There are several varieties of air filters in the market.

These filters are often classified based on the extent to which they can remove a particular size of solid particles present in air. When we say solid particles, it includes dust, pollen, bacteria, mold, etc.

For example, for a filter to be classified as a HEPA (High Efficiency Particulate Air) filter, it should remove 99.97% of particulates of size greater-than-or-equal-to 0.3 µm.

A ULPA (Ultra Low Particulate Air (filter) is a step further than HEPA. For a filter to be classified as ULPA it should remove 99.999% of particulates which are greater than or equal to 0.1 µm.

Who decides the standards of HEPA / ULPA filters

There are various agencies across the globe who do that.

In the case of the United States it is done by the American Society of Heating, Refrigerating, and Air Conditioning Experts (ASHRAE). These agencies provide guidelines for efficiency, testing and naming the filters.

Based on the MERV (Minimum Efficiency Reporting Value) ratings the filters are classified.

Here is a table with MERV ratings –

MERV ratings chart

Source: Mechanical Reps

What particles can a ULPA filter remove?

ULPA filter can remove a wide variety of particles from the air.

For example, it can completely remove – dust, bacteria, pigments, pollen, metallurgical dust/fumes, ground talc.

A ULPA filter can remove (to a large extent, not 100%) – oil smoke, tobacco smoke, rosin smoke, smog, insecticide dust.

It can also remove carbon black to some extent.

ULPA cannot remove viruses.

Here is a quick graphic to compare all of the above in terms of size on a single chart –

air containment size

Source: CDC

What is ULPA filter made of?

Both HEPA and ULPA filter media have similar designs.

The filter media is like an enormous web of randomly arranged fibres. When air passes through this dense web, the solid particles get attached to the fibres and thus eliminated from the air.

Porosity is one of the key consideration of these fibres. Lower porosity, while decreases the speed of filtration, increases the quality of filtered air. This parameter is measured in pores per linear inch.

How ULPA filter cleans the air?

Most people who are not aware of the air filtration process, believe that filtration works by physically blocking the material by a filter. Particles which are large do not get pass through and smaller one pass.

The above process called sieving is indeed used in air filtration but it is not the only process as it is not possible to use this method to remove smaller size particles.

The cleaning process is based on the particle size of the pollutant. Based on these, there are four –

  • Sieving
  • Diffusion
  • Inertial impaction
  • Interception

Here is a brief on all these 4 methods –

Sieving

This is simplest of all methods and literally involves blocking the large particles from entering. This is a great method for first level filtration to remove large particles from the air.

Interception

This phenomenon occurs when a particle moving along with the air comes very close to the filter fibre, to be precise, the distance between fibre and particle is less than equal to the radius of the particle. In such case, the particle touches the fibre and is removed from the airflow.

For Interception to occur the particle has to move very close to the filter fibre. If it is not, it will not be captured, but thanks to the dense mesh of the filter fibres, there is always some fibre which is close enough to the particle.

Inertial Impaction

This mechanism occurs when the particle moving with the air, literally hits the fibre as it is not able to change its direction and thus is trapped. A higher gas velocity with dense fibre web helps in this mechanism.

Diffusion

This phenomenon is slightly technical to understand. But in simple language, consider those very small particles do not move in a straight line but in zig-zag motions and keep colliding with other particles.

These zig zag moving particles then strike the fibres of the filter and captured. Slow air movement promotes diffusion.

There is an interesting part to ULPA filtration process. Research proves that removing particles below .1 micrometer nor removing particles between 0.1 and 0.4 micrometers is most difficult as none of the above 3 processes can do it efficiently.

The particles in the range – 0.1-0.4 micrometer are not small enough for diffusion nor are big enough for an interception. Thus, both these phenomena work for these particles, but less effectively. Thus making their removal a challenge.

Uses of ULPA filters

Today, ULPA filters are used widely wherever high level of air purity are required. Some of the most common places they are used are –

  • Biomedical/pharma facilities
  • Electronics / semiconductor manufacturing
  • Airline air filtration
  • Vacuum cleaners
  • Food processing
  • Hospitals

Both HEPA and ULPA have two major operating cost heads –

  • Filter
  • Energy consumption

The rate of air flow is generally inversely related to the air filter efficiency. For example, high rates diffusion process will not be optimal.

Making the filter more efficient can significantly decrease the energy cost associated with air filtration without compromising on the quality of air.

In most common cases a HEPA filter does a great job and leaving a few very pollutant sensitive applications HEPA filters can be used instead of ULPA.

ULPA filters are also expensive than HEPA and are known to have shorter lifespans than HEPA ones. This means higher operational costs.

For all home related applications, HEPA filters do an excellent job of cleaning the most common particles.

Team AirHealth.in
Author

This article is written by AirHealth's team which consists of writers with degrees in medicine, biotechnology, pharmacy, nutrition, engineering, etc. The work is peer-reviewed by the core team or by our Advisors before it is published. Check out more about us in "Our Team" section.

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