Can HEPA filters remove viruses?

Whether it be a facility manager, procurement personnel, or EHS team leader, when someone inquires about HEPA filters with respect to viruses, they want to know the answer—and not some vague marketing phrases. HEPA filtration is frequently mentioned in clean rooms, Hospitals, laboratories, and companies in the PharmaceutICal industry. But "whether HEPA filters are capable of eliminating viruses disseminated through the air" is a more exacting question than "how HEPA filters remove the partICulates." So let’s rather explain what HEPA filtration can do with the aerosols containing bacteria and how efficiency is tested than go into the engineering issues that usually determine whether a system is working as intended.

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Yes—But Capturing Viruses Is Not the Same as Eliminating Risk

To summarize it shortly: Certainly, HEPA filters can filter air particles which contain viruses. However, it doesn’t mean that simply using HEPA filter is enough to ensure safety inside the room, purify the air, and eliminate the chances for infections.

It's very important to understand the difference among capturing the culprit particles, eliminating microorganisms, reducing exposure, and preventing infection before choosing the equipment.

HEPA Captures Airborne Particles That Can Carry Viruses

HEPA filtration works by capturing the tiny particles in the air. This includes the tiny droplets and aerosols that viruses often travel on, thus ensuring that they do not reACH their intended host.

The size of viruses is far less than 0.3 μm, the size required for a HEPA filter to pass the tests for its efficiency. However, viruses do not travel alone but rather stick to a larger respiratory droplet or a larger particle in the air such as aerosolized saliva.

The focus of HEPA filters is on airborne transmission, which is not touching or surface infection.

Capture, Inactivation, and Infection Prevention Are Different Outcomes

It helps to separate four layers of what a system might ACHieve:

• Capture: The particle lands on or sticks to the filter fiber.
• Inactivation: The virus loses infectivity while on the filter or after capture.
• Exposure reduction: Fewer virus-laden particles remain suspended in the room air.
• Infection risk reduction: Fewer inhaled infectious doses reach susceptible people.

The main purpose of a HEPA filter is to filter out contaminants. However, a HEPA filter alone does not eliminate viruses. The translation of virus capture into lowered infection risk is dependent on factors such as speed of air flow, number of people in the room, effectiveness of the virus source, duration of use, and other strategies in place.

Never assume particle removal rate is equal to lowering the infection rate.

Performance Depends on the Installed System

A filter's rated efficiency is measured in a laboratory test rig. Real-world performance depends on:

• Actual airflow through the filter at design conditions
• Seal integrity around the frame and housing
• Air distribution patterns in the room
• How many hours per day the system runs
• Whether bypass paths exist around the filter
• Complementary measures like outside air ventilation and local exhaust

This article walks through each of those factors.

How HEPA Filters Capture Virus-Containing Aerosols

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How are viruses—which are less than 0.3 micrometers in size—being caught by a filter which is rated at such an extent? The answer lies in the mechanICs of aerosol physICs and fiber filtration, and not in something as simple as a sieve.

Just because a virus is small doesn’t mean that it is not being carried by other particles, which is where this misunderstanding comes from.

Three particle sizes are confused:

1. Substantially small virus particles: Viruses, like those which cause respiratory illnesses, are typically in the range of approximately 0.02 μm to 0.3 μm.
2. The virus-laden droplet: When someone coughs or sneezes, they send droplets of size as large as 100 μm into the air.
3. The evaporated droplet droplet: Once the droplets dry, their size shrinks to about 1 μm, where they stay suspended for hours.

It is the droplet nuclei that are smaller people are concerned about when we talk about airborne transmission.

These modes of filtration all operate by the same principle of capturing particles through their size and the flow of air.

Viruses are hardly ever circulating freely in the air.

Diffusion, Interception, and Inertial Impaction

Three main modes of capturing particles can be differentiated in HEPA filter:

• DiFFUsion: The smallest particles (less than 0.1 μm) diffuse through air due to Brownian motion, increasing the possibility of collision.
• Interception: In this case, the particle is close enough to the fiber to allow the particle to reach the filter fibers and get trapped.
• Inertial impaction: If the particle is sufficiently large, it cannot bend around the fiber when there is a change in airflow, and thus collides with the fibers.

Why HEPA Is Not a Simple Sieve

Many people mistakenly believe that HEPA operates as a mesh screen with holes sized 0.3 μm. However, this is incorrect.

Unlike mesh, HEPA media is made up of a thick mat of fibers which are arranged randomly. Air passes through this dense layer of fiber in a haphazard manner.

Particles become trapped when they collide with the fibers and not through the pores. This means that there is no automatic assurance that particles smaller than 0.3 μm will simply flow through the filter, and in fact, the efficiency of the filter sometimes gets better for very small particles.

The 0.3 μm figure does not indicate a size of the actual openings but is just a reference value.

What HEPA Efficiency Ratings Actually Demonstrate

A sticker that reads "H14" or "99.995%" appears to be conclusive. However, what has been tested, under which conditions, and is it indicative of effective working?

MPPS and the Limits of the 0.3 μm Reference

Most classifications of HEPA filters by EN 1822 and ISO 29463 are compared to the Most Penetrating Particle Size (MPPS) of the filter, and not a fixed 0.3 μm.

The MPPS is dependent on:

• Fiber diameter and composition of media
• The rate of flow through the filter
• The nature of the aerosol used in the test
• The condition of the filter before the test

The standard "99.97% at 0.3 μm" parameter is derived from obsolete specifications in the USA and DOP smoke tests. It cannot be accepted as a universal specification, a cutoff value, or an MPPS for a type of HEPA filter.

While assessing the specifications, one has to consider the testing method applied and the particle size used in determining efficiency.

Filter Media, Finished Filters, and Installed Systems

Performance has three different levels which should not be mixed up:

• Media sample efficiency - Testing of the material in a flat sheet rig.
• Finished filter efficiency - Testing of the pleated filter in a housing including the material used for packing and sealing the frame.
• Installed system efficiency - Evaluating real room performance taking into account bypass, air circulation, and time of operation.

Media that satisfies grade specifications does not automatically mean that the fraMed Device is free of leakage. A completed filter that passes factory tests does not mean that the installed housing has no leaks.

The same way the efficiency of one pass does not show how fast concentration in the room drops during continuous recirculation.

Particle Tests Versus Virus-Specific Tests

There are three categories of efficiency and they are distinct:

• Media efficiency: Testing on flat sheets.
• Filter efficiency: Tests with fully-assembled filters in place.
• Installed system efficiency: Efficiency of the actual system after bypass, distribution, and residence time corrections.

Media that satisfies the efficiency requirements does not imply that a framed product will not leak. A filter passing factory tests does not mean that the product is installed without any leakage.

Also, single-pass efficiency gives a good indication of what is the percentage of particles removed in a single pass, but it does not say anything about the speed of the concentration decrease in the room during recirculation.

Define the Application Before Selecting a HEPA System

Before comparing filter grades, decide what the system is supposed to control and where.

Set the Control Objective and System Boundary

Different targets require different configurations for high-efficiency particulate air (HEPA) filters and viruses:

• Supply air protection: It filters outdoor air or Return Air before it enters the cleanroom or controlled environment.
• Local source control: Capture of pollutants right at the source (e.g., fume hood discharge, biosafety cabinet).
• Room air recirculation: Continuous filtering of air in a room in order to achieve disruptive effect over the volume of time through reduction of airborne pollutants.
• Exhaust treatment: Protecting the environment from spoiled exhaust.

A system intended for fulfilling one purpose may not help in accomplishing the other. HEPA filters and viruses require installation of biosafety containment in case of high risk biological use since biosafety containment requirement, exhaust sterilization and constant pressure system connectiveness prevails over recirculating use of HEPA system.

Recognize Central HVAC, Local Filtration, and Portable Solutions

Three approaches vary in terms of efficiency, airflow process, and installation challenges:

• Central HVAC with HEPA filtration: Intended for use in ductwork and air handling units, the technology is responsible for filtering air throughout a whole space as long as fan power is sufficient, and plenum is available
• Localized HEPA filtration: It consists of HEPA filtration devices such as HEPA filters or ceiling-mounted units installed exactly at the points where the cleanest air is required. It is useful in clean rooms and has minimal capital investment; however, it only works in the area directly investigated
• Stand-alone HEPA filtration: It is a portable recycling system that facilitates visibility and emergency protection options but is not a substitute for thorough ventilation system; thus, HEPA filters should not be considered as substitutes for full ventilation systems.

Portable systems recycle the air from the same room, without taking in the fresh air from outside. If fresh air is required for dilution of CO2, odors, etc., it should be provided through a separate system.

CADR, ACH, and Equivalent Clean Air Use

A number of measures are often given but not understood well:

• CADR (Clean Air Delivery Rate): the amount of clean air achieved in terms of filtered air flow and filter effectiveness
• ACH (air changes per Minute): the number of changes made by incoming air (supply or outside) throughout the measured square footage
• Equivalent ventilation: the amount of filter-based fresh air flow

To establish this, you can use the actual flow through the filter, its efficiency with respect to the size of the particle, and the volume of the room.

These measures reveal the actual potential of treatment but do not ensure that infection risk is at the required level or any level in particular.

Engineer for Airflow, Pressure Drop, and Installation Integrity

If an HEPA filter is tested properly, it will fail to perform well if the accompanying components fail to provide the necessary airflow, or if some air manages to dodge the filter altogether. Let’s take a closer look at how to make sure the fan setup can do its job correctly.

HEPA filters are typically used in low resistance applications. However, later on, resistance levels will increase accordingly.

Before moving to a more expensive HEPA filter type, it is necessary to check the:

• Initial pressure level at the intended airflow
• Final pressure levels that the manufacturer has provided
• Resistance of the fan
• The point of resistance added by a HEPA air filter has after it is installed.

If the fan cannot handle the modifications, your airflow may decrease. A filter which delivers 50% of its designated flow rate may protect worse than a cheaper filter which is performing its functions correctly.

It is never safe to assume that changing your filter will not affect the flow rate at all.

Air naturally tends to flow along the path which offers the least constraints. This doctrine applies to air that could make it through the space surrounding the filter frame, or behind the panels of any frame that surrounds it.

Pay attention to the following gaps when inspecting for air leaks:

• How well the frame of the filter suits the filter and how tightly it is being held
• Connections between housing panels
• Area around the Doors in the housing
• Connections in the ducts
• Any leakages after the clean zone.

This is not the same as particles actually penetrating the HEPA filter media. This means that there are sections of the filter which did not perform any filtering process.

It is necessary to rely on specific values provided by filter manufacturers and casing design, as universal values may not be accurate.

Testing and commissioning have to be done correctly as well.

The points that have to be taken into account are:

• Actual supply and Return Airflow measurements.
• How well the air is reaching the various areas of the room.
• How fast the air moves in and out of the various sections of the room and the way it moves.

When passing through the filter, the air that is left will need to be recorded as well as the state of Doors and Windows when conducting the tests.

Common Misconceptions About HEPA Virus Removal

Several widely repeated claims about HEPA and viruses do not hold up under technical scrutiny.

Many of the often-quoted statements regarding HEPA and viruses turn out to be inaccurate from a technical viewpoint.

"Anything Smaller Than 0.3 µm Will Pass Through"

This is wrong. In section 2, it will be shown that diFFUsion is able to capture very small particles. MPPS is not equal to 0.3 µm for all filters, and particles smaller than MPPS are captured with even higher efficiency instead of being captured with lower efficiency.

"A HEPA Rating Means the Air Is Decontaminated"

HEPA captures particles, but it does not deactivate the virus on the filter itself. "Deactivation" means killing or neutralizing pathogens. Therefore, HEPA is simply a physics separation process that does not have a biocidal effect.

"A Higher Efficiency Always Means Better Protection"

Higher efficiency means that there is a higher rate of removal per passage of an air filter, but it also means that there will be a higher air pressure drop and higher energy consumption by fans and a low air flow rate if it was not designed for it.

"A Laboratory Test Result Will Be the Same in an Office"

Tests in laboratory chambers are done under controlled conditions, which means that the flow rate, test occasion, and aerosol types used in the experiment were established beforehand. Moreover, the real office and the laboratory have completely different characteristics. Therefore, although laboratory tests give an idea of the system performance, they cannot be substituted for field measurements.

Frequently Asked Questions

Can HEPA filters capture airborne viruses and also virus-containing aerosol particles?

HEPA filters are capable of capturing airborne particles and droplets that are carried by the infections of viruses. However, capturing is not synonymous with inactivating the viruses, and which means filtering should work in collaboration with good air flow, sealing and time spent filtering.

How do HEPA filters capture particles smaller than 0.3 μm?

Very small particles travel randomly, even colliding with the fibers of the HEPA filter, and the techniques of Brownian diffusion are used. The most penetrating particle size is not measured to be 0.3 microns for every type of filters, and the particles smaller than the size are usually captured more efficiently than those.

How efficient is a HEPA filter against a specific virus?

The efficiency of a HEPA filter against a certain virus can be different depending on the type of the virus-loaded aerosol and other factors like a type of the HEPA filter, airflow speed and quality of installation. The ratings established by the EN 1822 or ISO 29463 standard depend on finding the most effective particle size, but do not involve the analysis of infectious efficiency.

Does a HEPA filter kill the viruses that it catches?

No. A standard HEPA filter captures the particles physically due to a mesh of fibers used in its structure, without employing heat or any chemical substances that would destroy the virus.

Will using a more powerful HEPA filter lead to a better result?

No. The higher the HEPA filter is classified the more pressure drop it creates, so the airflow can be decreased because the fan will have problems to guarantee the airflow needed. Thus any upgrade should be supported by usage of appropriate parameters.

Can a standard HVAC system be retrofitted to work with HEPA filter?

It depends on availability of free space, fan strength, load-bearing capacity of construction walls, size of air duct and type of sealing method used. Thus a direct HEPA filter change will not give a desirable results.

What kind of proof do clients have to get in order to understand if the HEPA filter does indeed remove virus particles?

Clients have an opportunity to ask the biomass measurement system testing company to collect accredited documents that would state the method of the experiment, type of particle, result of analysis and the measure of the total candlepower.

What is the best way to maintain and change old filters?

The consumers will have to follow the indications of makers of the filters with respect to types and specifications of a certain HEPA filter.