health effects of air pollution
escape to the environment by a number of natural and/or anthropogenic
activities and may cause adverse effects on human health and the environment.
Increased combustion of fossil fuels in the last century is responsible for the
progressive change in the atmospheric composition. Air pollutants, such as
carbon monoxide (CO), sulfur dioxide (SO2), nitrogen oxides (NOx), volatile
organic compounds (VOCs), ozone (O3), heavy metals, and respirable particulate
matter (PM2.5 and PM10), differ in their chemical composition, reaction
properties, emission, time of disintegration and ability to diffuse in long or
Air pollution has both acute and chronic
effects on human health, affecting a number of different systems and organs. It
ranges from minor upper respiratory irritation to chronic respiratory and heart
disease, lung cancer, acute respiratory infections in children and chronic
bronchitis in adults, aggravating pre-existing heart and lung disease, or
asthmatic attacks. In addition, short- and long-term exposures have also been
linked with premature mortality and reduced life expectancy.
Although a number of
physical activities (volcanoes, fire, etc.) may release different pollutants in
the environment, anthropogenic activities are the major cause of environmental
air pollution. Hazardous chemicals can escape to the environment by accident,
but a number of air pollutants are released from industrial facilities and other
activities and may cause adverse effects on human health and the environment.
By definition, an air pollutant is any substance which may harm humans,
animals, vegetation or material. 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 noxious health effects may occur.
2. Pollutant categories
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:
pollutants (e.g. SO2, NOx, CO, ozone, Volatile Organic Compounds).
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 as
NO which rapidly reacts with ozone or radicals in the atmosphere forming NO2.
The main anthropogenic sources are mobile and stationary combustion sources.
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.
organic pollutants (e.g. dioxins).
organic pollutants form a toxic group of chemicals. They persist in the environment for long periods of time, and
their effects are magnified as they move up through the food chain (bio-magnification).
They include pesticides, as well as dioxins, furans and PCBs.
metals (e.g. lead, mercury). 4. Particulate Matter.
Heavy metals include basic metal
elements such as lead, mercury, cadmium silver nickel, vanadium, chromium and
manganese. They are natural components of the earth’s crust; they cannot be
degraded or destroyed, and can be transported by air, and enter water and human
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
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 mitochondrialDNA,
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.
Impact of air pollution
A basic and simple
criterion for assessing the importance of the health risk related to indoor
pollution makes reference to the severity of the effect concerned and to the
size of the population affected. The resulting 2 x 2 matrix is shown in Table
1. Important issues for the community may come from severe health impacts,
particularly when affecting a large segment of the population. Minor impacts,
such as those related to discomfort or annoyance may, however, become important
when a large number of individuals in the community are concerned.
Classification of environmental pollutants:
Toxicology of air pollutants
Not all air pollutants
have the same capacity for producing toxic effects, nor do they cause the same
damage. It is a logical conclusion that the differences are due to the physical
and chemical properties of these components. This report will briefly mention
the properties as they relate to toxicity. Beginning with the molecular
aggregation state, substances in aerosol form have been shown to be more toxic
than compounds in gaseous state. This is due to the fact that gaseous compounds
are eliminated by the respiratory system much more easily than aerosols, which
are rapidly deposited or absorbed. The particle size of an aerosol, between 1
nm and 2 µm, is easily deposited in the respiratory system (Wilson et al.
1996). Particle size determines the extent to which the particles can penetrate
into the respiratory system. Table 2 shows penetration ability of particles as
a function of size. Once particles have entered the respiratory tract, depending
on their size they can accumulate in different sites within the respiratory
Toxic effects of air pollutants
emitted into the atmosphere due to human activity or those compounds that are
byproducts of the interaction of chemical emissions have been shown to have
adverse effects on health. These effects, as discussed in this report, depend
fundamentally on the nature of the compound in question, the concentration in
the air and the time of individual exposure. Noxious health effects caused by
air pollution can be classified as due to either chronic or acute exposure.
2.4.1 Health effects due to acute exposure to air pollutants Toxic effects
attributable to acute exposure to air pollutants vary widely and have been
reported practically since the beginning of the industrial revolution where
episodes of high levels of pollutants were associated with increases in diverse
respiratory and heart diseases and death. These episodes have occurred on more
than a single occasion in different parts of the world, especially in highly
industrialised and/or populated areas .
The most studied toxic effect due to acute
exposure to environmental pollutants is mortality. Many reports describe an
increase in total mortality (not including accidental death) associated mainly
with exposure to particulate matter (PM), ozone and sulphates. This association
can be disputed, however, since the cause of death should be related to the
route of exposure (Schwartz 1994a , Dockery and Pope 1994).
Air pollution exposure
factors The major sources of human exposure to air pollution are, as mentioned
above, those produced by human activity. Pollutants can enter the organism in
various ways such as ingestion, absorption through the skin and inhalation (Möller
et al. 1994, Wilson et al. 1996). Inhalation is the major route of entry for
exposure to air pollution. An important aspect of inhalation that is often
ignored is oral breathing. When individuals breath through the mouth, the
physical and mechanical barriers of nasal breathing are absent, and oral
breathing has been shown to decrease the ability to eliminate particles
deposited in the respiratory tract, mainly in the upper air ways (Wilson et al.
1996). Until recently, only outdoor areas (exterior) were considered as
exposure sites since that was where an individual would contact the majority of
air pollutants. We now know that this is true only for certain types of
pollutants such as metals, which due to their particle size are found
essentially only outdoors (this is true for any particulate pollutant with a
particle diameter greater than 10 µm). Carbon monoxide (CO) and nitrogen
dioxide (NO2), on the other hand are found in greater quantity indoors (Möller
et al. 1994, Maynard 1999). A study in the United States showed that
individuals spend an average of 87.2% of their time indoors, 5.6% of their time
outdoors and 7.2% in transit (Wilson et al. 1996), and values for Mexico are
83.7%, 11.50% and 0.05% correspondingly (Rojas-Bracho 1994). These data demonstrate
the importance of determining indoor, as well as outdoor, exposure when
precisely defining an individual’s true exposure.
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