Why combine HEPA filtration with catalytic filters like DNO?

Key points

• HEPA filtration is designed to capture airborne particles (including fine particulate matter and many bioaerosols) by physically trapping them in filter media.
• Catalytic filters are designed to chemically transform certain pollutants at a surface, rather than only capturing them.
• Combining the two targets a broader mix of indoor pollutants: particles + some gases/VOCs + biological material.
• A multi-stage approach can also reduce concerns about secondary release from filters by adding an inactivation/breakdown step at or near captured material (depending on design and evidence).
• The meaningful question is not “how many stages,” but which pollutants each stage addresses, how performance is tested, and what happens over time.

Indoor air is a mixture problem. In most buildings, you do not have only one pollutant type: you have particles from outdoors and indoor activity, gases and VOCs from materials and products, and bioaerosols from occupants and damp surfaces.

That mix is why many air cleaning systems are multi-stage, commonly combining high-efficiency particle filtration with a gas/chemistry stage. This article explains what each stage does, why they are often paired, and how to interpret claims about “HEPA + catalyst” systems.

What does HEPA filtration do well?

A HEPA filter is a high-efficiency particle filter. In simple terms, it removes particles because air is forced through a dense fibre structure and particles collide with fibres and stick.

HEPA-class filtration is well suited to:

• PM₂.₅ and PM₁₀ control
• smoke and dust
• droplets and many bioaerosols carried on particles

This is one reason HEPA-based devices often show clear reductions in measured particle concentrations when they run continuously in a space.

What HEPA does not do by itself:

• It is not a strong solution for many gases and VOCs (because these are molecules, not particles).
• It does not “destroy” what it captures; it stores captured material in the filter until the filter is replaced.

That last point is not a criticism—filtration is fundamental—but it helps explain why other stages are sometimes added.

Why add a catalytic stage?

Catalytic filters are intended to do something different from “capture”: they aim to support chemical reactions at a surface so that certain pollutants are broken down into more stable end products.

In Healthy Air Technology’s terminology, D-orbital nano oxide (DNO) refers to a catalytic approach designed for surface-confined oxidation. The idea is that pollutants are held at/near a catalyst surface and oxygen activation and oxidation happen primarily at that surface, rather than creating reactive chemistry throughout the occupied air volume. (For a more in-depth explanation, see How do DNO catalysts work in air purification?).

A catalytic stage is typically discussed for:

• some VOCs and odorous compounds that are poorly addressed by particle filters alone
• certain reactive gases (depending on formulation and conditions)
• contributing to inactivation/breakdown of biological material at or near where it has been captured (depending on design and test evidence)

What does “HEPA + catalyst” mean in a real system?

In most multi-stage designs, the roles look like this:

Stage 1: particle capture (HEPA-class filtration)

• Removes airborne particles from circulation
• Reduces PM levels in the room air
• Captures particle-associated bioaerosols

Stage 2: surface chemistry (catalytic layer)

• Provides a surface environment for chemical breakdown of certain pollutants
• May contribute to inactivation/breakdown processes near captured material (where evidence supports it)
• Aims to keep reactive chemistry at a surface rather than in the bulk air

This division of labour is useful because it matches how pollutants behave: particles are best handled by filtration; gas-phase pollutants require adsorption and/or chemistry.

How does this relate to “secondary air pollution”?

One reason people care about catalytic approaches is the risk of unintended by-products in some oxidation-based technologies. In the Knowledge Hub, this is framed as secondary air pollution: pollutants that are created or re-released as a side effect of air cleaning or disinfection.

A combined HEPA + catalytic system can be evaluated with two complementary questions:

1. Does it reduce the pollutants you care about (particles, certain gases, bioaerosols)?
2. Does it avoid introducing new pollutants during operation (for example ozone or partial oxidation by-products), and do tests actually measure that?

Those questions are more informative than a generic “kills 99.9%” claim.

Does adding a catalyst solve filter “secondary release”?

A HEPA filter holds captured material. If filters are overloaded, damp, or poorly handled, a system can in principle re-entrain some material (or spread it during replacement). Adding a catalytic stage is often described as a way to move from “capture only” toward “capture plus inactivation/breakdown” at or near the surface.

• Filtration alone relocates contaminants into the filter.
• A catalytic layer may reduce viability/reactivity of what is captured, depending on design and evidence.

What should you look for in data when a system claims “HEPA + catalyst”?

1) Separate particle results from gas/VOC results

• Particle performance can often be described with CADR and translated into eACH (equivalent air changes per hour).
• Gas/VOC performance needs gas-specific or VOC-specific evidence (not just PM charts).

2) Look for operating mode and time

High airflows can look impressive in short tests. Ask:

• What fan setting was used?
• How long did the test run?
• Was there a baseline “device off” decay?

3) Check whether by-products were measured (when relevant)

If a system relies on oxidation chemistry, reports should check for common by-products appropriate to the method (e.g., ozone for some approaches, intermediates for some oxidation systems). “No by-products” is only meaningful if the report measured them.

4) Consider maintenance as part of performance

• Filter loading, replacement intervals, and access matter.
• A well-tested system should describe assumptions about maintenance, not just initial performance.

Summary

HEPA filtration and catalytic filters solve different parts of the indoor air problem. HEPA is a robust method for particle capture, while a catalytic stage is intended to support chemical breakdown/inactivation at a surface for certain pollutants that filtration alone does not address. The reason to combine them is straightforward: indoor air contains particles, gases, and bioaerosols, and no single mechanism handles all categories equally well.

When you evaluate “HEPA + catalyst” claims, focus on what was measured (particles vs gases vs microbes), the operating conditions, whether by-products were checked, and how performance is expected to hold up over time.