Introduction to Air Filtration
Pollutants in the air can generally be divided into two categories — gasses/vapours and particles. Particles include dust, pollen, dander, smoke, soot, mould spores, viruses, bacteria and respiratory particles. Particles can be removed by using a fan to pull air through a filter which collects the particles and allows the particle-free clean air to pass through.
Air filtration can happen in one of two places. Most buildings have a heating, ventilation and air conditioning (HVAC) system. This is also called mechanical ventilation. The units that supply air are called air handling units and in addition to supplying outdoor air and heating or cooling the air, they also have a filter. The air that is being supplied should have some outdoor air mixed in, should be heated or cooled to be comfortable and should be filtered.
The other way to filter the air is through a filtration unit in the space. It is a box with a fan and a filter inside that pulls in air from the room, passes it through a filter and supplies filtered air out. These units can be mounted on the walls or ceiling, or they can be portable and left on the floor.
A filter has two important properties:
1. Efficiency — How well the filter can remove particles from the air (that’s what the “E” stands for in HEPA and MERV). A more efficient filter can remove more particles while a less efficient filter will let more pass through.
2. Pressure drop — How much the filter restricts the airflow. A high pressure drop means there is a lot of restriction in the airflow and you need more powerful fans to ensure proper airflow.
HEPA Filters
HEPA stands for High Efficiency Particulate Air filter. These are the most efficient filters. Different agencies have different naming and ratings for high efficiency filters:
Most HEPA filters are H13 which removes 99.95% of particles. True HEPA or Medical Grade HEPA are just marketing names for H13. Sometimes you might see H14 advertised which removes 99.995% of particles. In any normal setting, going from 99.95% to 99.995% will have a negligeable impact on the clean air delivery rate, so it isn’t worth it — stick with H13.
HEPA filters have very high pressure drops so they cannot be used in most commercial air handling units. They would restrict the flow too much. However, there are some specially designed air handling units in hospital settings that are made to use HEPA filters.
In-room filtration, like portable air cleaners usually use HEPA filters with fans that are designed to handle the pressure drop. There is a lot of false marketing about HEPA filters, so make sure you buy one with a proper rated HEPA filter (H13). Here are some examples of false marketing:
HEPA-like, HEPA-style, 99% HEPA, HEPAsilent, HEPA Ultra, Ionic HEPA, HEPAFast, HEPA Efficiency, HEPA Functions, HEPA Action, Plasma HEPA, Super HEPA, HyperHEPA
You can get more information from: https://cleanaircrew.org/air-cleaners/
MERV Filters
MERV Definition
MERV stands for Minimum Efficiency Reporting Value. It goes from MERV 1 which is very poor to MERV 16 which is very efficient. What constitutes different ratings is based on the filter efficiency for different particle sizes. This is defined in ASHRAE standard 52.2. If a filter removes less than 20% of particles between 3.0–10.0 µm, then it can only be rated between MERV 1–4. A test is run to see how much dust gets removed by weight.
If the filter can remove more than 20% of particles between 3.0–10.0 µm, it will be rated MERV 5–16 based on its efficiency against different particle sizes.
Fine particulate matter, also known as PM2.5, are particles that are < 2.5 µm and are particularly hazardous to people. As you can see from the table, filters MERV 7 and lower are not designed to stop any PM2.5 (ranges 1 and 2). MERV 10 and lower are not designed to stop anything < 1 µm (range 1).
There are websites that state that MERV-17 = HEPA, but that is incorrect. MERV only goes up to 16.
There are other ratings (like MPR) that different companies have developed, but I would just view this as a marketing gimmick and convert the values to MERV. For example, MPR 1900 = MERV-13. There might be slight differences, but it would not make a significant practical difference.
Infectious Aerosols
ASHRAE 241, the standard for control of infectious aerosols provides a table indicating the efficiency of different MERV filters for infectious aerosols.
Standard HVAC applications use MERV-7 to MERV-10 which are not very effective against respiratory particles that can contain airborne pathogens. These filters can be upgraded to MERV 11 to 16 which are more effective. As you get to higher MERV, you are likely to have a higher pressure drop with more flow restriction and the filters will cost more. A sweet spot between efficiency and cost/pressure drop is MERV-13.
As you go to higher MERV than 13, the cost and pressure drop increase without significant increase in filtration efficiency. As you reduce efficiency below MERV 13, there is a meaningful increased risk of infection. Most air handling units can be upgraded to MERV-13 as the pressure drop is not that much higher than lower rated filters, but cheaper air handling units might not be able to handle them. It’s best to confirm with a HVAC professional.
MERV-A
In ASHRAE 52.2, appendix J provides an optional method of conditioning a filter “to demonstrate efficiency loss that might be realized in field applications”. To explain, high rated MERV filters do not use only a mechanical mechanism to capture particles, but also use electrostatic properties to capture fine particles. As time goes on and particles get captured, they can reduce the electrostatic properties, so the efficiency of these filters against fine particles does not necessarily remain over their lifetime. To ensure filters remain effective, appendix J provides a test procedure to condition the filters before measuring their effectiveness. After using this test the filter will be rated MERV-A (e.g. MERV-A 13).
In ASHRAE 241, before Jan 1, 2025, normal MERV filters can be used to comply with the standard. Starting Jan 1, 2025, the standard requires MERV-A filters. The reason for the delay is that currently, MERV-A filters are not commonly found, so ASHRAE has provided the industry time to properly rate their filters in accordance with appendix J. There has been criticism of this decision.
Use in Portable Air Cleaners
Although most portable air cleaners use HEPA filters, the Corsi-Rosenthal box uses MERV-13 filters. These are DIY portable air cleaners and the cheapest way to clean the air. MERV-13 filters are less efficient than HEPA filters, but ultimately you care about the clean air delivery rate = air flow x efficiency. Classic CR boxes use box fans which have a much higher airflow than most commercial HEPA filters, so the clean air delivery rate is still higher, even after accounting for the reduced efficiency. They are also easy to assemble and can be quickly deployed on a large scale. Newer style CR boxes use PC fans which are super quiet and also have very high air flows when multiple fans are used in parallel. They are more expensive and are currently more difficult to build.
Here’s a summary of how the different types of filters are used in different situations:
Advantages and Disadvantages of Filtration
In most buildings, ventilation is the primary tool to clean indoor air. Although filtration is not as commonly used, it has certain advantages over ventilation.
Comparing to Ventilation
Ventilation removes all indoor pollutants, but replaces them with outdoor pollutants, although usually at much lower concentrations. Filtration only removes particles from the air and does not have any possible adverse effects on air quality. Filtration is much less energy intensive than ventilation, as ventilation requires heating, cooling, humidifying or dehumidifying the outdoor air. Adding portable HEPA filters or improving filtration in air handling units does not require any structural changes to the building, while providing upgraded ventilation might require significant changes.
Filtration has some disadvantages as well. It cannot remove gases or vapours and is only effective against particulate matter. Unless well sealed, many filters have air bypassing them which significantly reduces their efficiency. When used in an air handling unit, filters can get loaded with dust and restrict airflow, leading to detrimental effects on ventilation. This is a more significant problem with high efficiency filters.
Issues with Portable HEPA Filters
For portable HEPA filters, the most significant issues are noise, air distribution and human operation. Filters are often sized without considering noise level, causing them to be run on lower speeds and be undersized for the space. Noise should be considered when selecting HEPA filters with the best option often being running multiple oversized HEPA filters on lower speeds. Recommendations for portable air filters with noise considerations can be found here: https://cleanairstars.com/
Providing clean air to the occupants of the room requires the clean air to be properly distributed throughout the room. Air speeds from HEPA filters are generally insufficient to allow the supplied air to mix well with the room. This can be mitigated by using fans to mix the air or spacing out multiple portable HEPA filters throughout the room. ASHRAE standard 62.1 states that if air is supplied and returns on the same side of the room, you should only assume that the air distribution is half as effective as if it were properly distributed throughout the space with diffusers (See ASHRAE 62.1–2022 Table 6–4 and equation 6–2). This would apply to portable air cleaners as well.
Portable air cleaners require occupants to operate them. If the person doesn’t care or the noise is bothering them, they are likely to turn it off and it becomes ineffective. For convenience, often they are placed in the corner and away from the people which can exacerbate air distribution issues. To ensure portable air cleaners are used properly, there should be follow up with the occupants of the space and adjustments of equipment or strategies if the portable air cleaners are not being used properly, but this is rarely done. It is for these reasons that while portable air cleaners can be effective, they are not an ideal long-term solution for indoor air quality improvement. A better alternative for in-room filtration is to mount air cleaners on the walls or ceiling, design it to have the proper clean air delivery rate and noise levels, remove occupant control and ensure the air is properly distributed.
