. An air pollutant is any substance which may cause harm to humans,
animals, vegetation or material. Anthropogenic activities are the major cause
of environmental air pollution although physical activities such as volcanoes,
fire, etc. may also release different pollutants in the environment. Hazardous
chemicals which are unhealthy to living organisms can escape to the environment
by accident, but a number of air pollutants can be released from industrial
facilities and other activities which may cause adverse effects on human health
and the environment (CEE,2012).
Air pollutants include suspended particles and gases including carbon
monoxide, volatile organic compounds (VOCs), nitrous oxides, sulphur dioxide,
and ozone. Suspended particles are called particulate matter (PM), which is a mixture
of tiny particles and liquid droplets that includes acids, organic chemicals,
metals, and dust. PM is measured by size: PM2.5 is 2.5 micrometres (?m) in diameter.
For comparison, a human hair is 70?m in diameter.
As far as humans are concerned an air pollutant may cause or
contribute to an increase in mortality or serious illness or may pose a present
or potential hazard to human health. The determination of whether or not a
substance poses a health risk to humans is based on clinical, epidemiological,
and/or animal studies which demonstrate that exposure to a substance is
associated with health effects. In the context of human health, ”risk” is the
probability that a detrimental health effects may occur.
The main change in the atmospheric composition is primarily due to
the combustion of fossil fuels, used for the generation of energy and
transportation. Variant air pollutants have been reported, differing in their
chemical composition, reaction properties, emission, persistence in the
environment, ability to be transported in long or short distances and their
eventual impacts on human and/or animal health. However, they share some
similarities and they can be grouped to four categories:
a. Gaseous pollutants (e.g. SO2, NOx,
CO, ozone, Volatile Organic Compounds)
b. Persistent organic pollutants (e.g. dioxins).
c. Heavy metals (e.g. lead, mercury).
d. Particulate Matter.
Gaseous pollutants contribute to a great extent in
composition variations of the atmosphere and are mainly due to combustion of
fossil fuels (Katsouyanni, 2003). nitrogen oxides are emitted from fossil
fuel burning and ammonia is emitted from agricultural activities. Nitrogen is a
nutrient and its increased deposition affects plant biodiversity. In addition,
nitrogen contributes to acidification of soils and waters. Nitrogen oxides are
emitted as NO which rapidly reacts with ozone or radicals in the atmosphere
forming NO2. The main anthropogenic sources are mobile and stationary
Moreover, ozone in the lower atmospheric layers is formed by a
series of reactions involving NO2 and volatile organic compounds, a process
initiated by sun light. CO, on the other hand, is a product of incomplete
combustion. Its major source is road transport too. While the anthropogenic SO2 emitted from fossil fuel burning
(industry, households, transport), sulphur dioxide causes acidification of
soils, streams and lakes and leads to erosion of building materials, including
cultural heritage. volcanoes and oceans are its major natural sources. The
latter contribute only ~2% of the total emissions.
Finally, a major class of compounds that fuel combustion and
especially combustion processes for energy production and road transport are
the major source of emission are the so called volatile organic compounds
(VOCs). Volatile organic compounds (VOCs) are emitted as gases from certain
solids or liquids. VOCs include a variety of chemicals, some of which may have
short- and long-term adverse health effects. They are compounds that have a
high vapor pressure and low water solubility. Many VOCs are human-made
chemicals that are used and produced in the manufacture of paints, pharmaceuticals,
VOCs typically are industrial solvents, such as trichloroethylene;
fuel oxygenates, such as methyl tert-butyl ether (MTBE); or by-products
produced by chlorination in water treatment, such as chloroform. VOCs are often
components of petroleum fuels, hydraulic fluids, paint thinners, and dry
cleaning agents. (EPA, 2006)
Persistent organic pollutants – Persistent organic
pollutants (POPs) are chemicals that have ability for long-range transport,
persistence in the environment, ability to bio-accumulate and bio-magnify in
ecosystems, as well as their significant detrimental effects on man’s health
and the environment. It is because of these reasons they are of global concern.
Exposure of human to these chemicals can take place in varieties of ways: majorly
through the food we eat, but also through the air we breathe, in the indoors, outdoors,
and at the workplace. Also, in our daily lives, the products we used may
contain POPs, which have been added to improve product characteristics, such as
surfactants or flame retardants. As a result, POPs can be found almost everywhere
on our planet in concentrations that are measurable.
Organochlorine pesticides, such as DDT, industrial chemicals, most
notably polychlorinated biphenyls (PCB), as well as unintentional by-products
of many industrial processes, especially polychlorinated dibenzofurans (PCDF) and
dibenzo-p-dioxins (PCDD) commonly known as ‘dioxins are all the most commonly
POPs can bio-accumulate in organisms and bio-magnify in the food
chain. The highest concentrations of POPs are thus found at the top of the food
chain in the organisms. Therefore, background levels of POPs can be found in
the human body.
Human exposure to both high levels and also in some cases low
levels of POPs can lead to increased birth defects. neurobehavioral impairment,
increased cancer risk, alteration of the immune system, reproductive disorders,
endocrine disruption and Geno toxicity
Heavy metals Heavy metals are natural components of
the Earth’s crust. They include metallic chemical element that has a relatively
high density and is poisonous or toxic at low concentrations. Cadmium (Cd), mercury
(Hg), arsenic (As), thallium (Tl), chromium (Cr), and lead (Pb) are examples of
heavy metals which can neither be degraded nor destroyed.
Heavy metals may enter our bodies in small quantities through air,
drinking water and food. As an essential trace element, some heavy metals (e.g.
copper, selenium, zinc) are important to regulate the human’s body metabolism.
However, at higher concentrations they can lead to poisoning. The poisoning be
from high ambient air concentrations, drinking-water contamination (e.g. lead
pipes), near emission sources, or intake via the food chain.
Heavy metals are lethal because they can easily bio accumulate.
Bioaccumulation means an increase in the concentration of a chemical in a
biological organism over time, compared to the chemical’s concentration in the
environment. In living things, compounds tend to accumulate in living things
any time they are taken up and stored faster than they are metabolized (broken
down) or excreted. Heavy metals can enter a water supply by industrial and
consumer waste, or even from acidic rain breaking down soils and releasing
heavy metals into streams, dams, ponds, lakes, rivers, and groundwater.
Particulate matter (PM) – Particulate Matter (PM) includes both liquid droplets and solid
particles found in air. Many natural sources and man-made emit PM directly or
emit other pollutants that react in the atmosphere to form PM. These liquid and
solid particles come in a wide range of sizes. Particles that are less than 10 micrometres
in diameter are likely to pose the greatest health concern because they can be
inhaled and accumulate in the respiratory system. Particles with diameters
between 2.5 and 10 micrometres are called coarse particles while fine particles
are particles less than 2.5 micrometres. Sources of fine particles include all
types of combustion (motor vehicles, power plants, wood burning, etc.) and some
industrial processes. Sources of coarse particles include crushing or grinding
operations, and dust from paved or unpaved roads.
The Environmental Protection Agency uses its Air Quality Index to
provide general information to the public about air quality and associated
health effects. An Air Quality Index (AQI) of 100 for any pollutant corresponds
to the level needed to violate the federal health standard for that pollutant.
For PM2.5, an AQI of 100 corresponds to 40 micrograms per cubic meter (averaged
over 24 hours) — the current federal standard. An AQI of 100 for PM10
corresponds to a PM10 level of 150 micrograms per cubic meter (averaged over 24
3. Routes of exposure
Humans enter in contact with different air pollutants primarily via
inhalation and ingestion, while dermal contact represents a minor route of
exposure. Air pollution contributes, to a great extent, to the contamination of
food and water, which makes ingestion in several cases the major route of
pollutant intake (Thron, 1996). Via the gastrointestinal and respiratory tract,
absorption of pollutants may occur, while a number of toxic substances can be
found in the general circulation and deposit to different tissues. Elimination
occurs to a certain degree by excretion (Madden and Fowler, 2000).
4. Health effects
Sporadic air pollution events, like the historic London fog in 1952
and a number of short and long term epidemiological studies investigated the
effects of air quality changes on human health. A constant finding is that air
pollutants contribute to increased mortality and hospital admissions
(Brunekreef and Holgate, 2002). The different composition of air pollutants,
the dose and time of exposure and the fact that humans are usually exposed to
pollutant mixtures than to single substances, can lead to diverse impacts on
human health. Human health effects can range from nausea and difficulty in
breathing or skin irritation, to cancer. They also include birth defects,
serious developmental delays in children, and reduced activity of the immune
system, leading to a number of diseases. Moreover, there exist several
susceptibility factors such as age, nutritional status and predisposing
conditions. Health effects can be distinguished to acute, chronic not including
cancer and cancerous. Epidemiological and animal model data indicate that
primarily affected systems are the cardiovascular and the respiratory system. However,
the function of several other organs can be also influenced (Cohen et al.,
2005; Huang and Ghio, 2006; Kunzli and Tager, 2005; Sharma and Agrawal, 2005).
4.1. Effects of air pollutants on different organs and systems
4.1.1. Respiratory system
Because most of the pollutants enter the body through the airways,
the respiratory system is in the first line of battle in the onset and
progression of diseases resulted from air pollutants. Depending on the dose of
inhaled pollutants, and deposition in target cells, they cause a different
level of damages in the respiratory system. In the upper respiratory tract, the
first effect is irritation, especially in trachea which induces voice
disturbances. Air pollution is also considered as the major environmental risk
factor for some respiratory diseases such as asthma and lung cancer.70,71 Air
pollutants, especially PMs and other respirable chemicals such as dust, O3, and
benzene cause serious damage to the respiratory tract.72,73,74,75,76,77
Asthma is a respiratory disease which may be developed as a result of exposure
to air toxicants.78 Some studies have validated associations between both
traffic-related and/or industrial air pollution and increasing the risk of
COPD.79,80,81 Treatment of respiratory diseases due to air pollution is
similar to the other toxic chemical induce respiratory disorders.
4.1.2. Cardiovascular system
Many experimental and epidemiologic studies have shown the direct
association of air pollutant exposure and cardiac-related
illnesses.82,83,84,85 Air pollution is also associated with changes in white
blood cell counts86 which also may affect the cardiovascular functions. On the
other hand, a study on animal models suggested the close relationship between
hypertension and air pollution exposure.87 The traffic-related air pollution,
especially exposure to high levels of NO2, is associated with right and left
ventricular hypertrophy.88,89 In addition to the antidote therapy that exists
only for a few cardiotoxic substances like CO, usual treatment of
cardiovascular diseases should be carried out.
4.1.3. Nervous system
The nervous system is mainly affected by heavy metals (lead,
mercury and arsenic) and dioxins. Neurotoxicity leading to neuropathies,
with symptoms such as memory disturbances, sleep disorders, anger,
fatigue, hand tremors, blurred vision, and slurred speech, have been
observed after arsenic, lead and mercury exposure (Ewan and Pamphlett,
1996; Ratnaike, 2003). Especially, lead exposure causes injury to the
dopamine system, glutamate system, and N-methyl-D-Aspartate (NMDA) receptor
complex, which play an important role in memory functions (Lasley and
Gilbert, 2000; Lasley et al., 2001). Mercury is also responsible for
certain cases of neurological cancer. Dioxins decrease nerve conduction
velocity and impaired mental development of children (Thomke et al.,
1999; Walkowiak et al., 2001).
4.1.4. Urinary system
Heavy metals can induce kidney damage such as an initial tubular
dysfunction evidenced by an increased excretion of low molecular weight
proteins, which progresses to decreased glomerular filtration rate (GFR). In addition,
they increase the risk of stone formation or nephrocalcinosis (Damek-Poprawa and
Sawicka-Kapusta, 2003; Jarup, 2003; Loghman-Adham, 1997) and renal cancer
(Boffetta et al., 1993; Vamvakas et al., 1993).
4.1.5. Digestive system
Dioxins induce liver cell damage (Kimbrough et al., 1977), as
indicated by an increase in levels of certain enzymes in the blood (see
following discussion on the underlying cellular mechanisms of action), as well
as gastrointestinal and liver cancer (Mandal, 2005).
4.2. Exposure during pregnancy
It is rather important to mention that air pollutants can also affect
the developing foetus (Schell et al., 2006). Maternal exposure to heavy metals
and specially to lead, increases the risks of spontaneous abortion and reduced
fetal growth (preterm delivery, low birth weight). There are also evidences suggesting
that parental lead exposure is also responsible for congenital malformations
(Bellinger, 2005), and lesions of the developing nervous system, causing
important impairment in new-born’s motor and cognitive abilities (Garza et al.,
Similarly, dioxins were found to be transferred from the mother to
the fetus via the placenta. They act as endocrine disruptors and affect growth
and development of the central nervous system of the foetus (Wang et al.,
2004). In this respect, TCDD is considered as a developmental toxin in all
5. Cellular mechanisms involved in air pollutants actions
Common cellular mechanism by which most air pollutants exert their
adverse effects is their ability to act directly as prooxidants of lipids and
proteins or as free radicals generators, promoting oxidative stress and the
induction of inflammatory responses (Menzel, 1994; Rahman and MacNee, 2000).
Free radicals (reactive oxygen and nitrogen species) are harmful to cellular
lipids, proteins, and nuclear- or mitochondrial- DNA, inhibiting their normal
function (Valko et al., 2006). In addition, they can interfere with signaling
pathways within cells (Valko et al., 2006). In eukaryotic aerobic organisms
including humans, free radicals are continuously generated during normal
metabolism and in response to exogenous environmental exposures (e.g.
irradiation, cigarette smoke, metals and ozone). When free radical
concentration increases, due to an overwhelming of organism’s defense, a state
of oxidative stress occurs. This oxidative state has been implicated in
a wide variety of degenerative diseases such as atherosclerosis, heart attacks,
stoke, chronic inflammatory diseases (rheumatoid arthritis), cataract, central
nervous system disorders (Parkinson’s, and Alzheimer’s disease), age related
disorders and finally cancer.
Furthermore, the toxic effects of heavy metals, apart from inducing
oxidative stress, can be also attributed to their ability to substitute diverse
polyvalent cations (calcium, zinc, and magnesium) that function as charge
carriers, intermediaries in catalysed reactions, or as structural elements in
the maintenance of protein conformation. Indeed, metals accumulate in cellular
organelles and interfere with their function. For example, it has been observed
that lead accumulation in mitochondria induces several changes such as
inhibition of Ca2þ uptake, reduction of the transmembrane potential, oxidation
of pyridine nucleotides, and a fast release of accumulated Ca2þ (Chavez et al.,
1987). Moreover, metals bind to proteins (Goering, 1993) and inhibit a large
number of enzymes, including the mitochondrial ones (Rossi et al., 1993).
Nucleic acid binding proteins are also involved, while it has been shown that
metals can also bind to DNA, affecting the expression of genes. For example,
nickel enters the nucleus, interacts with chromatin and silences the expression
of genes such as tumour suppressor genes, inducing carcinogenesis (Costa et al.,
2003). Finally, some metals interfere with various voltage- and ligand-gated
ionic channels exerting neurotoxic effects.
For instance, lead affects the N-methyl-D-aspartic acid (NMDA)
receptor, subtypes of voltage- and calcium-gated potassium channels,
cholinergic receptors and voltage-gated calcium channels (Garza et al., 2006;
Toscano and Guilarte, 2005).
Dioxin causes a broad range of adverse effects (Birnbaum, 1994):
they alter metabolism by inducing a number of metabolic enzymes (e.g. CYPs,
glutathione-transferase, tyrosine kinase etc.), homeostasis, through hormone
modulation (e.g. estrogens, androgens glucocorticoids, insulin, thyroid
hormones) and their receptors, and growth and differentiation by interfering
with growth factors (e.g. EGF, TGFa, TNFa) and their receptors. At the cellular
level, dioxins interact with the aryl hydrocarbon receptor (AhR) (Schwarz et
al., 2000) which has a basic helix-loop-helix domain, acting as a transcription
factor after nuclear translocation, allowing interaction of dioxins with DNA.
The receptor-ligand complex binds to specific sites on DNA, altering the
expression of a number of genes.
As far as cancer is concerned from the data presented above it
becomes clear that most pollutants play an important role in the initiation,
promotion and progression of cancer cells (Fig. 1).
6. Natural protection
In our day-to-day life we are exposed in different kinds of pollutants.
Health impacts, as already described above, depend on the pollutant type, its
concentration, length of exposure, other coexisting pollutants and individual
susceptibility. People living in cities are exposed to a greater extent, as a
consequence of increased industrialization and demands for energy and motor
vehicles. Occupational exposure is also an important factor that should be
taken into consideration. During the last decade, health effects of air
pollution are studied more in developed countries, while more and better
environmental monitoring data are required in order to setup threshold levels.
In addition, efforts should be intensified by taking the appropriate measures,
in order to reduce the possibility of human pollutant exposure. The human body,
in order to protect itself against the potential harmful insults from the
environment, is equipped with drug or xenobiotic metabolising enzymes (DMEs or
XMEs) that play a central role in the biotransformation, metabolism and/or
detoxification of xenobiotics or foreign compounds, including different kinds
of pollutants. XMEs include a variety of enzymes such as cytochrome P450 (P450
or CYP), epoxide hydrolase, glutathione transferase,
UDP-glucuronosyltransferase, sulfotransferase, NAD(P)H quinone oxidoreductase
1, and aldo-keto reductase. These enzymes mainly participate in the conversion
of xenobiotics to more polar and water-soluble metabolites, which are readily
excreted from the body. Finally, it should be noted that, in many cases, the
chemically reactive metabolites produced during metabolism, are equally harmful
and therefore undergo additional metabolism to inactive products.
Hence, the final outcome of a compound modulating the
detoxification enzyme systems is the result the effects on the different
metabolic pathways. A number of substances of dietary nature are beneficial,
protective, and supportive of good health and the body’s own natural chelation
mechanisms. They include nutrients with natural chelating properties, which may
help to detoxify the body, such as antioxidants, herbs, minerals, essential
amino acids, other detoxifying or protective agents, and fiber (Kelly, 2004).
Among them dietary antioxidants contribute to the organism’s antioxidant
defence system, that includes a series of antioxidant enzymatic (e.g.
peroxidase) and nonenzymatic compounds (such as glutathione, or food-derived
like vitamin E, or polyphenols), as well as damage removal / repair enzymes.
Several natural compounds, such as vitamins C, E, and A and
polyphenols, found in the majority of plant foods, interfere with or scavenge
ROS concentration within cells and subsequently protect the organism from the
adverse effects of oxidative stress. Indeed, as it has been shown by our group
that the antioxidant activity of plasma in humans following a diet rich in
vegetables, fruits and olive oil was increased in comparison to a normal diet
(Kampa et al., 2002). This increase can be mainly attributed to polyphenols
which exhibit a wide range of biological activities, including
anti-tumorigenic, anti-mutagenic, anti-inflammatory, and antiviral actions
(Bravo, 1998; Hertog and Hollman, 1996) mainly due to their antioxidant
properties and their ability to exert inhibitory effects by affecting basic
cellular functions. Indeed, the beneficial role of polyphenols in preventing
cancer can be in part attributed to their ability to modify enzymes that
activate or detoxify environmental carcinogens.
This brief review presents the adverse effects of a number of (air)
pollutants in human health. As shown, major impairments of different organs can
be observed. The main conclusion drawn is that, in view of increased exposure
of humans in a diversity of pollutants, dietary interventions, rich in
plant-derived foods, may protect or decrease their effects on different organs.
This conclusion is supported by a number of epidemiological studies on the
beneficial effect of a Mediterranean- type diet on human health.
1) EPA http://www.health.state.mn.us/divs/eh/indoorair/voc/