American Journal of Plant Sciences, 2013, 4, 1990-1997 Published Online October 2013 (
Depletion of the Ozone Layer and Its Consequences:
A Review
Anjali Aggarwal1, Reeta Kumari1, Neeti Mehla1, Deepali1, Rishi Pal Singh2, Sonal Bhatnagar3,
Kameshwar Sharma4, Kuldeep Sharma5, Amit Vashishtha1, Brijesh Rathi2*
1Department of Botany, Sri Venkateswara College, University of Delhi, New Delhi, India; 2Department of Chemistry, Sri Venkates-
wara College, University of Delhi, New Delhi, India; 3Department of Botany, Deshbandhu College, University of Delhi, New Delhi,
India; 4Department of Biochemistry Sri Venkateswara College, University of Delhi, New Delhi, India; 5Department of Botany, Uni-
versity of Delhi, New Delhi, India.
Email: *
Received July 11th, 2013; revised August 12th, 2013; accepted September 12th, 2013
Copyright © 2013 Anjali Aggarwal et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Ozone (O3) is a stratospheric layer that plays important role in providing support to humans for their survival. It is an
essential factor for many global, biological and environmental phenomena. The ultra-violet (UV) rays emitted from sun
are captured by ozone and thereby provide a stable ontological structure in the biosphere. Various anthropogenic activi-
ties such as emissions of CFCs, HCFCs and other organo-halogens lead to the depletion of ozone. The ozone depletion
resulted in secondary production of an ozone layer near the ground (terrestrial ozone layer), which is responsible for
adverse effects on plants, humans and environment with increased number of bronchial diseases in humans. The muta-
tions caused by UV rays result in variation in morphogenic traits of plants which ultimately decreases crop productiv-
ity. However, UV radiation is required in optimum intensity for both plants and animals. This review takes into an ac-
count the wide ranging effects of ozone depletion with a majority of them being detrimental to the plant system.
Keywords: Ozone Depletion; Ultra-Violet Radiation; Chlorofluorocarbons; Plants; Ecosystem
1. Introduction
Some 2 billion years ago, rising atmospheric oxygen con-
centrations helped Earth’s atmosphere to build up ozone
and gradually led to the formation of the stratosphere.
The photo-dissociation of oxygen molecules by high-
energy solar photons (175 - 242 nm) in the stratosphere
results in the production of ozone. This process leads to
the release of single oxygen atoms which later combine
with intact oxygen molecules to form ozone [1].
Ozone (O3) forms a protective layer present in the
earth’s atmosphere and it is found in the lower portion of
stratosphere, which is ~12 to 50 km above earth’s surface.
Discovered first by Charles Fabry and Henri Buisson in
1913, its properties were explored by G. M. B. Dobson.
This layer acts as a shield to protect the earth against
harmful ultraviolet (UV) radiation from the sun [2,3].
UV radiation is part of the solar electromagnetic spec-
trum, with wavelengths shorter than those of visible light,
but longer than X-rays, and is widely known as a geno-
toxic environmental agent that affects ecosystem and hu-
man population [4]. Ozone, a variant of oxygen is a poi-
sonous gas; and its formation and destruction is a conti-
nuous phenomenon. Ozone occurs at two levels, the stra-
tospheric ozone and the tropospheric ozone. The tropo-
spheric (ground) ozone varies with the daylight varia-
tions. Ozone near-ground is a pollutant and its production
is enhanced due to air pollutants, like, nitrogen oxides
(NOx) and volatile organic compounds (VOCs). The in-
crease in terrestrial ozone particulates results in their en-
hanced scattering and improved absorption of UV-B ra-
diations, which contributes to global warming by acting
as a greenhouse gas and also shows harmful effects on
both animals and plants. An increase in the UV-B radia-
tion is one of the major causes for enhanced production
of carbon monoxide from dead organic matter and re-
lease of nitrogen oxides. The ground ozone along with
carbon monoxide is responsible for acid rain which caus-
es damage to lung tissue and its long-term exposure can
cause permanent tissue damage. Tree leaf and needle
losses are linked to acidification and high percentage of
*Corresponding author.
Copyright © 2013 SciRes. AJPS
Depletion of the Ozone Layer and Its Consequences: A Review 1991
ground ozone. Ground ozone concentration is lower in
Polar and equatorial regions. The sub-tropical ground
ozone concentration in Northern hemisphere is twice the
corresponding region in Southern hemisphere [5].
Although, O3 is present in low concentration (~0.6
ppm) in the atmosphere, it plays an important role by ef-
ficiently screening out harmful radiations. The UV rays
are of shorter wavelengths ranging from 100 - 280 nm
(UV-C), 280 - 315 nm (UV-B) to 315 - 400 nm (UV-A).
Of the UV rays, UV-C is completely absorbed by the
ozone layer and only 5% of UV-B reaches the earth sur-
face, while nearly 95% of UV-A is able to penetrate the
atmospheric layers [6]. UV radiations affect all the ele-
ments of the biome including plants, pathogens, herbi-
vores, carnivores and microorganisms. These radiations
are harmless to DNA but can cause genetic damage to
the skin and are responsible for increasing the total reac-
tive oxygen level. UV-A can be further divided into UV-
A1 (340 - 400 nm) and UV-A2 (320 - 340 nm). UV-A1
radiations damage DNA through the generation of radical
and non-radical reactive oxygen species (ROS) as an in-
direct response, while UV-A2 radiations can cause dam-
age both indirectly as well as directly through generation
of ROS and DNA photoproducts, respectively. The ROS
so formed can actually lead to oxidative damage of DNA
by the formation of 8-oxo-7, 8-dihydro-2’-deoxyguano-
sine and thymidine glycol, lipid peroxidation, and by
cross-linking of proteins such as collagen. When human
skin is exposed to UV-A radiation, cyclobutane pyrimi-
dine dimers are produced in significant amount, leading
to photo-carcinogenesis of the skin [7]. The UV-B radia-
tions play a vital role in the synthesis of vitamin D,
which involves two steps: formation of pre-vitamin D and
its thermo conversion. UV-B radiations exhibit the abil-
ity to transmute the biochemical and physiological path-
ways of cells by structural changes in biomolecules, which
ultimately cause diseases such as skin cancer, photo-ag-
ing, immuno-suppression and cataracts in the human po-
pulation. The ability of UV-B to penetrate water bodies
affects the cellular DNA in phytoplankton, zooplankton
and ichthyoplankton that led to increased mortality due
to physiological anomalies [6]. The sunspot cycle leads
to an increase in UV-B influxes during its various stages,
which causes stratospheric temperature fluctuations. The
solar maxima of the sun-spot cycle are responsible for
increased UV-C radiation which stimulates the formation
of stratospheric ozone.
The ozone depletion over the Antarctic has been no-
ticed since 1970s and the Arctic region has also been wit-
nessing the occurrence of an ozone-hole during the last
decade. The overall depletion has been increasing at the
rate of 0.5% per year since 2000, because of the exten-
sive use of ozone depleting substances (ODSs) such as
propellants (in the manufacture of soft and hard foams),
refrigeration, air conditioning and as cleaning solvents
[8]. The atmospheric release of ODSs such as halocar-
bons including chlorouorocarbons (CFCs), hydro-chlo-
rofluorocarbons (HCFCs), hydro-fluorocarbons (HFCs)
and bromofluorocarbons (BFCs) has led to a significant
decrease of the ozone layer. Halocarbons are artificially
synthesized gases consisting of carbon and one or more
halogens (fluorine, chlorine, iodine and bromine) releas-
ed in enormous amounts and they are responsible for an
increased concentration of Cl and Br in the atmosphere
[9]. CFCs (Freons) are a group of colorless, non-combu-
stible liquids which are highly volatile substances and
poorly soluble in water. Hence, they are mainly released
into the air through evaporation during their production
and use. These do not bind to soil strongly and thus they
can easily leach to the groundwater. The use of these
chemicals has been phased out because of their deleteri-
ous effects on ozone layer but they may still be found as
an environmental hazard as they degrade slowly in
CFCs are also found to have health effects which in-
clude short-term (acute) and long-term (chronic) effects.
Exposure to pressurized CFCs can cause frostbites to the
skin and to the upper airway if inhaled. At high tempera-
ture, they can degrade to more acutely toxic gases such
as chlorine and phosgene. At high concentrations, inhala-
tion of CFCs affects the central nervous system (CNS)
with symptoms of alcohol-like intoxication, reduced co-
ordination, light headedness, headaches and convulsions.
Disturbances in heart rhythm can occur at very high con-
centrations and had even caused some deaths from inten-
tional sniffing. Increased health impacts had been ob-
served with the increase in CFCs concentration [10].
Apart from this, traces of gaseous nitrogen compounds,
such as NO, NO2 and N2O, present in small quantities in
the atmosphere are considered to be the largest ozone-
depleting substances emitted by human activities exceed-
ing the contribution of chlorofluorocarbons [11]. If these
chemicals escape into the environment, they drift up the
stratosphere where Cl and Br radicals are liberated by the
action of ultraviolet light on their molecule and act as a
catalyst affecting the ozone layer at 78˚C (critical tem-
perature required by chlorine to breakdown ozone at sur-
face of polar stratospheric cloud crystals), where they
lead to a complete breakdown of ozone and thus reduce it
to oxygen molecules. One chlorine or CFC molecule can
destroy 100,000 ozone molecules. As a result the ozone
layer becomes incapable of absorbing UV radiations
which enter the earth’s surface and affect various living
The CFCs have been phased out in both developed and
developing countries since 1996 and 2010, respectively.
Alternative to CFCs, HCFCs will also be phased out in
both developed and developing nations by the year 2020
Copyright © 2013 SciRes. AJPS
Depletion of the Ozone Layer and Its Consequences: A Review
and 2030, respectively. The World Meteorological Or-
ganization (WMO), 1995 predicted that the depletion of
the ozone layer peaked around 1998 and the layer would
slowly recover by 2045 [12]. But many researchers do
not agree with these predictions [13,14] and express their
concern regarding a delayed recovery of stratospheric
ozone [15]. Thus, at present the anthropogenic damage to
the ozone layer strongly exceeds its recovery. There is a
burgeoning need to reduce the production of industrial
products causing ozone depletion and global warming.
The Vienna Convention for protection of Ozone layer
was adopted by 43 nations in 1985. It addressed the im-
portance of conservation of Ozone layer and established
global mechanism for research, monitoring and exchange
of information. Two months later, its adoption by the Bri-
tish scientists announced the presence of Ozone hole
over Antarctic triggering concern about human safety.
Nearly 60 plus countries met at Montreal in 1987 to come
up with a protocol on curbing the Ozone Depleting Sub-
stances (ODSs). For the first time the CFCs were identi-
fied as a major culprit and CFCs-11, 12, 13, 114 and 115
and Halons-1211, 1301 and 2402 were targeted for re-
duction. The onus for reduction was more on developed
countries, but to encourage developing countries for join-
ing the protocol it was incentivized through favorable
trade benefits. The 1990 London Amendment brought
other ODSs into ambit for a total phase-out by 2000, viz.,
Carbon tetrachloride, and trichloroethane. The signato-
ries have been given ten year time for total phase out for
enlisted ODSs. It has targeted to 2040 for a total phase
out of all kinds of ODSs [16]. The Kyoto Protocol sought
reduction of CO2 emissions and was signed in 1997. But,
it became a trading house of carbon credits, which allow-
ed developed countries to pass off their commitments on-
to the less developed countries, which had low emissions
due to low development. The Kyoto takes 1990 as the
base year for green-house gases emission levels (GHGs).
In the case of hydrofluorocarbons (HFCs) and perfluoro-
carbons (PFCs) it is 1995. But, the base year has been
fluctuating for individual countries [17].
2. Consequences of Ozone Layer Depletion
The ozone layer plays an important role in the biology
and climatology of the earth’s environment. Radiations
below the wavelength of 3000 Å are biologically harmful
and ozone helps to filter-out these radiations. The strato-
spheric ozone layer protects life on earth by absorbing
the damaging, high-energy UV-C radiation. Depletion of
stratospheric ozone increases the concentration of terres-
trial ozone, which is considered harmful for health. Ozone
depletion resulted in global warming by increase of the
atmospheric temperature by 5.5˚C [18]. Exposure to UV
rays due to ozone depletion causes innumerable biologi-
cal hazards such as variation in the physiological and de-
velopmental processes, reduced growth and productivity
of plants. Indirect damage caused by the UV-B includes
changes in the plant form and distribution of nutrients
within the plant. These changes have important implica-
tions for plant competitive balance, herbivory, plant dis-
eases, and biogeochemical cycles. Exposure to solar UV-
B radiation has been shown to affect both orientation me-
chanisms and mortality in phytoplankton, resulting in re-
duced survival rates for these organisms. Solar UV-B ra-
diation has also been found to cause damage to the early
developmental stages of fish, shrimp, crab, amphibians
and other animals. Most severe effects are decreased re-
productive capacity and impaired larval development. In-
crease in solar UV radiations affect terrestrial and aquatic
biogeochemical cycles, thus altering both sources and
sinks of greenhouse and chemically important trace gases
such as carbon dioxide (CO2), carbon monoxide (CO),
carbonyl sulphide (COS) and possibly other gases, in-
cluding ozone. These potential changes would contribute
to the biosphere-atmosphere feedbacks that attenuate or
reinforce the atmospheric build-up of these gases. Syn-
thetic polymers, naturally occurring biopolymers, as well
as some other materials of commercial interest are ad-
versely affected by solar UV radiation [19].
2.1. Effects of Ozone Depletion on Plants
A large number of negative effects of UV-B radiations
on the global plant productivity due to stratospheric ozone
depletion have been observed. Earlier studies report the
loss of 50% crop plants in European countries due to
UV-radiations that enter the earth’s surface. It adversely
affects the rate of photosynthesis in plants resulting in
decreased agriculture production. UV-B radiations affect
the plant’s height, fresh weight, dry weight and its ash
contents which reflect the deleterious effects of UV-B on
crop plants [20]. UV enhances the rate of evaporation
through stomata and results in decreased soil moisture
content thus, ultimately affects the growth and develop-
ment of crop plants. Ozone depletion adversely affects
the weather which influences the crop production due to
plant injury and development of various diseases [13].
The leaf expansion is also inhibited by UV radiations
[21-23]. Other morphogenetic effects include reduced
leaf size, increased leaf thickness [24-26] and leaf mass
per unit area [27,28], accumulation of leaf surface waxes
[29] and reduction in the total number of leaf (observed
in species Cucumis sativus and Lactuca sativus) [30,31].
Besides morphological variations in response to UV-B,
anatomical changes in plants such as injury or death of
epidermal cells have also been reported. Leaf browning
or bronzing in beans, leaf desiccation in collards, radish,
cucumber, squash and epinasty of leaves in beans have
also been observed [21]. However, recent studies focus
on utilization of UV-B radiations, for modified resistance
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Depletion of the Ozone Layer and Its Consequences: A Review 1993
to pest and disease attack, and increased global crop pro-
duction by alterations in the secondary metabolism, en-
hanced photo-protection, and up-regulation of antioxida-
tive response [32]. Bacteria are sensitive to ultraviolet ra-
diations and hence are killed instantly in the presence of
UV light. The ozone reduction affects the cyanobacterial
growth in rhizospheric zone of legumenous plants, which
helps in retaining nitrogen content and thus gets adver-
sely affected [33]. Recent studies clearly indicate that
UV radiations can be exploited as a major tool for en-
hancing crop growth and production by exogenous ap-
plication of phytohormones on growing plants and seeds
Extensive studies have been carried out to know the
adverse effects of UV-B radiations on the plant morpho-
logy, physiological processes and on biologically impor-
tant molecules such as the nucleic acid, proteins, pig-
ments and lipids [37-41]. UV-B radiations do not affect
the seed germination in most of weedy species due to
presence of hard seed coat [42]. Lactuca sativa showed
improved seed germination when exposed to 254, 265,
334 and 405 nm of radiations as compared to those
grown in dark [43]. However, rapid seed germination and
highly branched roots were observed in kale, cabbage,
radish and agave seeds when treated with UV radiation
when compared to the control [44]. A study on cotton
(Gossypium hirsutum) showed that excess UV-B dam-
ages the developing shoots of cotton resulting in the re-
duction of dry matter, Zn mobilization and leaf expan-
sion [45]. UV-B radiations inhibit the radical elongation
but shoot growth is not affected by these radiations
which indicate that roots are more sensitive to UV light.
Exposure to some UV-B radiations during the day time
may result in the selection of more efficient UV-protec-
tion mechanism which makes shoots less susceptible to
the harmful effects of these radiations.
All the species respond differently to UV-B radiation
under different experimental approaches as the effects are
either stimulating, depressing, or have no effect on their
growth and physiology [8,19,42-46]. Availability of wa-
ter influences the effect of UV-B on bryophytes as these
are poikilohydric and unavailability of water causes leaf
cells to dry out and ceases the metabolism [47]. Desicca-
tion tolerance in many bryophytes might be assisted by
the development of antioxidant and photo-protection me-
chanisms that scavenges or minimizes the production of
reactive oxygen species. Various experiments have been
carried out that showed variability in the bryophyte per-
centage cover, sporophyte abundance, annual growth,
sclerophylly index (quotient between shoot mass and sur-
face area of fresh prostrate apex), and chlorophyll con-
centration of different species under drier and mesic sites
[48]. There have been several studies during the last de-
cade which elucidate the negative implications of UV re-
sponse on the plant development. These inferences are
based on the studies that have frequently used unbalanc-
ed spectral manipulations, unrealistically high supple-
mentary fluxes and in vitro exposures of single cellular
components. This has introduced ambiguity to overall
perception of UV radiation as a stimulus for the plant
development. The significant questions remain with re-
gard to the various consequences of UV radiation for
agro-ecosystems and other ecological systems. Many stu-
dies using a wide range of pathogens have demonstrated
that UV can also kill fungal spores and inhibit their ger-
mination [49,50].
2.2. Effects of UV-B Radiations on Aquatic and
Terrestrial Ecosystem
Ozone layer plays an important role in the evolution of
terrestrial plants by the development of phenolic polymer
metabolism induced by UV-B [51]. Flavonoids and lig-
nin present in gymnosperms and angiosperms (but absent
in algae) are the major products of this metabolism. The
solar UV radiations stimulate the enzymes such as PAL
(Phenylalanine Ammonium Lyase) and CHS (Chalcone
Synthase) that catalyses transformation of phenylalanine
to trans-cinnamic acid. This results in the formation of
complex phenolic compounds such as flavonoids, lignins
and tannins. Accumulation of these compounds reduces
the penetration of UV wavelengths deeper into the leaves
thus protects photosynthetic machinery and other essen-
tial components from damage [24,52]. Hence these com-
pounds acts as UV screen and make plants resistant to
solar UV. It has been inferred that plant cells receive da-
mage from exposure to UV-B as it induces change in the
proteins and nuclear DNA. Young bud and leaves are
considered more susceptible than the mature plant parts.
Although there are still inconclusive observations of UV
induced photo-morphogenesis particularly with regard to
signal transduction and other early stage responses. As
DNA is a strong UV absorbent [53], accumulation of fla-
vonoids provides photo-protection to limit DNA damage
[51,54,55] and the photo-repair of DNA lesions through
photo-reactivation processes [56]. The variability in UV
radiations on earth had partly governed the evolution of
plants and animals [51]. Eco-physiological studies have
provided sufficient evidence suggesting that the plant
growth inhibition, caused by the high and ambient doses
of ultraviolet radiations could be related to DNA damage
leading to various mutations and neoplasia [18,19]. DNA
damage indicates acute effects of short exposures to UV-
B because short-wave UV radiation can disturb most bio-
logical macromolecules, including proteins, lipids, and
nucleic acids. UV-B effects on DNA are also responsible
for cryptic transposable elements in some species, which
might result into mutations beyond the extent of immedi-
ate DNA damage [15]. Studies in animal systems suggest
Copyright © 2013 SciRes. AJPS
Depletion of the Ozone Layer and Its Consequences: A Review
that damage to DNA is the principal cause of cell death
and degeneration.
2.3. Effects of Ozone Depletion on Human
Exposure of UV radiations leads to the formation of pa-
tches on skin and weakens human immune system. The
UV radiations damage skin either by damaging melano-
cyte cells or by causing sun-burns due to faster flow of
blood in capillaries of exposed areas. Malignant melano-
ma, a type of skin cancer is also caused by UV exposure
which is less common but far more dangerous. Its rela-
tionship with UV exposures has not been understood yet
but it is thought both UV-A and UV-B are involved [57].
Studies showed 10% increase in UV-B resulted in 19%
increase in melanomas in men and 16% in women. More
than one million new cases of non-melanoma skin can-
cers are reported in the US only. The susceptibility to
cancer is often conspicuous in xeroderma pigmentosum,
a disorder leading to extreme photosensitivity and early
onset of cutaneous malignancies. It may also cause leu-
kemia and breast cancer. UV exposure to human eye da-
mages cornea and lens leading to photokeratitis, cataract
and/or even blindness. Emphysema, bronchitis, asthma
and even obstruction of lungs may be caused on exposure
of UV light to human beings. Excess of UV light expo-
sure causes DNA breakage, inhibition and alteration of
DNA replication and premature ageing in humans [32].
Basal and squamous cell carcinomas are the most com-
mon type of cancers in humans due to excess UV expo-
sure. The mechanism involved for the induction of these
cancers by UV light includes absorption of UV-B radia-
tion causes the pyrimidine bases in the DNA molecule to
form dimers, resulting in transcriptional errors during
DNA replication. These cancers are rarely fatal. Scien-
tists estimate that every 1% decrease in stratospheric ozone
would increase the incidences of these cancers by 2%
[58]. Increased surface UV leads to increased troposphe-
ric ozone which is a health risk as ozone is toxic due to
its strong oxidant properties [59]. Besides producing vi-
tamin D, UV-B radiation itself is correlated with skin
cancer, photoaging, immuno-suppression and cataracts,
to mention just a few of the harmful effects. Nevertheless,
the overproduction, leads to the degradation of already
formed vitamins, thereby attaining toxic levels and is as-
sociated with high mortality [60].
3. Conclusion
A large number of environmental problems such as ozone
depletion and global warming are associated with in-
creased development and economic growth throughout
the world during the last century. The halocarbon refri-
gerants used in the refrigeration and air-conditioning
systems have become a subject of great concern for the
last few decades. The earth is the only planet that sup-
ports life, and thus preserving ozone layer and reducing
the release of greenhouse gasses are the essential steps
required for the protection of life. The stratospheric ozone
helps in limiting the influx of harmful UV-B and green-
house gas. UV radiation imposes a significant influence
on the growth and development of fungi, plants and hu-
mans. The fungal diseases on plants have receding ef-
fects due to the inhibition of sporulation caused by expo-
sure to UV radiation. In plants, UV radiations resulted in
reduced plant height, fresh-weight, dry-weight, seed ger-
mination and seedling growth. The plants also showed
mutant formation that alters the growth properties which
are detrimental to optimal utilisation of the plant prod-
ucts. The exposure of humans to UV can lead to various
diseases such as skin cancer, cataract and mutant DNA.
There have been a significant number of studies till date
which have described negative implications of UV re-
sponse for plant development. However, numerous stud-
ies have also reported the positive aspects of UV radia-
tions wherein it plays an important role in the evolution
of plant and animal species. Therefore, one has to take
the larger argument of the protective role of ozone layer
along with its phytogenic response. For this purpose, dif-
ferent conventions and protocols have been adopted to
control ozone depletion and its impacts on all life forms.
These include Vienna Convention in 1985 followed by
the Montreal Protocol in 1987 and the Kyoto Protocol in
1997. These protocols banned the use of ozone depleting
substances (ODSs) in both developed and developing
countries. Chlorofluorocarbons (CFCs) have been found
to be the main cause of ozone depletion and have many
health impacts. Stratospheric ozone depletion leads to the
formation of a secondary ozone layer near ground called
terrestrial ozone. Air pollutants enhance the production
of ground ozone. Terrestrial ozone acts as a green-house
gas and leads to global warming by the absorption of so-
lar UV-B radiations.
4. Acknowledgements
The authors gratefully acknowledge Dr. P. Hemalatha
Reddy, Principal (Sri Venkateswara College) University
of Delhi for providing institutional support.
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List of Abbreviations Used
ROS Reactive Oxygen Species
CFC Chlorofluorocarbon
UV Ultraviolet
HCFC Hydro Chlorofluorocarbon
WMO World Meteorological Organisation
ODS Ozone Depleting Substances
BFC Bromofluorocarbon
CO2 Carbon Dioxide
CO Carbon Monoxide
COS Carbonyl Sulphide
PAL Phenylalanine Ammonium Lyase
CHS Chalcone Synthase
HFC Hydro-Fluorocarbon
DNA Deoxyribonucleic Acid
Cl Chlorine
Br Bromine
Zn Zinc
NO Nitrogen Oxide
NO2 Nitrogen dioxide
N2O Nitrous Oxide
C Carbon
US United States
CNS Central Nervous System
VOC Volatile Organic Compounds
GHGs Green-House Gases
PFCs Perfluorocarbons
O Ozone
nm Nanometer
ppm Particle per Million
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