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IAQ & Ultrafine Particles — The Next Challenge for the Air Purification Industry?
For years, the indoor air quality conversation has focused on pollutants we can measure relatively easily — particularly PM10, PM2.5, CO₂ and VOCs.
But there is another part of the particle story
that is receiving increasing scientific attention:
ultrafine particles, or UFPs.
These particles are so small that they
contribute very little to the overall mass of
particulate matter, yet they can account for a
very large proportion of the total number of particles in the air.
That creates an interesting challenge.
If we measure PM2.5 but don't measure particle number, could we be missing an important part of what people are actually breathing?
And if we don't routinely measure UFPs in buildings, how do we know whether our existing approaches to air purification are effectively controlling them?
For an industry increasingly focused on measurable, performance-based IAQ, UFPs may represent the next significant challenge — both technically
and from a measurement and standards perspective.
What are UFPs?
Ultrafine particles (UFPs) are generally defined as airborne particles smaller than 0.1 micrometres (100 nanometres) in diameter.
To put that into perspective:
Particle category
Approximate diameter
PM10
≤ 10 µm
PM2.5
≤ 2.5 µm
PM1
≤ 1 µm
Ultrafine particles (UFPs)
< 0.1 µm (100 nm)
Nanoparticles are also generally defined around the sub-100-nanometre range, although terminology can vary depending on the scientific context.
An important point is that UFPs are not simply “very small PM2.5.”
They occupy a different size range and behave differently in the air and in the respiratory system.
For example, a 0.1 µm particle is 25 times smaller in diameter than a 2.5 µm particle.
There is also an important measurement distinction.
PM10 and PM2.5 are conventionally expressed as mass concentrations, such as micrograms per cubic metre (µg/m³).
UFPs are more commonly characterised using particle number concentration (PNC) — typically expressed as particles per cubic centimetre (particles/cm³).
This matters because a very large number of ultrafine particles can contribute relatively little to total particle mass.
In other words, mass tells us one part of the particle story. Particle number tells us another.
Are VOCs nanoparticles?
No.
This is an important distinction because VOCs and UFPs are sometimes discussed together, but they are fundamentally different types of pollutants.
VOCs — volatile organic compounds — are chemicals.
They are organic compounds that readily evaporate into the air and are generally present as gases or vapours.
Examples include formaldehyde, benzene, toluene and many solvents.
UFPs, by contrast, are particles.
They are defined primarily by their physical size — generally below 100 nanometres.
However, there is an interesting connection.
Some VOCs and other gaseous compounds can participate in chemical reactions in indoor or outdoor air and contribute to the formation of secondary organic aerosol (SOA). These newly formed particles can fall within the ultrafine size range.
So the relationship can look something like this:
VOC → chemical reaction/condensation → ultrafine particle
But it is important not to confuse the two.
A VOC is not itself a nanoparticle.
This distinction also has an important implication for IAQ monitoring.
A PM2.5 monitor does not tell you the concentration of VOCs.
A TVOC sensor does not tell you the concentration of UFPs.
And a UFP counter does not tell you the concentration of VOCs.
Different pollutants require different measurement approaches.
Why are UFPs attracting attention?
One of the reasons UFPs are attracting attention is
their enormous particle numbers.
UFPs can represent a very large proportion of the total
number of particles present in an environment,
particularly near combustion sources.
Common sources include vehicle exhaust,
combustion, cooking, candles and incense, tobacco
smoke and certain industrial processes.
And this is where the health discussion becomes
important.
Because of their small size, UFPs can penetrate deeply into the respiratory system. Research has also raised concerns about their potential to contribute to cardiovascular and respiratory effects.
The WHO identifies UFPs as a pollutant of health concern and notes that exposure can increase the likelihood of pulmonary, cardiovascular and ischaemic heart disease.
However, we should be careful not to overstate what is currently known.
The evidence base for UFPs is developing, and establishing a simple concentration-response relationship is more difficult than it is for pollutants such as PM2.5.
That is one reason why UFPs remain an emerging area of IAQ research, measurement and regulation.
Does meeting the PM2.5 guideline mean UFPs are under control?
Not necessarily.
This is perhaps one of the most interesting questions.
PM2.5 remains an extremely important IAQ metric and controlling PM2.5 is essential.
But PM2.5 is a mass-based measurement.
It does not tell us how many ultrafine particles are present.
Two environments could potentially have similar PM2.5 mass concentrations while having quite different particle-number concentrations and particle-size distributions.
That means PM2.5 should not be regarded as a complete proxy for UFP exposure.
This is one reason UFP monitoring is becoming increasingly interesting
What does WHO say about UFPs?
Here we need to be precise.
The WHO's 2021 Global Air Quality Guidelines do not establish a
formal numerical guideline value for UFPs comparable to the
PM2.5 guideline.
Instead, WHO provides a good-practice statement for managing ultrafine particles because there is currently insufficient quantitative evidence to establish a health-based guideline concentration.
WHO does, however, identify particle-number concentrations that can be used to characterise low and high exposure:
These should therefore be understood as WHO reference points for describing exposure, rather than two separate “WHO standards.”
That distinction is important.
And it illustrates one of the challenges facing the industry.
We have growing scientific concern about UFPs, increasingly sophisticated measurement technology, but we do not yet have the same globally established regulatory framework that exists for PM10 and PM2.5.
-
Low PNC: below 1,000 particles/cm³ as a 24-hour mean
-
High PNC: above 10,000 particles/cm³ as a 24-hour mean
-
High PNC: above 20,000 particles/cm³ as a 1-hour mean
Can we easily measure UFPs?
This is where the problem becomes practical.
Measuring PM2.5 has become relatively straightforward. There are now inexpensive sensors and monitors that can provide continuous readings.
UFP measurement is more technically demanding.
Accurate particle-number measurements can require specialised instruments, appropriate calibration and an understanding of particle-size distributions and measurement methodology.
This makes routine UFP monitoring much less accessible than conventional PM monitoring.
For building owners and facilities managers, this creates an obvious problem:
If we don't measure UFPs, how do we know whether we are controlling them?
This is particularly relevant in environments where there may be significant sources of ultrafine particles — such as buildings close to heavy traffic, buildings with significant cooking operations, or spaces where
combustion-related pollutants can enter the occupied environment.
What options do we have?
There is no single answer.
The first principle should always be source control.
If you can prevent a pollutant from entering or being generated within a building, that is generally preferable to trying to remove it later.
The second consideration is ventilation.
In the GCC, however, ventilation presents a particular challenge.
Buildings often require significant quantities of outside air, but that outside air can contain particulate pollution from traffic, construction, dust and other sources.
And before that outside air enters the occupied space, it may need to be cooled and dehumidified.
So increasing ventilation is not necessarily a simple or energy-neutral solution.
What about HEPA filtration?
HEPA filtration can be highly effective at removing airborne particles when correctly selected, installed and operated.
For UFPs, however, there is an important
engineering consideration.
A HEPA H13 filter is a high-efficiency filter with
relatively high resistance to airflow.
That creates pressure drop across the HVAC system.
And pressure drop has consequences.
More pressure drop → greater fan effort → potentially greater fan energy consumption.
If an existing HVAC system does not have sufficient fan capacity or available static pressure, installing a higher-efficiency filter can also reduce airflow.
That can affect both the air-cleaning performance and the ability of the HVAC system to deliver the required cooling and ventilation.
The issue becomes particularly important in dusty environments such as the GCC.
A sensible filtration design may therefore use multiple stages, with progressively higher-efficiency filters.
Pre-filtration can protect the final high-efficiency filter from excessive dust loading.
But every additional filtration stage also introduces additional resistance, cost and maintenance requirements.
So the engineering challenge is not simply:
“How efficient is the filter?”
It is:
“How do we achieve the required air-cleaning performance while maintaining the required airflow and energy efficiency?”
There is another important point.
HEPA is a particle-control technology.
It does not inherently remove gaseous pollutants such as VOCs, formaldehyde or CO₂.
Those require other approaches, including source control, ventilation or appropriate gas-phase filtration.
And finally, filter efficiency is not the same thing as whole-room effectiveness.
A filter can have extremely high removal efficiency when air passes through it.
But the question for an occupied building is:
How much of the air in the occupied space actually passes through that filter — and how often?
That is why airflow, air distribution, bypass leakage, filter loading, maintenance and room mixing all matter.
So what about active air cleaning?
This is where the discussion becomes particularly interesting.
One approach being investigated is the use of bipolar ionisation to supplement mechanical filtration.
The basic principle is that positive and negative ions are introduced into the airstream. These ions can interact with airborne particles, changing their electrical characteristics and potentially promoting interactions between particles.
The objective is not simply to “replace” the filter.
Rather, the ionisation process can potentially make particles easier for the existing filtration system to capture.
This makes the relationship between active air cleaning and passive
filtration particularly interesting.
What does research tell us?
A 2023 study by Pranav Muthukrishnan and Faramarz
Farahi investigated the effect of bipolar ionisation on the
filtration of fine and ultrafine particles.
The researchers used a bipolar ioniser with a
recirculating air-handling system and compared filter
performance with and without ionisation.
They reported a 275% increase in removal efficiency in
the most penetrating particle-size range of 100–500
nanometres when ionisation was used.
The study also found that ionisation changed the particle-size behaviour and could improve filtration performance without increasing the filter's resistance in the same way that simply installing a more restrictive filter would.
That is an interesting finding because the most penetrating particle size range is precisely where conventional filtration presents one of its greatest challenges.
Similar findings are contained in research conducted by Deify Law and Elbert Ho at the Department of Mechanical Engineering, California State University, Fresno, and published by the American Society of Mechanical Engineers in May 2025.
But we should be scientifically disciplined here.
One study does not establish that every bipolar-ionisation system will produce the same result.
Performance can depend on the specific ionisation technology, ion concentration, filter characteristics, airflow, particle size and operating conditions.
So the right question isn't: “Does ionisation work?”
It's: “Under what conditions, with which technology, and with what measured result?”
That is a much more useful question for the IAQ industry.
The opportunity: Measure and Manage
And perhaps this is where the UFP discussion ultimately leads us.
If ultrafine particles are important, but we rarely measure them in buildings, we have a potential IAQ blind spot.
The answer isn't necessarily to install another piece of equipment.
The answer is to start with measurement.
Measure.
Understand.
Control.
Verify.
Measure the particle environment.
Understand the sources.
Evaluate the available combination of source control, ventilation, filtration and active air-cleaning technologies.
Then verify the result under actual operating conditions.
That approach is consistent with the broader direction of IAQ management: moving away from assumptions and towards measurable, evidence-based performance.
Could UFPs become the next IAQ challenge?
It is too early to say that UFPs will replace PM2.5 as the dominant IAQ metric.
They shouldn't.
PM2.5 remains an important and well-established measure of particulate pollution.
But UFPs could become an increasingly important additional dimension of IAQ assessment.
The technology to measure them is improving.
The scientific evidence is growing.
And interest in particle-number concentration is increasing.
What remains less developed is a universally adopted framework for routine building measurement, reporting and performance targets.
That creates an opportunity for researchers, standards organisations, regulators and the air-purification industry to work together.
Rather than waiting for regulation to catch up, the industry can begin asking a more fundamental question:
What should we actually be measuring if our objective is to understand the air people breathe?
PM10?
PM2.5?
PM1?
UFPs?
VOCs?
CO₂?
The answer may ultimately be:
All of the pollutants that matter — measured appropriately for the building, its occupants and its sources.
And that brings us back to a principle that should underpin modern IAQ management:
You can't effectively manage what you don't measure.
UFPs may well be one of the next areas where indoor air quality moves from assumption to evidence.
And if that happens, the next challenge for the air-purification industry won't simply be developing better technology.
It will be developing better ways to measure, demonstrate and verify performance.
Measure. Understand. Control. Verify.
This may be the future of IAQ and UFPs.
Clean Air Associates
A subsidiary of Strategic Brand Solutions FZ-E
The IAQ authority for the GCC & Middle East
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