What Makes a HEPA Filter: Physics, Standards and Real-World Performance

A filter born in the nuclear age

The high-efficiency particulate air filter was developed in the 1940s, when the American nuclear programmes needed a way to keep radioactive particles out of the air that workers breathed and out of the air released from laboratories and plants. The early filters were known as absolute filters. The name HEPA came into general use later, and the US Department of Energy's history of air cleaning describes how the technology grew from these programmes into an industry [1].

The original idea still sets the US definition. The Department of Energy specification for HEPA filters requires a minimum efficiency of 99.97% for particles of 0.3 µm, and every filter is tested individually before use [2]. What began as protection against radioactive dust is now the core of cleanrooms, operating theatres, pharmaceutical plants, biosafety cabinets and room air cleaners.

Not a sieve: how fibres catch particles

It is natural to imagine a filter as a sieve with holes smaller than the particles it stops. A HEPA filter does not work like that. Its media is a mat of glass microfibres, many of them less than a micrometre thick, laid in random directions. The gaps between fibres are much larger than the particles the filter catches. Particles are caught because they touch a fibre and stick to it, held by van der Waals forces, and there are three main ways they come to touch it [3].

• Inertial impaction: large, heavy particles cannot follow the air as it bends around a fibre and run straight into it. This works best for particles larger than about 1 µm.
• Interception: a particle follows the airflow but passes close enough to a fibre that its edge touches it. This works best for mid-sized particles.
• Diffusion: very small particles are knocked about by air molecules (Brownian motion) and wander across the airflow until they hit a fibre. This works best for particles smaller than about 0.1 µm, and better still at low air velocity.

The hardest particle to stop

Each mechanism is strong at one end of the size range and weak at the other. Impaction and interception get stronger as particles get bigger, and diffusion gets stronger as they get smaller. Between them is a size where the filter is least efficient, called the most penetrating particle size (MPPS). For HEPA glass-fibre media it typically lies between about 0.1 and 0.3 µm, depending on the media and the air velocity [3].

This has a practical meaning that is often misunderstood. A filter rated at its weakest point is more efficient for everything else. A HEPA filter that stops 99.97% of 0.3 µm particles stops a higher share of both larger dust and much smaller particles, including the smallest virus-sized particles, which diffusion captures very effectively.

Two ways of rating a HEPA filter

There are two main rating systems, and they are not the same test. The US approach, used by the Department of Energy and in IEST recommended practice, tests at a fixed 0.3 µm [2]. The European standard EN 1822 and the international standard ISO 29463 test at the MPPS of each filter, scan every filter for local leaks, and sort filters into classes [5][6]. Because the MPPS is harder to catch than 0.3 µm, the numbers cannot be converted exactly. A specification that names an EN 1822 class should be answered with a test report to that class.

EN 1822 also defines E10, U16 and U17. ISO 29463 uses a parallel scheme in which ISO 35 H corresponds to H13 and ISO 45 H to H14.

Thinking in penetration, not efficiency

Efficiency figures close to 100% hide large differences. It is clearer to look at penetration, the share of particles that gets through. An H13 filter may let through up to 50 particles in every 100,000 at its MPPS; an H14 filter up to 5. Moving from H13 to H14 therefore means ten times fewer particles downstream, even though the efficiency figures differ only in the third decimal place.

The same view explains why the step costs something. Holding back more particles needs denser media or more of it, which usually means a higher pressure drop or a deeper filter, and more fan energy. That is why H13 remains the usual choice for ISO 7 and ISO 8 rooms, while H14 is chosen where the room class or the specification calls for it.

From a good filter to a good installation

A filter that passes its factory test can still fail in the ceiling. Air takes the easiest path, and the easiest path is often around the filter rather than through it: a gap in the gasket, a damaged corner, a frame that is not tight against the housing. Most leaks found during commissioning are at the seal and frame, not in the media. For that reason, ISO 14644-3 describes an in-place leak test in which an aerosol is released upstream and the whole downstream face and seal are scanned with a probe [4].

Three other things decide how well an installed HEPA filter performs over time. Good prefilters upstream keep the HEPA media from loading quickly. Airflow must stay within the filter's rated range, because efficiency for the smallest particles falls as velocity rises. And the filter must be handled with care: unpacked on site, inspected, never touched on the media, and installed straight away.

What the evidence shows in real rooms

Laboratory tests show what a filter can do; field studies show what it does in use. During the COVID-19 pandemic, a team at Addenbrooke's Hospital in Cambridge installed portable HEPA filtration units with UV sterilisation on a COVID-19 ward and an intensive care unit and sampled the air with the units switched off, on and off again. On the ward, SARS-CoV-2 was detected in the air in the weeks the units were off and was not detected in the week they were running, and other bacterial, fungal and viral bioaerosols also fell [7].

The authors were careful about the limits: it was a small study, and it measured virus in the air, not infections. That is a fair way to read most evidence about HEPA filtration. Removing particles from the air is well proven; how much that changes an outcome depends on the airflow, the room, and everything else that happens in it.

How to specify a HEPA filter

• Name the class and the standard, for example H14 to EN 1822-1, or 99.97% at 0.3 µm if your specification follows US practice.
• Ask for an individual test report for each filter, showing efficiency and the leak scan result.
• Give the airflow per filter and the maximum initial pressure drop you can accept.
• Choose the construction for the duty: mini-pleat panels for ceilings and FFUs, V-bank or box filters for high airflow, separator filters for rugged or hot service.
• Specify the frame material and the seal (gasket or gel) to match the housing or ceiling grid.
• Plan the in-place leak test with the installer before the filters arrive.

What a HEPA filter will not do

A HEPA filter removes particles. It does not remove gases: odours, solvent vapours, ammonia or acid gases pass through it, because molecules are far smaller than particles and do not stick to glass fibres. Where gases matter, a chemical filter with activated carbon or impregnated media is needed alongside the HEPA stage. And a HEPA filter cannot make up for a poor air pattern: clean air delivered to the wrong place, or swept past the work by a door or a heat source, protects nothing.