January 4, 2013
Cleaning air the natural way
by Arnfinn Christensen
A new air purification unit requires little energy in removing
toxic gasses, dust and bacteria, much the way air is cleansed in
the atmosphere. The new invention is out of the test lab and now
cleaning industrial emissions.
Making neighbours happy: The photochemical accelerator removes
foul smells from the recycling of oil and waste water at the
Jysk Miljørens plant in the harbour area of Aarhus,
Denmark. (Photo: Matthew Johnson)
Metal sections have been mounted on the roof of the company Jysk
Miljørens in Aarhus, Danmark which literally are making the owners
Until now the company’s neighbours have been protesting the big
stink it made. The firm recycles oil from the water in ballast
tanks of ships and the process noticeably fouled the air, despite
a high chimney. Complaints led to threats of closing the plant
But the stench is now history. The metal crates contain the first
commercial unit for a new type of air purifier developed by the
chemist Matthew Johnson at the University of Copenhagen.
Variations of the purifier can be put to use cleaning emissions
and recycling polluted air indoors in factories as well as homes.
The purifier has several advantages. A major one is that it uses
little energy. By recycling indoor air it also saves energy that
would be lost in warming up outdoor air in winter.
Certain factories can avoid building tall chimneys to disperse
polluted discharges high above the heads of immediate neighbours.
The new technology was first presented to the public in the autumn
of 2010. Johnson has spent the last two years on the perilous path
from the laboratory to the market and industrial use.
In getting there, the researcher had to pass what he calls the
“death zone”. This is the zone between labs and markets,
where so many great ideas perish before becoming viable products.
“We crossed the death zone with a prototype. I bought a shipping
container with a little funding from the industrial development
firm Copenhagen Cleantech Cluster,” says Johnson.
As in the atmosphere
This diagram shows how air is sucked in and disinfected with
bacteria-killing ozone and ultraviolet radiation, before water
vapour and/or ammoniac is condensed into droplets, which pick up
dust and chemicals. Then the tiny droplets forming an aerosol is
given an electrical charge and thus is attracted to a field with
an opposite charge, before the remaining ozone is cleaned out in a
And what did Matthew Johnson place in the container? A unit that
mimics the air cleaning processes which naturally occurs high in
In the atmosphere, gasses are heated by the sun and they float to
higher air strata. At lofty altitudes they react with ozone and
create tiny particles that wash away with precipitation.
Much of that, plus a little more, is what goes on in what Johnson
calls an atmospheric photochemical accelerator.
At one end dirty air is sucked in. It can contain toxic gasses,
dust, smoke, fungal spores, bacteria and viruses, i.e. every
health hazard that needs to be removed from the air before it can
be safely breathed.
Ozone is first added to the foul air. Ozone kills microorganisms
like bacteria and viruses and reacts with certain offensive
smelling organic compounds. Ozone purifiers have been on the
market doing this job for some time.
But ozone is a poisonous gas. It’s removed in the photochemical
accelerator before the air is released from this chamber.
After ozone has killed pathogens and removed bad smells, the
suspended residue is collected as specks of dust and water vapour
is added to form an aerosol suspension. This aerosol of compound
particles and vapour condenses into tiny droplets.
It’s the same process as when clouds form in the atmosphere.
Then the air is irradiated with ultraviolet rays. These UV rays
have shorter wavelengths than the solar rays that tan or burn our
Matthew Johnson has developed a new, energy-saving air
purification unit that removes bacteria and viruses, dust and
toxic gasses from the air, similar to the way the Earth’s
atmosphere cleans itself, using sunlight, ozone and rain.
These kill bacteria too. Any microorganisms that survived the
ozone shower encounter a deadly challenge here.
However, the UV rays escalate the attack with another function.
They react with ozone and water to form bactericidal molecules,
Anything that survives ozone and strong ultraviolet light will
call it quits when it comes in contact with these hydroxyls.
After this initial microbiological death trap the dirty air
continues to another large chamber. It is given time to circulate
here while more and more toxic gasses react with ozone and
hydroxyls and attach to aerosol droplets of ammoniac and water.
This is much the same as what occurs in the atmosphere when water
vapour collects into raindrops around tiny particles and thus
cleans the air.
But raindrops fall to the ground carrying pollutants back down to
us with them. The photochemical accelerator has to do better
Given a charge
The dirty fog continues past a powerful electrode and the droplets
and particles are given a charge.
This electrical charge is fateful. Soon afterwards the polluted
droplets are drawn into metal plates with an opposite charge. It’s
an electrostatic filter. The impurities are condensed here and
An electrostatic filter has a large advantage on ordinary filters,
such as charcoal filters. It lets air pass through much more
freely. It doesn’t require powerful fans to press air through and
thus saves more energy.
The first commercial model of the air purifier is now working at
Jysk Miljørens in Aarhus.
The air is now cleansed and nearly ready to be released at the
opposite end. But first it passes through a catalyst which starts
chemical reactions that remove any residual poisonous ozone.
Testing this principle in a lab is one thing. Getting it to work
in the chaotic real world is another.
Matthew Johnson took the container with his purification system to
a firm that was struggling to meet stringent emissions standards.
The company demanded anonymity and the tests were done in secret,
according to Johnson.
Johnson also wanted to be sure the purifier worked before
broadcasting news of the tests.
“We cut a hole in their factory chimney and led the polluted air
through the container. I could demonstrate to them that all the
pollution had been removed from the airflow,” says Johnson.
The experiment was a success. The photochemical accelerator was on
its way out of the death zone. A group of investors led by the
Danish firm Infuser had faith in the invention.
In June 2012 they signed a contract with the University of
Copenhagen and in July the first unit was sold to Jysk
“In November the first commercial unit was mounted on the
recirculation plant in Aarhus and we plan to rebuild the container
for new demonstrations of the purifier,” says Matthew Johnson.
METHOD AND DEVICE FOR CLEANING AIR
Inventor: JOHNSON MATTHEW STANLEY // ARLEMARK JAN [SE]
Applicant: KOBENHAVNS UNI [DK]
A method and device for cleaning air. The air to be cleaned is
directed as a continuous flow in succession through a) a first
zone wherein the air is treated with ozone and possibly also
water, ammonia or other aerosol growth promoters; b) a second zone
wherein the air is subjected to ultraviolet light; c) a third zone
wherein the air is maintained for a sufficient time to allow
aerosol growth; d) a fourth zone where particles in the air are
given an electrical charge; e) a fifth zone wherein the air is
passed through an electrostatic filter; and f) a sixth zone
wherein the air flows over a catalyst to break down residual
ozone.; The air in confined spaces including indoor rooms, public
vehicles with limited access to fresh air is subjected to a low
energy consuming universal broad-spectrum removal of the various
types of indoor air pollution including toxic gases, organic
compounds, microorganisms and liquid and solid particles being
hazardous to health and detrimental to quality of life.
 The present invention relates to a method and device for
cleaning pollution from air wherein the air to be cleaned is
subjected to a sequence of physical and chemical treatments.
BACKGROUND OF THE INVENTION
 Indoor air pollution is produced by many sources including
furniture and building materials, industrial activity, cooking and
human metabolism. If nothing is done this pollution is detrimental
to health and the quality of life. The main method for improving
indoor air quality is dilution: fresh air is brought in from the
outside. This is expensive because in a cold climate the air must
be heated and in a warm climate it must be cooled and
 Presently there is not a device available for cleaning
large volumes of indoor air cheaply and efficiently. Mechanical
filtration involves limited conductance and a pressure drop,
necessitating large fans involving large energy consumption. In
addition the filter must be changed and can itself become a source
of bacteria. Electrostatic filtration does not cause a large
pressure drop, but only removes pre-existing particles; it does
not act on gas-phase pollution. Ozonolysis is used to remove
odours but the chemical products of ozonolysis are often more
hazardous than the original compounds, in addition there are
important components of indoor air pollution that do not react
with ozone. UV light is used to sterilize air in hospitals but
this method removes only a very few specific types of pollution
from the airstream.
 U.S. Pat. No. 6,589,486 (Spanton) discloses an air
purifying apparatus and method suitable for use in a standard
forced air building Heating, Ventilating and/or Air Conditioning
system (HVAC) of a building. The air is treated with ultraviolet
(UV) radiation and ozone. The UV is germicidal and kills
microorganisms, including both bacteria and viruses. The ozone
cleans air and removes odours from air. Ozone in combination with
UV radiation destroys microorganisms which are not killed by the
UV radiation. Spanton does not disclose how to remove toxic
gaseous contaminants and particles such as smoke and dust from the
air. Thus Spanton does not suggest to control the process in order
to optimize aerosol formation inter alia by ensuring a sufficient
time to aerosol growth and/or by addition of aerosol formation
accelerators such as water and ammonia.
 US Patent Application 2004/0120845 (Potember et al.)
discloses a method and apparatus for neutralizing airborne
pathogens in ventilated air, and in heating or air conditioning
systems. The system has a flow-through reaction chamber that
contains a UV light source that emits short intense flashes of
broad-spectrum UV light, a source of water vapour or spray, and an
ozone generator. After the treatment with UV and ozone the air
passes through a porous matrix and a solid support coated with an
ozone removal catalyst. The passage through such matrix requires a
sufficient pressure requiring substantial fan energy consumption.
Furthermore Potember at al. do not disclose how to remove
pollution from the air steam using the aerosol particle growth
 U.S. Pat. No. 5,656,242 (Morrow et al.) discloses an air
purifier having a perforated plate between UV lamps and a porous
air filter. Biological material is trapped by the filter and
killed by the low dose of UV radiation which passes through the
perforations in the plate. Filtered air passing through the plate
is subjected to a high dose of UV radiation which sterilizes
remaining biological material in the air. An electrostatic filter
at the outlet may trap viruses which have been positively charged
either by the action of the UV lamps or by positively charging the
plate in order to strip electrons from the viruses. The UV lamps
may be mercury lamps which are allowed to emit at both their ozone
forming wavelength as well as the ozone breakdown wavelength. In
such instance, a light filter surrounds the lamps which pass light
only at the ozone breakdown wavelength. Air subjected to the
unfiltered light is consequently exposed to ozone, which is a
known biocide. The filtered light is in a zone which is filled
with water mist such that hydroxyl radicals result. Air passing
through this zone is scrubbed by the hydroxyl radicals. Morrow et
al do not disclose how to treat the air with ozone before
treatment with UV radiation. Morrow does not disclose removal of
pollution via formation of aerosols. They do not control the
dosage of ozone.
 Electrostatic air purifiers are used to remove particles
produced by welding. They are also marketed to clean indoor air,
for example in offices and homes. These systems remove
pre-existing particles (for example smoke), but do not remove
toxic gases and other polluting compounds which are not in the
form of particles.
 Ozone is currently used to remove smells in many fields.
Examples are kitchen exhaust, livestock barns and wastewater
treatment plants. However the products of ozonolysis are typically
more irritating and toxic than the original compounds so this is
not a satisfactory solution as the air must be diluted
substantially before it is safe to breathe.
 UV light is used in air circulation systems of some
hospitals to sterilize air. However, this application is not able
to remove many types of pollution including most gas phase
chemicals and particles.
 The object of the present invention is to meet the demand
of a universal or "broad-spectrum" removal of air pollution in an
efficient and simple way with minimum energy consumption.
SUMMARY OF THE INVENTION
 Accordingly, the present invention relates to a method for
cleaning air wherein the air to be cleaned is directed as a
continuous flow in succession through
 a) a first zone wherein the air is treated with ozone;
b) a second zone wherein the air is subjected to ultraviolet
c) a third zone wherein the air is maintained for a sufficient
time to allow aerosol growth;
d) a fourth zone wherein particles in the air are provided with an
e) a fifth zone wherein the air is passed through an electrostatic
f) a sixth zone wherein the air flows over a catalyst to break
down residual ozone.
 By "sufficient time to allow aerosol growth" as used
herein, is meant a time in which a significant fraction of the
actual pollution to be removed such as at least 60, 70 or 80% by
weight, preferably 90% by weight, most preferably 99% by weight,
of pollution which is able itself or whose reaction products are
able to be removed by aerosol formation are removed from the
airstream. In addition, pollution may be removed by concomitant
photochemical processes, including ozonolysis, photolysis and
radical reactions. The time required for aerosol formation may be
dependent on the actual type and amount of pollution to be removed
and can be estimated by the person skilled in the art based on
relevant analyses. The required time may be ensured by proper
design of the dimensions of the third zone (c) and/or the
temperature in the third zone (c) and/or the airflow velocity in
the third zone (c).
 The present invention also relates to an air cleaning
device including a channel with an air inlet for air to be cleaned
and an air outlet for cleaned air and means for leading air
through the channel from the inlet to the outlet wherein the
channel has following zones in succession:
 a) a first zone having a source of ozone;
b) a second zone having a source of ultraviolet light;
c) a third zone having a dimension which allows aerosol growth;
d) a fourth zone having an electrical discharge generator;
e) a fifth zone having one or more electrostatic filters; and
f) a sixth zone having a catalyst for removal of residual ozone.
 The present invention provides a desirable "broad-spectrum"
removal of the various types of indoor air pollution including
irritating and/or toxic gases, organic compounds, microorganisms
and liquid and solid particles being hazardous to health and
detrimental to quality of life. Thus, the combined treatments not
only kill microorganisms but also subject toxic and hazardous
compounds to chemical reactions followed by agglomeration or
capture of the resulting compounds together with small particles
including dust and smoke in an aerosol growth zone resulting in
aerosol particles sufficiently large to be removed from the air in
an electrostatic filter after the aerosol particles have been
 An advantage by the present inventive method is that the
pressure drop through the device is relatively small whereby the
energy consumption for the necessary air flow is low.
 A further advantage is that the air can be cleaned
sufficiently for reuse in larger and smaller rooms without or with
a minimum of fresh air supply. Accordingly, it is possible to
reduce the need for fresh air supply, and thereby reduce the
energy required to heat or cool and dehumidify said fresh air.
This makes the present invention suitable for use in rooms where
access to fresh air is difficult or impossible.
 In an embodiment of the inventive method the amount of
ozone delivered into the first zone (a) is regulated through a
feed-back system based on measurements by an ozone sensor situated
in the third zone (c) and/or an ozone sensor situated in the fifth
zone (e) and/or an ozone sensor situated in the sixth zone (f)
which ozone sensor(s) ensure that no residual ozone escapes into
the environment from the sixth zone (f).
 Thus in one embodiment of the inventive method the amount
of ozone delivered into the first zone (a) is regulated through a
feed-back system based on measurements by an ozone sensor situated
after the sixth zone (f) ensuring that no residual ozone escapes
into the environment from the sixth zone.
 To ensure a suitable amount of ozone in the first, second
and third zones the amount of ozone delivered into the first zone
(a) may be regulated through a feed-back system based on
measurements by an ozone sensor situated in the third zone (c).
 In a further embodiment the amount of ozone delivered into
the first zone (a) may be regulated through a feed-back system
based on combined measurements by ozone sensors after the sixth
zone (f) and/or between the fifth (e) and the sixth (f) zone
and/or in the third zone (c).
 The electrical charge provided to the particles in the
fourth zone (d) may be provided by a corona discharge or exposure
to ionizing radiation from a radioactive source. The corona
discharge is a simple, cheap and effective method for providing
electrical charge to the particles.
 Thus, in a preferred embodiment the electrical charge in
the fourth zone (d) is provided by a corona discharge.
 As explained above an important feature of the inventive
method is the formation and growth of an aerosol. In case the air
to be treated already contains sufficient moisture or other
aerosol growth promoters further addition of such promoters may be
unnecessary. However, depending on the air to be treated it is
often preferred to add one or more aerosol growth promoters such
as water and/or ammonia in the first zone (a).
 Accordingly, the air cleaning device according to the
invention preferably includes an injection system for the
injection of ammonia and/or water vapour in the first zone (a).
 A preferred catalyst for removal of residual ozone is
manganese dioxide or cerium oxide.
 The extent of applicability of the invention appears from
the following detailed description. It should, however, be
understood that the detailed description and the specific examples
are merely included to illustrate the preferred embodiments and
that various alterations and modifications within the scope of
protection will be obvious to persons skilled in the art on the
basis of the detailed description.
BRIEF DESCRIPTION OF THE DRAWING
 The invention is explained in detail below with reference
to the drawing, in which
 FIG. 1 is a schematic view of the inventive air cleaning
DETAILED DESCRIPTION OF THE INVENTION
 The present invention relates to a method and device for
removing pollution from air within a building ventilation system,
air in a room or air in connection with a local source of
pollution. In the following description, numerous specific details
are set forth in order to provide a thorough understanding of the
present invention. It will, however, be apparent to a person
skilled in the art that the present invention may be practiced
without these specific details.
 One application of the present invention is to clean air in
buildings, but a person skilled in the art would know that the
method may equally well be used in other installations, such as
without limitation, a small unit to clean air in a room or office,
a train, an airplane or any other confined space with no or
limited access to clean/fresh air. This small unit may or may not
 To assure that all contaminated air enters and passes
through the air cleaning system, the chamber air inlet and outlet
may be adapted to fit the existing ducts using methods known in
the art so that no air is allowed to bypass the system.
 In one embodiment, the chamber air inlet/outlet is adapted
to fit an existing HVAC air circulation system of, for example, a
 FIG. 1 shows schematically an embodiment of the air
cleaning device according to the invention arranged as a channel 1
with an inlet 2 for the air to be cleaned and an outlet 3 for the
cleaned air. The airflow is provided with per se known means such
as a fan (not shown). The inner side of the channel 1 comprises a
first zone (a), a second zone (b) a third zone (c), a fourth zone
(d), a fifth zone (e) and a sixth zone (f) placed successively in
the airflow direction from left to right in FIG. 1.
 A source of ozone 4 feeds ozone to the first zone (a) for
ozone treatment of the air. Furthermore, the first zone (a) is
optionally provided with one or more sources 5 of aerosol growth
accelerators feeding water, ammonia and/or other aerosol growth
accelerators into the first zone (a).
 The next zone, the second zone (b), has a source of
ultraviolet light for example UV lamps 6.
 After the treatment first with ozone and then with UV
radiation the air is maintained for sufficient time ensuring the
desired chemical reactions including aerosol formation and growth
in a third zone for aerosol growth (c). In the embodiment shown in
FIG. 1 the necessary time is obtained by a broadening of the
channel 1 giving a slower air velocity through the third zone (c).
In an alternative embodiment the third zone (c) could be elongated
using the same channel diameter throughout the zones (a)-(f).
 The fourth zone (d) contains a source of electrical charge
such as corona wires 7. By this means the solid and liquid
particles in the aerosol as well as larger molecules will be
electrically charged and caught in the fifth zone (e) containing
an electrostatic filter element 8.
 After the fifth zone (e) possible excess of ozone is
removed from the air in the sixth zone (f) containing an ozone
removal catalyst 9 where after the cleaned air leaves the channel
1 through the outlet 3.
 To control the ozone treatment and ensure that the air
leaving through the outlet 3 is substantially free of ozone, i.e.
below the acceptable threshold limiting value, one or more ozone
sensors are provided in the channel 1. Thus a first ozone sensor
10 may be placed near the end of the third zone (c), a second
ozone sensor 11 may be placed between the fifth and sixth zones
(e) and (f) and/or a third ozone sensor 12 after the sixth zone
(f) at the outlet 3.
 The amount of ozone provided by the ozone source 4 may be
regulated through a feed-back system based on measurements by one
or more of the sensors 10, 11 and 12.
 The channel 1 can also be provided with one or more further
sensors (not shown) for the measurement of other relevant
conditions such as temperature, relative humidity and
concentration of relevant contaminants. Together with the ozone
measurements such measurements may be usable for regulation,
control and monitoring purposes.
 a) First Zone: Treatment with Ozone
 In principle any source of ozone may be used in the
treatment with ozone in the first zone provided the ozone can be
delivered or generated in the desired amounts and in a safe way.
 In a preferred embodiment the ozone source is an ozone
generator as for example a corona discharge generator. While ozone
can be generated using UV light, this is too inefficient and
expensive at the present time. However, should the technology
advance for generating ozone using UV or other methods, it may be
incorporated into the present invention. Electric corona discharge
generators produce large quantities of ozone in a short time. The
passage of a high voltage, alternating electric current through an
air stream containing oxygen breaks down molecular oxygen to
atomic oxygen. These oxygen atoms may react to form ozone.
Commercial ozone generators are available in various shapes and
sizes with various capacities for generating ozone.
 In another preferred embodiment the ozone source is an
ozone generator from O3 Technology which is based on a technology
wherein oxygen or air is passed through parallel plates, and a
charge is maintained by an AC voltage. The amount of ozone that
enters the system is controlled by controlling the gas flow from
this generator. This unit will produce the majority of the ozone
in the system/reactor.
 Ozone oxidizes aromatic- and unsaturated-hydrocarbons.
However many kinds of compounds/chemicals, such as saturated
hydrocarbons and material trapped in the liquid or solid phases of
aerosols, do not react with ozone. Other indoor air pollutants
that do not react with ozone include carbon monoxide and
formaldehyde. Pollen and cigarette smoke react with ozone, but are
not removed by ozonolysis.
 The ozone acts as a biocide killing biological material,
such as bacteria, moulds and the like in the air. Ozone is a
naturally occurring substance which cleans air and removes odours
 The photochemical oxidation by O3, OH and other species in
the reaction region will mainly result in additional oxygen
containing functional groups (e.g. alcohols, carbonyls, acids,
etc.) on organic pollution molecules. Each functional group will
reduce the vapour pressure of the organic molecule increasing its
propensity for forming aerosols.
 The first zone may in addition to an ozone source also,
optionally, contain an injection system, i.e. a water source, for
the introduction of water vapour (which also includes humid air),
or small water droplets and the step of introducing ozone to the
zone is performed by forming either a mixture of water vapour
and/or water droplets and ozone before introducing the mixture
into the zone.
 A further, optional, addition may comprise an injection
system for the introduction of ammonia, i.e. an ammonia source.
 Addition of water or ammonia promotes aerosol formation.
 The dimensions of the first zone should be designed to
ensure the necessary treatment time t1 in the zone defined as the
time from the point where the air contacts the ozone to the point
immediately before it is subjected to UV. The necessary time t1
depends on various factors including the type of contamination,
the ozone source and the temperature. Based on the required flow
rates in an HVAC system and the dimensions, t1 should typically be
less than 15 seconds, preferably less than 10 seconds, such as
less than 8 seconds. Typically t1 should be above 1 second,
preferably above 5 seconds.
 b) Second Zone: Irradiation with Ultraviolet Light
 The ultraviolet light source used in the second zone may be
any conventional source providing UV-C light. Such UV-C light is
per se a biocide because it denatures DNA.
 Broad-spectrum ultraviolet light with a wavelength between
100 and 330 nm causes ozone and water to react forming highly
reactive ozone-based free radical intermediates, such as hydroxyl
radicals, that in turn react with and neutralize airborne
 UV-C light initially breaks down the ozone, exiting from
the first zone, into oxygen (O2) and an electronically excited
oxygen atom (O*) also termed an oxygen radical.
 In the presence of water this excited oxygen radical may
react with water (moisture) in the air and form hydroxyl radicals:
 Furthermore, the excited oxygen radical may react with a
hydrocarbon or with an oxygen molecule to reform ozone:
h[nu] is photon with a wavelength below 330 nm,
.OH is a hydroxyl radical,
RH is a hydrocarbon and
M is a collision partner, usually N2 or O2.
 Hydrocarbon radicals (.R) may react by addition or
fragmentation to obtain aldehydes, ketones, acids, alcohols or
other functionalised hydrocarbons.
 Thus in the second zone, some of the ozone will be broken
down into oxygen gas and hydroxyl radicals. It will also be
recognised by those skilled in the art that hydroxyl radicals can
also form peroxides, which themselves can act as biocides.
Therefore, these peroxides, in addition to the hydroxyl radicals,
assist in killing any living biological material which may enter
 Hydrocarbons may react with hydroxyl radicals:
 NO is present in the background air. Any kind of
hydrocarbon will make an oxy radical like the methoxy radical
above, and this radical can donate an H to O2 to form a stable
aldehyde/ketone and .HO2. Another source of H2O2 will be:
 Ozone in combination with UV radiation, which may form
hydroxyl radicals and/or peroxides, destroys microorganisms which
are not killed by the UV radiation as such.
 The free radicals formed by the interaction of ozone with
water in the presence of UV light, act as oxidants on cell walls
even before they penetrate inside the microorganisms where they
oxidize essential components such as enzymes and proteins.
 Ozone does not itself react significantly with either water
or oxygen in the absence of UV irradiation. Water and air merely
provide the medium in which ozone diffuses to react with organic
molecules such as those on the outside of the cell wall of
pathogens such as bacteria, viruses, moulds or pollen. UV
irradiation causes ozone to react with water and to decompose into
various highly reactive free radicals, such as hydroxyl radicals.
 The dimensions of the second zone should be designed to
ensure the necessary treatment time t2 in this zone defined as the
time from the point where the air it is subjected to UV to the
point where it leaves the UV radiation. The required time t2
depends on various factors including the type of contamination,
the treatment in the first zone, the UV radiation source and the
temperature. Typically t2 should be less than five minutes,
preferably less than 10 seconds, such as less than 8 seconds.
Typically t2 should be longer than 50 ms, preferably above 0.1
second, such as above 5 seconds.
 t2 should be relatively long time as carbon monoxide reacts
rather slowly with OH, in order to remove it a treatment time of
up to five minutes may be needed.
c) Third Zone: Aerosol Growth
 An important feature of the inventive method is that an
aerosol growth zone is provided after the second zone of UV
treatment. The purpose of this zone is to allow particles to grow,
removing pollution from the gas phase. One problem by prior art
air purification is that smells of cooking oil and diesel or
heating oil cannot be removed, even by ozone. Due to the use and
formation of .OH, other radicals derived from ozone or other
sources, and aerosols these pollutants can be removed by the
 The aerosol growth chamber may involve increasing the cross
sectional area of the flow duct in order to decrease the flow
rate, allowing time for the aerosols to grow.
 Accordingly, the dimensions of the third zone should be
designed to ensure the necessary retention time t3 in this zone
defined as the time from the point where the air leaves the second
zone of UV radiation to the point immediately before it enters
into the electrostatic filter in the fourth zone. The necessary
time t3 depends on various factors including the type of
contamination, the treatments in the first and second zones, and
the temperature. Typically t3 should be less than five minutes,
preferably less than 10 seconds, such as less than 8 seconds.
Typically t3 should be longer than 50 ms, preferably above 0.1
second, such as above 5 seconds.
d) Fourth Zone: Electrical Charge
 In the fourth zone the air leaving the second zone is
subjected to a source providing the molecules, particles and
droplets with an electrical charge enabling removal thereof with
an electrostatic filter in the following zone. In a preferred
embodiment the source of electrical charge is a corona discharge.
 A small, negligible, amount of ozone is produced by the
corona discharge wires. However, this is a by-product and it
contributes only a minor amount of the total ozone.
 The corona discharge in the fourth zone gives an electrical
charge to particles in the airstream allowing them to be removed
by the electrostatic filter in the fifth zone.
 It is possible that charging the particles will improve
aerosol particle trapping because the agglomeration of oppositely
charged particles will increase particle size, and the presence of
charge will improve the thermodynamics of particle growth.
 Heavier combined particles may precipitate (fall) out of
the air when two smaller particles agglomerate.
 The dimensions of the fourth zone should be designed to
ensure the necessary treatment time t4 in this zone defined as the
time from the point where the air it is subjected to an electrical
charge to the point immediately before it enters into the
electrostatic filter. The necessary time t4 depends on various
factors including the type of contamination, the treatments in the
first and second zones, the source of the electrical charge and
the temperature. This time need not be long. The requirement is
that the aerosols are charged before the electrostatic filter in
the fifth zone, and this is a fast process. In most cases t4
should be between 0.01 and 2 seconds, preferably 0.05-1.0 seconds,
such as 0.1-0.8 second.
e) Fifth Zone: Passage Through Electrostatic Filter
 Any electrostatic precipitator can be used in the present
invention. An electrostatic precipitator is a particulate
collection device that removes particles from a flowing gas (such
as air) using the force of an induced electrostatic charge.
Electrostatic precipitators are highly efficient filtration
devices that minimally impede the flow of gases through the zone,
and can efficiently remove fine particulate matter such as smoke
or dust from the air stream.
 Smaller particles, which are not heavy enough to
precipitate, are forced out through electrostatic filtration. The
electrostatic filtration comprises charged metal plates, with
alternating positive and negative charges, where positive
aerosol-particles will accelerate into the negative plates and
negative aerosol-particles will accelerate into the positive
 The addition of moisture, ammonia and/or other agents to
the incoming air improves the efficiency of trapping pollution and
pollution oxidation products through the mechanism of aerosol
 The dimensions of the fifth zone should be designed to
ensure the necessary treatment time t5 in this zone defined as the
time from the point where the air enters into the electrostatic
filter to the point immediately before it contacts the catalyst
for removal of residual ozone. The necessary time t5 depends on
various factors including the type of contamination, the
treatments and reactions in the first, second, third and fourth
zones, the type of electrostatic filter and the temperature.
Generally this time need not be long.
f) Sixth Zone: Removal of Residual Ozone
 A catalyst for removal of residual ozone is essential for
the present invention, since prolonged exposure to elevated
concentrations of ozone may irritate the respiratory system and
harm the lungs. The U.S. Environmental Protection Agency
classifies average 8-hour exposures of 85 to 105 parts per billion
as unhealthy for sensitive groups. Concentrations higher than this
increase the risks.
 To ensure that no harmful residual ozone will contaminate
the air that exits the sixth zone, one or more ozone removal
catalysts known in the art may be placed in that zone. Ozone
removal catalysts that can be used in various embodiments include,
manganese dioxide, all-aluminium catalyst, a carbon-supported
metal oxide, copper chloride-coated carbon fibres, carbon-iron
aerosol particles, and metal catalysts. CARULITE(R) (an inorganic
oxide) made by Carus Chemical Company is another ozone removal
catalyst. The catalyst may be solid-supported, and any solid
support may be used, especially glass or silica which substances
can catalyze ozone decomposition. The catalyst could also comprise
manganese dioxide containing paint.
 The catalyst for removal of residual ozone will have a
large surface area for contacting the air containing the residual
ozone. The sixth zone should also have a minimum pressure drop. To
this end the catalyst material may be applied in the shape of a
honeycomb (hexagonal shape).
 The unstable and highly reactive free radical intermediates
obtained from ozone, e.g. hydroxyl radicals, form stable products
including water and carbon dioxide that are not associated with
health risks when present in air at small concentrations. The
decomposition of ozone into stable oxygen is accelerated by
surfaces that act as substrates and/or reaction-sites for the
Feed-Back System/Ozone Sensors:
 The device will be equipped with ozone sensors for safety
and efficient control. For example an ozone sensor in the aerosol
growth chamber (third zone) would help to control ozone dosage in
response to changing pollution levels, and a sensor at the exit of
the device will ensure the overall safe operation.
 Examples of the chemical and/or physical reactions believed
to occur during the inventive method include:
 A. Unsaturated (including aromatic) hydrocarbons, for
example benzene and isoprene, react directly with ozone in the
first zone (a) where ozone is added, or with ozone found later in
the system for example in the aerosol growth chamber of third zone
(c). In principle, the reaction with ozone could take place
anywhere where ozone is present that is from the first zone (a)
where ozone is added until it is removed by the catalyst in the
sixth zone (f).
B. Saturated hydrocarbons including small species such as methane
or propane and larger molecules such as diesel or cooking oil, and
some other species including carbon monoxide and formaldehyde will
react with the hydroxyl radical .OH or other ozone-based radicals
formed when O3 is photolysed by the UV lamps in the presence of
C. The products of the reactions of type A and B will typically
condense onto aerosol particles, either pre-existing particles or
newly nucleated particles. (Sometimes however the products will
include volatile species as CO2 that are less polluting than the
precursors). All particles will be removed from the airstream by
the electrostatic filter.
 The present invention can be applicable in a variety of
places such as, but not limited to:
 1) cleaning air in buildings or rooms, for example offices,
kitchens and apartments, to improve air quality and reduce cost of
bringing in fresh air;
2) cleaning air in airplanes or other vehicles having confined
spaces with no or limited access to fresh/clean air which would
reduce disease transmission, reduce amount of fresh air brought in
3) provide clean air for people with allergies to chemicals or
4) providing clean sterile air in hospitals; and
5) at point sources of air pollution, to remove for example oil
used as part of a manufacturing process or fuel oil/diesel fumes.
 In the foregoing description, the invention has been
described with reference to specific embodiments thereof. It will,
however be evident that various modifications and changes may be
made to the invention without departing from the broader scope of
the present invention. The present application will be described
in further detail, by the following non-limiting examples.
 In a building with a total volume of free space of 60.000
m<3 >and provided with a standard Heating, Ventilating
and/or Air Conditioning system (HVAC) the air cleaning device as
described in FIG. 1 is installed in the HVAC system. The air flow
through of the device is 7200 m<3>/h giving a flow velocity
of 5 m/s in the first and second zones ensuring the required
treatment times in the first, second, fourth and fifth zones.
 The ozone source in the first zone provides 200 g/h ozone
giving an ozone concentration of 14 ppm inside the device.
 The outlet air, in comparison to the inlet air, shows
significantly reduced concentrations of several classes of
pollution including particles, scents including perfumes,
hydrocarbons, volatile organic compounds, carbon monoxide and
 The air cleaning system described in FIG. 1 is installed in
an office, apartment, hospital room, kitchen or other smaller room
with a volume of roughly 100 cubic meters. A freestanding enclosed
air cleaning unit as described in FIG. 1 including a fan is placed
in the room. It has a flow rate of 2 m/s and cross section of 0.02
square meters giving a flow rate of 0.04 cubic meters per second,
meaning that on average air will circulate through the device
every 40 minutes. The device can be used to eliminate air
pollution from the room, including volatile chemicals, smoke,
odours and allergens.
 The air cleaning system described in FIG. 1 is used to
treat air from a local source of pollution, for example oil
vapours given off by the cutting tool of a lathe or mill, in
association with use of diesel or heating oil in a building, or
when solvents are used for painting or gluing. The air from the
process is drawn through the device by a fan and the pollution is
prevented from entering the rest of the room or building.
 The above description of the invention reveals that it is
obvious that it can be varied in many ways. Such variations are
not to be considered a deviation from the scope of the invention,
and all such modifications which are obvious to persons skilled in
the art are also to be considered comprised by the scope of the