HEPA filters and portable air cleaners/purifiers have been subject to a lot of talk due to the need to boost ventilation from COVID-19 pandemic times.

While HEPA filters and air cleaners/purifiers have their uses in specialist settings, standard filters can prove effective instead of HEPA filters; while portable air cleaners/purifiers should only be used in small spaces, where additional fresh air ventilation is impossible to provide.

Mechanical heat recovery ventilation units, however, are an effective instrument at reaching difficult to ventilate locations, i.e. where there are no openable windows, providing fresh outdoor air, in an energy efficient manner, without compromising safety in terms of indoor air quality.

Uses of HEPA filters and portable air cleaners/purifiers

There is an overall focus on indoor air quality, which aims to boost the supply of fresh outdoor air into a building, benefitting both the health of occupants and the productivity of the business.

In a study by Environmental Health Perspectives (EHP) in an office-like environment, it was found that performance diminished by 66 percent with a carbon dioxide (CO2) level of 1,000 parts per million (PPM) when compared with the same space at 600 PPM. In the same study, performance output reduced by more than 75 percent when CO2 levels reached 2,500 PPM, versus 600 PPM levels. Study results, therefore, show increased fresh air supply rates can productively and economically benefit workplaces.

Both HEPA filters – and standard air filters – and air cleaners/purifiers are tried and tested in indoor environments. They are protective devices, used to both improve indoor air quality and clean the air. They aim to protect indoor spaces from airborne pollutants for the benefit of people, products, processes and expensive ventilation systems, to reduce the risk of airborne pollutant spread and malfunction of ventilation systems.

It should be noted at the outset that portable air cleaners/purifiers do not provide any fresh air ventilation, they merely clean the re-circulated air that passes through the unit. Consequently, these units should not be used to boost ventilation levels as they are not a substitute for fresh air ventilation.


Filters are composed of a fine pleated fibre structure that is porous enough to allow air to pass through, but tight enough to block most airborne pollutants. That structure is also referred to as the filter medium/media. Depending on the specific filter class, the filter medium physically captures pollutants, such as, virus particles, dust, pollen, bacteria and mould spoors, depending on their size.

Filters are classed according to their efficiency and ability to block/capture the most penetrating particle size (MPPS) of airborne pollutants. Some of these pollutants can penetrate deep into the lungs and enter the bloodstream, bypassing the human body’s natural defences, due to their microscopic size.

The MPPS is the particle size that filters find most difficult to capture, including:

  • virus particles, i.e. COVID-19 or flu
  • dust
  • dirt
  • pollen
  • bacteria
  • mould spoors
  • soot

Air filters, such as, coarse filters are the least efficient, while in the high efficiency air filters range, ULPA filters are the most efficient.

Filters efficiency to block most penetrating particle size (MPPS)

Filter classifications

Air filtersCoarse19 filter types
ePM1010 filter types
ePM2.510 filter types
ePM110 filter types
High efficiency air filtersEPAE10

Filters protect people products processes and plant

Protect people, products, processes and plant

Housed within mechanical ventilation systems, filters are generally used for two purposes:

1. Protect people, products and processes

Traditionally, filters are used to clean fresh supply air – incoming air from outside – from airborne pollutants/contaminants entering a building. They also filter the air that is re-used through mechanical heat recovery ventilation (MHRV) or re-circulated via the re-circulation damper in an air handling unit (AHU).

Please note, in the AHU re-circulation process, re-circulated air is only filtered using the least efficient filters – coarse or PM10 – therefore, re-circulation dampers should be closed during COVID-19 times or periods when seasonal illness are more prevalent in the community.

Filters in a mechanical heat recovery ventilation (MHRV) unit

High efficiency air filters, such as, HEPA filters play an essential role in biomedical, pharmaceutical, manufacturing and food production settings in protecting, mainly products, from contamination. In other settings, such as hospitals and medical clinics, high efficiency air filters are used to protect the human organs from infection. They can also capture and filter harmful particles from leaving a building for health or regulatory reasons.

2. Protect plant (equipment)

Filters also protect the interior of mechanical ventilation equipment/systems from dust and dirt damage. This enables the mechanical ventilation plant/equipment to operate efficiently, as excessive dust or dirt could slow the system down and increase energy running costs.

Filters capture airborne particles

How do filters work?

As air passes through filters – housed within a mechanical ventilation system – the filter captures airborne particles within the filter medium and stops them from entering the indoor space, where ultimately the cleaned air is distributed. The volume of airborne pollutants captured in filters depend on the efficiency of the filter and the size of the pollutant or particle.
The filter also captures dust and dirt and stops them from entering the unit.

COVID-19 pandemic use

Further use of filters during the COVID-19 pandemic were in portable air cleaners/purifiers, which mainly employed HEPA filters. These units were used to clean the air that was circulating within a room. Their effectiveness for reducing the risk of COVID-19 virus spread for all persons present, however, is unproven due to data availability.

Portable air cleaners/purifiers do not introduce any fresh air and

“cannot ensure adequate indoor air quality, particularly where significant pollutant sources are present and ventilation is insufficient”.

US Environmental Protection Agency

HEPA filters located within portable air cleaners/purifiers will only filter air in the immediate vicinity of the unit. While a higher velocity fan speed could pull a greater volume of air through the unit, the noise associated with a higher fan speed could exceed acceptable workplace noise levels. As the units are most effective when located close to people, increasing noise levels could upset concentration levels of staff and workplace productivity.

Mechanical ventilation systems on the other hand, were used to deliver greater quantities of fresh cleaned air, in accordance with the ventilation rate to achieve the appropriate room air changes.

Sections of a typical air handling unit (AHU)

Location of filters

Filters are placed within mechanical ventilation systems, such as, air handling units (AHUs) or mechanical heat recovery ventilation (MHRV) units and clean the air that is passed through. They also protect the inside of the unit from dust and dirt to prolong the unit’s life and maintain its efficiency.

Generally, AHUs will have a two-stage filter setup. The first filter collects the larger particles, the second filter, the finer particles. Fresh outdoor air is pulled through these mechanical ventilation systems to supply clean air to a room. The first filter (or pre-filter) will usually be of the lowest efficiency, i.e. coarse or PM10 standard, prolonging the life and protecting the more expensive and efficient higher grade second filter.

The mechanical ventilation systems are usually located at ceiling level, either visible or within a false a ceiling.

HEPA filters within portable air cleaners/purifiers are located centrally within the unit – which operates as an all-in-one portable device.

Capturing COVID-19

The smallest airborne pollutants, such as COVID-19 virus particles, can be the most challenging for a filter to intercept due to their microscopic size. However, both the interception method a filter uses and the viruses’ adherence to a host cell makes it slightly less difficult for a filter to capture.

When virus particles spread – due to breathing or coughing – they are attached to microscopic water droplets or aerosols, made up of biological material – enlarging the virus particle.

Ways filters capture airborne particles

Four ways filters capture

Filters are designed and constructed to incept particles that travel through the air in several ways.

  1. Inertia – as the air is drawn through the pleated and randomly arranged filter medium, particles find it difficult to follow the air stream and inevitably collide and adhere to the filter.
  2. Interception – particles in the air flow collide and then adhere to the filter medium, physically blocking their passage.
  3. Diffusion – the smallest particles – such as virus particles – by their very nature are highly active and are constantly moving, leading to almost inevitable collision with the filter medium.
  4. Electrostatic – particles become stuck to the medium due to the electrostatic charge on the filter fibre.

Different filters capture different particle sizes

Filters are made to a range of efficiencies; therefore, only certain filters are efficient at capturing a large volume of pollutants as small as virus particles. The higher the filter efficiency, the less the MPPS will pass through the medium. With standard coarse filters – the least efficient – the smallest particles can evade capture, passing through the filter. The most efficient filter, the ultra low penetration air (ULPA) filter can capture up to 99.99995% of the smallest particles.

Relative size of particles captured in filters

Particles measured in microns

While we measure objects in our everyday life in terms of centimetres, metres and kilometres, when measuring virus particles and airborne contaminants, micrometres or microns (µm) are used.

The COVID-19 virus particle is roughly 0.125 microns (µm). One micron is one thousandth of a millimetre – 0.001 mm – and is invisible to the human eye. The COVID-19 virus particle is then enlarged due to its adherence to a host cell.

High efficiency air filters - EPA, HEPA and ULPA

High efficiency air filters

High efficiency air filters are a category of filter containing three different filter groups:

  • efficient particulate air (EPA)
  • high efficiency particulate air (HEPA)
  • ultra low penetration air (ULPA)

HEPA filters are grouped as H13 and H14 and remove at least 99.95% and 99.995% of the most penetrating particle size of contaminate – 0.3 µm. ULPA filters can remove particles as small as 0.12 µm, but require higher fan velocity due to the more dense filter medium, which enable it to capture smaller particles.

The table below shows the three most efficient filter groups – EPA, HEPA and ULPA – and their efficiencies against the MPPS.

Filter groupClassCollection efficiency
EPAE10≥ 85%
E11≥ 95%
E12≥ 99.5%
HEPAH13≥ 99.95%
H14≥ 99.995%
ULPAU15≥ 99.9995%
U16≥ 99.99995%
U17≥ 99.999995%

HEPA filters’ use during the COVID-19 pandemic

Due to HEPA filters’ high efficiency – capturing at least 99.95% of particle sizes of 0.3 µm – they were widely sought for air cleaners/purifiers during the COVID-19 pandemic. However, due to data availability, their effectiveness at reducing the risk of COVID-19 transmission is unproven.

History of HEPA filters

HEPA filters were originally used in masks during World War Ⅱ and then in nuclear testing – to filter airborne chemical and radioactive contaminants. They were later manufactured for commercial use in the 1950s.

Not required on outside air

In the context of the COVID-19 pandemic, it should be noted that HEPA class filters are not required on outside fresh air intakes into settings where they weren’t required before the pandemic. Standard air filters will suffice, as there isn’t a high risk of virus aerosols entering the mechanical ventilation system from outside the building.

Retro-fitting HEPA filters

If you wanted to retrofit HEPA filters to an existing mechanical ventilation system, caution is advised. Not all mechanical ventilation systems can support HEPA filters without alteration. This is because HEPA filters comprise of specific:

  • frames
  • gaskets – helps to create an air tight seal
  • media separators
  • sealant – prevents any bypass by binding the medium to the frame, thus helping create an air tight seal around the filter

Testing EPA, HEPA and ULPA filters

The ability and efficiency of high efficiency air filters – classed as E, H and U – are individually examined using a test aerosol and are carried out in accordance with the EN 1822-1 standard. This factory test is based on retention of the most penetrating particle size (MPPS) through the filter medium and it is carried before the filter departs the manufacturing factory.

Air filters particle capture efficiency

Air filters

Air filters are a category of filter containing four different class of filter – coarse, ePM10, ePM2.5 and ePM1 – efficient at collecting the following particle sizes:

  • coarse – greater than 10 µm
  • ePM10 – 0.3 to 10 µm
  • ePM2.5 – 0.3 to 2.5 µm
  • ePM1 – 0.3 to 1 µm

These air filters are based on efficiency particle mass (ePM) and are segmented into four classes – as shown in the table below.

Filter groupClassCollection efficiency
Air filterCoarse< 50% of PM10
ePM10≥ 50% of PM10
ePM2.5≥ 50% of PM2.5
ePM1≥ 50% of PM1

Protecting human health

Air filter testing centres around the smallest particles, as they are the most damaging to human health. This is because the human body doesn’t have the same protections for all particulate mass sizes.

For example, particles of 10 µm and smaller can lodge and enter, deep inside the lungs, coarse particles cannot. Generally, coarse particles of roughly 10 to 40 µm are visible to the human eye.

Particles of 2.5 µm and smaller can enter the blood system and according to the World Health Organisation:

“chronic exposure to particles contributes to the risk of developing cardiovascular and respiratory diseases, as well as of lung cancer.”

Air filter collection efficiency testing

The air filter must capture at least 50% of the particle size within the class of filter to achieve certification to the test standard. For example, if an air filter is tested at ePM2.5 does not collect 50 percent or more of particles of 0.3 to 2.5 µm, then it is rated to the lower ePM10 class.

ePM10, ePM2.5 and ePM1 class filters have 10 filter types within each class, each with a collection efficiency of between 50 to 95 percent, rounded to the nearest five percent.

Coarse filters contain 19 different filter types from 5 to 95 percent collection efficiency, rounded to the nearest five percent – as exhibited in the testing and collection efficiency image below.

Air filter testing and collection efficiency

Test standard – ISO 16890

The international test standard for air filters is ISO 16890, replacing the previously used filter groups G and F.

ISO 16890 tests a filters ability to capture particles across a range of different sizes, rather than just one size, as was the case historically – better representing real life situations.

Filters service and maintenance

Manufacturer’s recommendations

Filters should be inspected and replaced periodically, based on manufacturer’s recommendations, energy consumption (pressure drop) or as per the filters’ maximum operating time – to minimise the risk of microbial growth, such as, bacteria.

Dangers of not changing filters

Failure to change filters could lead to increased energy use, due to increased difficultly in air trying to pass through the filter medium or worse – leakages. A leakage could occur where air tries to force its way around the filter, bypassing the medium, leading to a potential escape of harmful particles.

Clean room testing – ISO 14644

In clean rooms and associated environments, i.e. hospitals, the periodic testing and inspection of filters is carried out on-site annually, in accordance with ISO 14644-1:2015. This is to confirm and certify their ongoing correct operation in critical environments.

Portable air cleaners/purifiers for and against use

Portable air cleaners/purifiers

Protect against pollutants

Air cleaning units are tried and tested, mainly in hospital, biomedical, pharmaceutical, manufacturing or food preparation environments to clean the air and protect products, such as, medicine, microchips and food from microscopic dust, dirt and harmful contaminates.

Clean air in small spaces

Air cleaners are “a short-term measure” and “less effective in large spaces”, according to the Irish government’s Expert Group on COVID. They are helpful when additional fresh air ventilation is not possible.

Less effective in large spaces

Portable air cleaners/purifiers, also known as COVID air purifiers or HEPA units are a cost effective short term measure to clean indoor air in small spaces. They are less effective in larger spaces, due to the limited airflow through units. For these units to be effective in capturing virus particles, they need to be physically located close to people.

Located close to people

HEPA filters are advised as efficient enough to capture COVID-19 virus particles, due to their ability to capture at least 99.95 and 99.995 percent of 0.3 µm. However, the filter can only clean the air that is drawn through the filter medium. Therefore, portable air cleaners or HEPA units are not deemed sufficient where the potential person omitting COVID-19 virus particles/aerosols is not in the immediate vicinity of the unit.

HEPA, True HEPA, HEPA-type, HEPA-like filters

True HEFA, HEPA-type and HEPA-like units

Be aware of subtleties in language around air cleaning units. “HEPA-type” or “HEPA-like” units, in all probability, do not contain HEPA filters. They will not operate to the same efficiency as a true HEPA filter. Units containing HEPA filters may also be known as “true HEPA”.

Air cleaners/purifiers in schools

These portable air cleaners can prove noisy due to the sound of ventilation fans within the units.

Depending on the area situated, their proximity to people and the activities within those areas, fan speeds will have to be restricted, particularly in schools and libraries.

Restricting fan speed because of noise could adversely affect performance and efficiency of the unit, negatively impacting on indoor air quality (IAQ).

Schools in Ireland have recommended indoor ambient noise levels for classrooms and general teaching spaces. Consequently, acceptable sound levels should not exceed 35 decibels (dB).

Air cleaners/purifiers will not remove exhaled carbon dioxide from air, therefore, classroom CO2 monitors will not show a decrease in carbon dioxide levels unless air purifiers are supported by fresh air or natural ventilation.

Thermodial’s approach

Installing/retrofitting high efficiency air filters, such as HEPA filters, into existing mechanical ventilation systems is not required. Once natural ventilation, such as openable windows are utilised to support mechanical ventilation during COVID times, ePM1 class filters will suffice. HEPA filters are for specialised environments, where advised by a ventilation specialist. Moreover, Irish government’s Expert Group on COVID state, “there is no direct evidence that use of high efficiency particulate filters can reduce the transmission of SARS-CoV-2”.

Likewise, the use of portable air cleaners with HEPA filters in non-specialist environments are ineffective and unnecessary, unless there is no other way of providing additional fresh outdoor air. These units are only a short-term solution and aren’t effective in large spaces, as they provide no fresh air ventilation.

If you do not have a mechanical ventilation system in your rooms or building and do not want the large capital outlay of installing, an air handling unit (AHU), Thermodial recommends installing mechanical heat recovery ventilation. Heat recovery ventilation, when ducted to the outside air, will not only supply fresh cleaned air into a room, but it will do so in an energy efficient manner – recovering the energy from the stale extracted (exhaust) air to heat or cool the incoming air.

It is also recommended that the method of heat recovery is through a plate heat exchanger. Unlike rotary or thermal wheel heat recovery systems, the supply and extract air will not mix, thus reducing the risk of possible re-introduction of airborne pollutants back into a building – very relevant in COVID-19 times.

Both air handling units and mechanical heat recovery ventilation can supply outside fresh air into a building to achieve the required minimum number of air changes per hour for the relevant building type.

Thermodial recommend a review your current ventilation setup, taking a risk averse approach to indoor air quality. Our engineers can inspect and remedy the following pressing issues that are negatively affecting indoor air quality during COVID times.

Talk to Thermodial about ventilation service, maintenance or inspection to support your businesses’ critical needs or explore our full capabilities in HVAC upgrades.

Special ventilation advice for COVID-19 and seasonal illness times

  • AHUs should have re-circulation dampers closed, to reduce the risk of possible re-introduction of airborne pollutants back into a building from the re-circulated stale extract (exhaust) air.
  • All mechanical ventilation systems should be serviced and maintained according to manufacturer’s recommendations and any remedial repair works should be prioritised and carried out without delay.
  • Mechanical heat recovery ventilation systems should utilise the bypass function – if available – to maximise air changes.
  • Mechanical heat recovery ventilation should have a purging sector fitted to systems that contain a rotary or thermal wheel. This virtually eliminates cross contamination between supply and extract air in the heat recovery process. Thereby reducing the risk of possible re-introduction of airborne pollutants back into a building when recovering the energy from the stale extract (exhaust) air.
  • General inspection of mechanical heat recovery ventilation systems to ensure they are operating efficiently and as intended.

Key guidance source materials

Camfil | Understanding your HEPA filter
Camfil | The international test standard ISO 16890 for air filters for general ventilation
Environmental Health Perspectives (EHP) | Is CO2 an indoor pollutant? Direct effects of low-to-moderate CO2 concentrations on human decision-making performance
Environmental Protection Agency (United States) | Air cleaners, HVAC filters, and coronavirus (COVID-19)
EN 1822:2019 | High efficiency air filters (EPA, HEPA and ULPA) – classification, performance testing and marking
Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA) | Filter class conversion between EN 779 and EN ISO 16890-1
Government of Ireland | Expert Group on the role of ventilation in reducing transmission of COVID-19: second report (02 March 2021)
ISO 14644:2015 | Cleanrooms and associated controlled environments – air cleanliness in terms of concentration of airborne particles
ISO 16890:2016 | Air filters for general ventilation – international test standard classification
ISO 29462:2013 | Field testing of general filtration devices and systems – measuring the performance of general ventilation devices in their end use installed configuration
ISO 29463:2017 | High efficiency filters and filter media – classification, performance testing and marking
United Kingdom Government | EMG: potential application of air cleaning devices and personal decontamination to manage transmission of COVID-19
World Health Organisation (WHO) | Ambient (outdoor) air pollution