Photocatalytic air purifiers
by Chris Woodford. Last updated: January 19, 2017.
Light is an amazing source of energy—the power behind virtually everything
that happens on Earth. Light from the Sun brightens the dark depths of space, makes plants leap to life, and (indirectly) powers our bodies. But did you know it can help to clean things as well? In air purifiers that
work using a method called photocatalysis, light energy kick-starts a
process that zaps all kinds of nasty air pollutants and turns them
into harmless substances instead. For people who suffer from asthma
and allergies, light-powered air purifiers like these are another
weapon in the fight for cleaner air and better health. Now photocatalysis
might sound horribly complicated, but it works in a relatively simple way. Let's
take a closer look!
Photo: Designed by US DOE/NREL (National Renewable Energy Laboratory), this photocatalyser unit uses ultraviolet light to help purify the air inside cars, turning harmful volatile compounds (from such things as petroleum and exhaust fumes), bacteria, and viruses into less harmful substances instead. Domestic air purifiers that use photocatalysers in a similar way are made by companies
such as Daikin and Zander. Photo by Warren Gretz courtesy of US DOE/NREL.
What is a catalyst?
Artwork: The structure of a typical zeolite catalyst. Photo by courtesy of NASA Marshall Space Flight Center (NASA-MSFC).
You've probably heard of catalysts already—in things like
catalytic converters (the exhaust-cleaning systems fitted to cars)
and zeolites (rock-like crystals used in all kinds of products and
industrial processes). Catalysts are hugely important in
industry; there are many types, they work in many
different ways, and they're used in the manufacture of almost every
chemical product you can think of.
Simply speaking, a catalyst is a substance that makes a chemical
reaction more likely to happen by reducing the energy needed to kick
start it ("activation energy," as it's known). A catalyst can speed up a chemical reaction
or make it happen at a lower temperature. Once the reaction has finished, the catalyst isn't used up, though it may
be physically changed in some way.
What about photocatalysts? When you see "photo" attached to a
word (as in photocopier, photograph, photomultiplier,
photoelectric...), you can be pretty sure light is involved:
phōtos is the Greek word for "light." Photocatalysis means light is involved in
making a catalyst do its job. In other words, light provides the
energy that allows the catalyst to work.
How does a photocatalytic air purifier work?
In a nutshell, ultraviolet light shines onto a catalyst, which converts water in the air into a form that turns
molecules of pollution into more harmless substances. Here's how...
In photocatalytic air purifiers, the catalyst that cleans the air is typically titanium
dioxide (sometimes called titania) and it's energized by ultraviolet
(UV) light. UV is the short-wavelength light just beyond the
blue/violet part of the electromagnetic spectrum that our eyes can detect. The bad
thing about it is that it gives you sunburn. The good thing is that it has much more energy than ordinary, visible light—and exactly the right amount of energy to get titanium dioxide excited.
Titanium dioxide is a semiconductor (a bit like materials such as
silicon, used in integrated circuits).
You don't actually need much titanium dioxide: just a thin
film covering the surface of a backing material called a substrate,
which is usually made from a ceramic or a piece of metal (such as
Here's how the titanium dioxide catalyst in an air purifier breaks apart molecules of air pollution:
When UV light (the big yellow arrow shown here) shines on the titanium dioxide, electrons (the tiny, negatively charged particles inside atoms) are released at its surface. It's the electrons that do the useful work
- The electrons interact with water
molecules (H2O) in the air, breaking them up into
(OH·), which are highly reactive, short-lived, uncharged forms of
hydroxide ions (OH−).
- These small, agile hydroxyl radicals then attack bigger organic
(carbon-based) pollutant molecules, breaking apart their chemical
bonds and turning them into harmless substances such as carbon
dioxide and water. This is an example of oxidation—and that's why
air purifiers that work this way are sometimes also described as PCO (photocatalytic oxidation) air cleaners.
Here, then, is the big advantage that photocatalytic air purifiers have
over other air-cleaning technologies, such as filters: instead
of simply trapping pollutants (which still have to be disposed of somehow),
they completely transform the harmful chemicals and effectively destroy them.
The disadvantage of this process is that photocatalytic purifiers also produce tiny
amounts of ozone (O3), a chemical variant of the oxygen in the air
that is, in itself, a toxic air pollutant. Purifier makers claim the amounts of ozone produced are well within the guideline limit (0.05 parts per million) suggested by the US FDA but, even so,
this is something to bear in mind. Although hydroxyl radicals occur naturally in the atmosphere, they can themselves pose dangers; there is some debate and uncertainty over whether those produced by photocatalytic air purifiers could pose a greater risk to human health than the pollutants they are designed to remove.
Another thing worth noting is that all the interesting stuff happens on the surface of the titanium dioxide catalyst. That's why air purifiers need fans to suck polluted air in at one end and blow clean out out of the other. It's also why air purifiers take some time (typically up to 30 minutes) to clean a large room properly.
Photocatalysis in practice
Photocatalysis only tackles certain, chemical forms of air pollution and doesn't solve the problem of
particulates (soot and dirt). That's why photocatalytic air purifiers combine UV-activated,
titanium-based catalysers with other cleaning and filtering technologies to form a
comprehensive system that can tackle a whole range of dirt and
A typical purifier draws incoming air past a series of
different cleaning stages, each of which tackles a different kind of
- A relatively coarse prefilter captures large particles of household
dust, hair, and pet hair. This filter is made of polypropylene
netting coated with catechin (a bitter-tasting natural substance, found in green tea,
that works as an antibacterial agent and deodorant).
- A fine HEPA filter removes airborne viruses, bacteria, spores,
- A plasma ionizer gives a positive electrical charge to any
remaining dust and pollen particles so they stick to a negatively
charged metal grid (or something like a roll of disposable filter paper) further along the machine. (This is much like the
system used in an electrostatic smoke precipitator that scrubs soot from smokestacks.)
- A photocatalyst made from titanium apatite (similar to but more effective than titanium dioxide) chemically destroys
remaining organic pollutants such as exhaust fumes, volatile organic compounds, and so on.
Artwork: An exploded view of a typical Daikin air purifier, which reads from right to left. Air enters on the right (blue arrow) through the front grille (gray, 2). It passes through a coarse prefilter (orange, 4), an ionizer that charges dirty particles (blue 5), and a roll filter (gray, 6). Then it enters the photocatalytic cleaning section in the middle, where there are two honeycomb-shaped photocatalytic surfaces (green) energized by UV lamps (yellow, 12). The back of the machine contains the fan and motor (orange, 13 and 14). Artwork from US Patent: 6,761,859: Air cleaner by Yasuhiro Oda, Daikin Industries, Ltd, courtesy of US Patent and Trademark Office (with colors added for clarity).
Find out more
On this website
On other sites
- Indoor Air Quality (IAQ): A comprehensive, trustworthy, independent introduction to improving air quality from the US EPA Office of Air and Radiation.
- Guide to Air Cleaners in the Home: A short, handy factsheet from the US EPA. This one includes some discussion of photocatalytic air cleaners.
- Air Purifier Power: This excellent site is packed with refreshing, first-hand knowledge about air purifying products and breezily sweeps aside—indeed, you could even say purifies!—the exaggerations and unsubstantiated claims that manufacturers enjoy making.
- Daikin Industries: Air Purifier MC707: Mechanism: Daikin's simple explanation of its photocatalytic MC707 air purifier includes a clear diagram showing the different stages of air cleaning and filtration. [Via the Wayback Machine; the original link has been deleted.]
- Eat My Dust: An Allergy Sufferer Tests Six Air Purifiers by Steven Kurutz. The New York Times, May 15, 2013. A look at the controversial claims made by air purifiers and a comparison of six recent models.
- Odor-Destroying Robots Solve Problems You Didn't Know You Had by Erico Guizzo, IEEE Spectrum, January 18, 2012. A mobile air purifier that nips around your house, zapping nasty smells!
- How to Select an Air Cleaner by Jay Romano. The New York Times, February 11, 2007. This article suggests that performance, room coverage, and noise are the three most important criteria on which to compare cleaners.
If you're looking for a more detailed technical description of real-world air and water purifiers, start here:
- US Patent: 5,045,288: Gas-solid photocatalytic oxidation of environmental pollutants by Gregory B. Raupp et al. Issued September 3, 1991. Describes a photocatalytic air and water purifier that can tackle volatile organic compounds (VOCs) and non-volatile organic compounds.
- US Patent: 6,761,859: Air cleaner by Yasuhiro Oda, Daikin Industries, Ltd. Issued July 13, 2004. A very clear description of what you'll find inside a typical household air purifier that combines photocatalysis with traditional air filters.
- US Patent: 6,884,399: Modular photocatalytic air purifier by Bradley Reisfeld et al, Carrier Corporation. Issued April 26, 2005. Describes a titanium-dioxide photocatalytic air cleaner broadly similar to the one I've sketched out above.
- US Patent: 7,300,634: Photocatalytic process by Zvi Yaniv et al, Nano-Proprietary, Inc. Issued April 27, 2007. This patent goes into a bit more detail about the mechanism of photocatalysis involving titanium dioxide and ultraviolet light.