How HEPA Filters Perform Against Viruses of Different Sizes

First, How Large Are Viruses?

Viruses are among the smallest microorganisms in nature. Their sizes vary considerably by type, but most are in the range of 20–300 nm (nanometers), where 1 μm equals 1,000 nm. This is at or below the 0.3 μm (300 nm) reference particle size commonly associated with HEPA testing. Examples include:

Larger viruses: poxviruses at 200–300 nm and adenoviruses at 70–90 nm.

Medium-sized viruses: influenza viruses at 80–120 nm, SARS-CoV-2 at 80–120 nm, and respiratory syncytial virus at 120–300 nm.

Smaller viruses: rhinoviruses at 20–30 nm, which can cause the common cold, and polioviruses at 27–30 nm.

All of these viruses are smaller than or close to HEPA's 0.3 μm reference size. How, then, can HEPA filters capture them so effectively?

How HEPA Filters Capture Viruses: Dynamic Mechanisms, Not Just Pore Blocking

HEPA filtration does not work like a simple sieve that stops only contaminants larger than its openings. Four mechanisms act together, and the final three are especially important for very small particles below 0.3 μm.

Inertial impaction: This mechanism primarily affects larger particles above 1 μm. When viruses are carried on droplets of 1–10 μm or aerosols of 0.1–1 μm, their inertia can pull them away from the airflow path and into filter fibers, where they are captured.

Diffusion: This mechanism is important for extremely small particles below 0.1 μm, including rhinoviruses. Brownian motion causes these particles to move randomly and collide frequently with filter fibers, where they are adsorbed. The smaller the particle, the stronger its Brownian motion and the higher the probability of capture.

Interception: When particles in the approximate range of 0.1–1 μm, including influenza-virus-containing particles, follow the airflow close to a filter fiber, contact with the fiber surface causes them to be retained. Capture does not depend on the particle being larger than the spaces in the filter.

Electrostatic attraction: When HEPA media are made from permanently charged electret material, electrostatic forces attract charged or polar particles. Most viruses carry electrical charges through their protein shells, so this mechanism can further increase filtration efficiency.

The key point is that 0.3 μm is commonly treated as the most penetrating particle size (MPPS) for a HEPA filter. Particles around this size are influenced least by diffusion and inertial impaction, making them relatively difficult to capture. Viruses are generally smaller than 0.3 μm, so diffusion, interception, and—in charged media—electrostatic attraction can produce filtration performance that is at least as strong as the filter's rated efficiency for 0.3 μm particles.

Filtration Performance for Viruses of Different Sizes

Laboratory tests of HEPA filtration efficiency against viruses often use a virus-aerosol challenge, in which viruses are dispersed into the air as aerosols and the fraction retained by the filter is measured. Published research is commonly summarized as follows:

Virus type | Size range | Typical HEPA filtration efficiency | Main mechanism

Rhinovirus | 20–30 nm | Above 99.99% | Diffusion and electrostatic attraction

Influenza virus | 80–120 nm | Above 99.98% | Interception and electrostatic attraction

SARS-CoV-2 | 80–120 nm | Above 99.98% | Interception and electrostatic attraction

Adenovirus | 70–90 nm | Above 99.97% | Primarily interception

Poxvirus | 200–300 nm | Above 99.97% | Interception with some inertial impaction

These results show that very small viruses can be captured highly efficiently. A 20–30 nm rhinovirus, for example, undergoes intense diffusion, moving randomly and repeatedly into filter fibers. Even without electrostatic charge, HEPA media can therefore achieve very high capture efficiency for particles in this range. Viruses closer to 0.3 μm experience less diffusion but can still be captured efficiently through interception and other mechanisms.

In Real-World Conditions, Viruses Are Even Easier to Filter When Carried on Larger Particles

Airborne viruses rarely occur only as isolated individual particles. They are more often carried on respiratory droplets of 5–100 μm or aerosols of 0.01–10 μm. A sneeze, for example, can produce virus-containing droplet nuclei of 1–5 μm, while coughing can generate virus-containing aerosols of 0.1–1 μm.

These virus-and-carrier combinations can further improve HEPA filtration performance.

When a virus is attached to a droplet larger than 5 μm, inertial impaction and interception can capture it with efficiency approaching 100%.

When it is attached to an aerosol of 0.1–1 μm, HEPA performance is already high because diffusion and interception work together. The carrier particle effectively makes the virus still easier to capture.

This is why HEPA filters are a central defense against viral transmission in settings such as hospital negative-pressure rooms and biosafety laboratories. They not only capture free virus particles but also efficiently remove the droplets and aerosols that carry them.

Correcting a Common Misconception: A Filter Opening Larger Than a Virus Does Not Make HEPA Ineffective

The claim that “a HEPA filter's pores are larger than viruses, so viruses pass straight through” misunderstands how filtration works.

First, a HEPA filter does not have one fixed pore size such as 0.3 μm. Its fibers form an irregular three-dimensional network, and filtration depends heavily on interaction with the fiber surfaces rather than on a single absolute opening size.

Second, 0.3 μm is close to the low point in the filtration-efficiency curve, not a cutoff below which particles cannot be captured. Smaller particles, including very small viruses, can be more readily captured through diffusion.

Conclusion: HEPA Virus Filtration Is the Result of Deliberate Engineering

HEPA filters capture viruses of different sizes through the combined effects of their materials, such as ultrafine glass fibers or electret media; their structure, including a large pleated surface area; and filtration mechanisms such as diffusion, interception, and electrostatic attraction. Whether a virus is as small as 20 nm or as large as 300 nm, it is unlikely to pass through easily. In real-world environments, the droplets and aerosols that carry viruses further increase the probability of capture.

This also explains why public-health guidance in many countries recommends air purifiers equipped with HEPA filters for enclosed spaces such as elevators and meeting rooms. The basic rationale is that HEPA filtration creates a highly effective barrier against airborne viruses and the particles that carry them.