IAQ Byproducts of Electronic Air Purifiers Using UV Light, Ionizers, or PCO Technology>>>

By INDEX Editorial Team | Based on peer-reviewed research –

Some air-cleaning devices may reduce certain pollutants while creating others. Here’s how to evaluate the tradeoffs using a science-first, criteria-based approach.

If you are shopping for an air purifier because someone in your home has asthma, odor sensitivity, recurring irritation, or concerns about wildfire smoke, dust, or indoor contaminants, the label “air purifier” can sound reassuring. But that label covers very different technologies.

Some devices mainly capture particles. Others use electricity, UV light, ion generation, or catalytic reactions to transform pollutants in the air. That distinction matters. In real buildings, certain electronic air-cleaning technologies can produce secondary pollutants or reaction byproducts that may affect indoor air quality (IAQ) in ways most consumers are never clearly told.

The main concern is not that every device is automatically dangerous. The real issue is that performance claims often focus on what a device is intended to remove, while giving much less attention to what the device may add to the air as a byproduct. Depending on the technology and operating conditions, those byproducts can include ozone, formaldehyde, acetaldehyde, nitrogen oxides, reactive oxygen species, or partially oxidized VOCs.

For homeowners, renters, school leaders, office managers, and facility teams, the practical question is simple:

Will this device improve IAQ overall, or could it solve one problem while creating another?

This guide takes a criteria-first approach. We’ll explain what to look for, why it matters, and how UV, ionizer, and photocatalytic oxidation (PCO) devices differ from more established filtration-based strategies.

The 8 Criteria to Use Before Considering an Electronic Air Cleaner

Before looking at claims like “destroys viruses,” “neutralizes odors,” or “active purification,” start with these eight criteria.

1) Ask whether the device captures pollutants or chemically transforms them

This is the first screening question.

• Mechanical filtration primarily captures particles.
• Electronic air cleaners often rely on ionization, UV exposure, oxidation, or plasma-related reactions.
• When a device transforms pollutants rather than capturing them, the chemistry does not always stop at harmless end products.

If the technology depends on reactions in the air stream, byproduct risk deserves extra scrutiny.

2) Look for independent emissions testing, not just removal claims

A manufacturer may show that a device reduced one target pollutant in a chamber test. That is not the same as proving it does not emit or generate other compounds.

Look for evidence on:

• ozone emissions
• aldehyde formation
• secondary VOC formation
• testing under realistic airflow and humidity conditions
• third-party rather than internal manufacturer testing

3) Separate microbe inactivation claims from overall IAQ improvement

A device may inactivate some microorganisms under certain conditions and still fail to improve overall exposure for occupants.

For example:

• UV-C can inactivate microbes when dose and exposure time are sufficient.
• But UV systems do not remove particles from the air.
• Dead particles, fragments, or other contaminants may still remain airborne.

4) Be cautious with devices marketed for odor removal

Odor reduction is often used as a proxy for “clean air,” but that can be misleading.

Some technologies do not truly remove gaseous pollutants well. Others may alter odor-causing compounds into different chemicals, some of which may be more irritating or more concerning from an IAQ perspective.

5) Check whether the technology can produce ozone, even indirectly

This is one of the most important consumer-protection questions.

The U.S. EPA states that ion generators and some other electronic air cleaners can produce ozone, a lung irritant. Ozone is not just an outdoor smog problem; indoors, it can worsen respiratory irritation and react with other indoor chemicals.

6) Ask how the device performs under real-world airflow

Many marketing claims are strongest in small test chambers, long contact times, or tightly controlled pollutant conditions.

In actual homes, schools, and offices:

• airflow is faster,
• pollutant mixtures are more complex,
• humidity varies,
• catalyst surfaces age,
• and contact time may be too short for complete reactions.

That gap between lab promise and field performance is especially important for PCO and similar technologies.

7) Prefer technologies with a stronger evidence base for the specific problem

If the problem is:

• particles → filtration is usually the central strategy
• gases/odors → sorbent media such as activated carbon may be part of the solution
• infection risk → ventilation, filtration, and correctly designed supplemental strategies matter more than broad marketing language

The goal is not to buy the most advanced-sounding device. It is to choose the least risky effective pathway.

8) Consider the vulnerability of the people in the space

Byproduct risk matters more when occupants include:

• children
• older adults
• people with asthma
• people with COPD
• chemically sensitive individuals
• people recovering from respiratory illness

A device that introduces reactive compounds into indoor air may be a poor fit for precisely the people most likely to be seeking cleaner air.

Why These Criteria Matter for IAQ

Indoor air is not a blank space. It is a dynamic chemical environment containing:

• particles
• humidity
• cleaning product residues
• cooking emissions
• fragrances
• building material off-gassing
• outdoor pollution that infiltrates indoors

When an electronic device adds electrical charge, UV energy, or oxidation chemistry, it can trigger secondary reactions. That means a device may not just remove a pollutant; it may change it into something else.

This is why public-health and building guidance continues to urge caution around some “active” air-cleaning technologies. The concern is not theoretical. It is based on evidence that certain devices can create undesirable byproducts under realistic indoor conditions.

Ionizers: What They Do and Why Ozone Is a Concern

Ionizers work by electrically charging particles in the air. Those charged particles may then:

• settle onto room surfaces,
• attach to walls, floors, and furnishings,
• or in some designs be drawn back to a collection plate.

That sounds useful at first glance, but there are several limitations.

What the EPA says

EPA guidance notes that ion generators may remove some small particles from indoor air, but:

• they do not remove gases or odors, and
• they may be relatively ineffective for larger particles such as pollen and dust allergens.

EPA also warns that ion generators and some other electronic air cleaners can produce ozone, a lung irritant.

Why ozone matters indoors

Ozone can:

• irritate the respiratory tract,
• aggravate asthma,
• contribute to coughing or chest discomfort,
• and react with other compounds already present indoors.

That last point is often overlooked. Ozone is not only a pollutant by itself. It can also react with indoor chemicals from cleaning products, fragrances, and building materials to create additional byproducts, including aldehydes and ultrafine particles.

Another limitation: deposition is not the same as removal

If particles are pushed out of the air onto surfaces, that is not the same as permanently solving the exposure problem. Some deposited particles may later be resuspended by walking, cleaning, or air movement.

Bottom line on ionizers

The practical issue with ionizers is not only uncertain real-world benefit. It is that the technology may introduce ozone and other reactive chemistry into the space while offering limited help for gases and mixed-source IAQ problems.

UV Air Purifiers: Useful in Some Designs, But Not Automatically Low-Risk

UV-based air cleaning is often marketed as a more scientific option, especially when it uses terms like “germicidal,” “hospital-grade,” or “advanced pathogen control.” But the details matter.

What UV can do

Ultraviolet systems, particularly UV-C, can inactivate microorganisms when the design provides:

• adequate lamp intensity,
• adequate dose,
• sufficient exposure time,
• and correct engineering controls.

That means UV can have a role in certain applications. But it is not a shortcut around core IAQ principles.

What UV does not do by itself

UV does not remove dust, smoke particles, allergens, or many chemical pollutants from the air. Even when it inactivates microbes, the physical material may remain present.

Can UV systems create ozone?

Some UV devices have the potential to generate ozone, depending on wavelength and design. Public guidance documents have flagged this possibility and treat UV as a supplemental rather than primary strategy when ventilation and filtration are limited.

Why marketing can outpace performance

A common consumer misunderstanding is to equate “kills germs” with “cleans the air.” Those are not identical outcomes.

A UV unit may be useful in a tightly defined engineering context yet still be a weak choice for a home trying to address:

• wildfire smoke,
• dust,
• pet dander,
• or mixed particle-and-gas exposure.

Bottom line on UV devices

UV technology should be evaluated narrowly:

• What is the target?
• What dose is delivered?
• Is there ozone risk?
• Is the system paired with filtration?
• Is it solving the problem you actually have?

If those answers are unclear, caution is warranted.

PCO Technology: The Biggest Byproduct Question Mark

Photocatalytic oxidation, or PCO, typically uses UV light and a catalyst such as titanium dioxide to oxidize airborne chemicals. On paper, the concept is attractive: convert pollutants into harmless end products such as carbon dioxide and water.

In practice, that ideal outcome is not always what happens indoors.

The core problem: incomplete oxidation

Peer-reviewed reviews and technical guidance have repeatedly noted that PCO can generate intermediate byproducts rather than fully neutralizing pollutants. These byproducts may include:

• formaldehyde
• acetaldehyde
• ozone
• other partially oxidized VOCs

That matters because formaldehyde is itself a significant indoor air contaminant.

Why real-world conditions matter so much

PCO performance depends heavily on:

• airflow rate
• humidity
• pollutant type and concentration
• catalyst condition
• residence time
• reactor design

In real buildings, those variables are rarely ideal. Short contact times and mixed contaminant loads can make complete oxidation less likely.

The evidence pattern

Recent reviews and public guidance have converged on several concerns:

• relatively low removal efficiency for many indoor gases
• lack of standardized real-world performance testing
• inconsistent field effectiveness
• potential formation of harmful secondary pollutants

Connecticut’s public guidance, drawing on EPA, CDC, ASHRAE, academic, and state sources, explicitly notes that PCO can generate harmful byproducts such as formaldehyde, ozone, and acetaldehyde and that it often lacks strong performance evidence in practice.

Why this matters for consumers

PCO is often marketed in terms that sound highly reassuring:

• destroys VOCs
• neutralizes odors
• active oxidation
• molecular purification

But if a device reduces one parent VOC while increasing aldehydes or other oxidized compounds, the net IAQ benefit may be uncertain or negative.

Bottom line on PCO

Among consumer-facing electronic air-cleaning technologies, PCO deserves especially careful scrutiny because the gap between marketing promise and real-world chemistry can be large.

The Most Common Byproducts and IAQ Concerns

Here are the major byproducts and secondary effects consumers should understand.

Ozone

Associated with:

• ionizers
• ozone generators
• some UV-based devices
• some other electronic air cleaners

Why it matters:

• respiratory irritant
• can worsen asthma and airway symptoms
• reacts with indoor chemicals to create more pollutants

Formaldehyde

Associated with:

• PCO and oxidation-related reactions
• ozone reactions with indoor chemicals under some conditions

Why it matters:

• common indoor irritant
• important IAQ pollutant
• not something you want added while trying to “clean” indoor air

Acetaldehyde and other aldehydes

Associated with:

• incomplete oxidation of VOCs
• catalytic or reactive air-cleaning processes

Why it matters:

• may contribute to irritation and degraded IAQ
• signals that pollutant transformation may be incomplete

Reactive oxygen species and other oxidants

Associated with:

• ionization and related “active purification” technologies

Why it matters:

• exposure implications are still being evaluated
• can add chemical reactivity to occupied spaces

Particle deposition and resuspension

Associated with:

• ionization-based systems

Why it matters:

• moving particles onto surfaces is not the same as removal
• exposure can return later through disturbance and resuspension

A More Practical IAQ Decision Framework

If your goal is healthier indoor air, start with the basics in this order:

1) Source control

Reduce the pollutant at its source whenever possible:

• choose lower-emission products
• manage moisture
• address combustion issues
• improve housekeeping for settled dust
• reduce fragrance load

2) Ventilation

Bring in cleaner outdoor air when conditions and building design allow.

3) Filtration

For particles, properly sized filtration remains a more established pathway than many reactive electronic technologies.

4) Gas-phase media when needed

For some gaseous pollutants or odors, sorbent media may be part of the solution.

5) Supplemental technologies only after the basics are addressed

If you are considering UV or another advanced technology, ask whether it is:

• supplemental,
• independently tested,
• appropriate for the pollutant profile,
• and unlikely to generate harmful byproducts.

Questions to Ask Before You Buy

Use these questions with any seller or manufacturer:

• Does the device emit or generate ozone under any operating mode?
• Has it been independently tested for byproducts, not just removal efficiency?
• What pollutants is it actually designed to address: particles, gases, microbes, or odors?
• Was testing done under realistic airflow and humidity conditions?
• Does it create formaldehyde, acetaldehyde, nitrogen oxides, or other secondary compounds?
• Is there peer-reviewed evidence for real occupied spaces, not just lab chambers?
• Is the technology supplemental, or is it being sold as a replacement for ventilation and filtration?
• Is the device appropriate for children, older adults, or people with asthma?

If clear answers are not available, that is useful information.

What INDEX Would Treat as a More Reliable Consumer Signal

From a consumer-protection standpoint, a stronger signal is not flashy chemistry. It is transparency.

A more trustworthy pathway includes:

• clearly stated pollutant targets
• independent emissions and performance data
• no vague “purifies everything” claims
• realistic room or airflow sizing
• acknowledgment of limitations
• compatibility with ventilation and filtration rather than replacing them

That is often less exciting than aggressive marketing. It is also more consistent with good IAQ decision-making.

Practical Pathways

Practical options to consider first

For this topic, the more practical pathway is usually to prioritize solutions that:

• improve ventilation,
• use effective particle filtration for particle problems,
• use appropriately selected gas-phase media for relevant gases or odors,
• and provide transparent test data on emissions and limitations.