass will reduce efficiency, and a filter should be used according to its design values. For example air flying through the filter media faster than designed will dramatically reduce filtration of ultrafine particles.

To learn more about testing of air filters please see e.g.

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