Theme: The Air We Breathe
Topic 2. Impacts of human activities on air quality
A. Air pollution

The presence of one or more chemicals in the atmosphere in quantities and duration that
cause harm to humans, other forms of life, and materials.
 Pollutants could be removed or diluted by self-cleaning processes of the nature.
 However, if human activities produce air pollutants at a faster rate than the self – cleaning
processes, then harmful substances will be accumulated and the air quality will be
deteriorated.
B. Major Air Pollutants and their effects on human health
1. Organism

Example: virus, pollen, insects, etc.

Virus is invisible and borne by aerosols expired by patients or carriers. These
aerosols are spread by wind. Some disease-causing bacteria, e.g. anthrax, and viruses
e.g. smallpox, are spread in close distance through air. Thus, patients must be
isolated to prevent from epidemics.
2. Physical pollutant

Example: radiation, heat, fire, etc.
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
Radiation is emitted from nuclear electric power station while heat is emitted from
machines.

High dose of radiation is fatal. Medium dose of it would cause disease, induction or
mutation.
3. Chemical

Industrial, agricultural and social activities exploited a lot of natural resources.

Burning of fossil fuel (e.g. petrol) contributes a lot to air pollution (Refer to topic
“Petroleum as energy source” of theme “Our Earth Resources” for typical
composition of car exhaust)

Produces pollutants such as sulphur dioxide, nitrogen dioxide, ozone and particulate
matters.
Examples of air pollutants
1. Carbon monoxide

Common air pollutant in modern cities, consists of a carbon atom and an oxygen atom,

A colorless, odorless and poisonous gas.

Results from incomplete combustion (inadequate amount of oxygen is supplied in the
combustion process) of fuel
Complete combustion:
Incomplete combustion:
C (s)
2C (s)
+ O2 (g)
+ O2 (g)
CO2 (g)
2CO (g)

Emitted from vehicle exhaust pipes, power stations and domestic cooking.

Competes with oxygen molecule for the binding site on haemoglobin (red blood cell
responsible for “transporting” oxygen) in blood.

Initial symptoms of CO poisoning include headache, fatigue, shortness of breath and
nausea
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
Few concentration of CO could cause death (0.5% of CO in air is sufficient to kill people
in short time).
Irreversible
CO (g)
+ haemoglobin
carboxyhaemoglobin
Reversible
O2 (g)
+ haemoglobin
oxyhaemoglobin
2. Nitrogen oxides

Nitrogen oxides (NOX) is a general term for a group of highly reactive gases that made up
of nitrogen and oxygen in varying amounts.

Most of nitrogen oxides are odorless and colorless except NO2, a reddish brown gas with
irritating odor.

The sources (Figure 1) of NOX are motor vehicles, electric power stations (utilities), and
other industrial, commercial and residential sources that burn fuels under high
temperature.
Figure 1. US primary
sources of Nitrogen
oxides (U.S. EPA web
site:
http://www.epa.gov/air/
urbanair/nox/what.html
).
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
NO2 contributes to the formation of acid rain

Nitrogen oxides are acidic gases that attack human respiratory system

NOx and volatile organic compounds (VOCs) under UV radiation (from sunlight) and
high temperature (hot climate) form photochemical smog.
3. Sulphur dioxide

Sulphur is common in raw materials, including crude oil, coal, and ore.

Sulphur dioxide (SO2) is formed when the fossil fuels (especially in power plant) are
burned or metals are extracted from ore.

Causes temporary breathing difficulty for people with asthma.

Long-term exposures to high levels of SO2 cause respiratory illness and aggravate
existing heart disease.

Interacts with other gases and particles in the air to form products such as sulphurous acid,
that can be harmful to people and the environment.
— SO2 dissolves in water vapor to form acid.
SO2 (g)

+
H2O (l)
H2SO3 (aq)
SO2 and nitrogen oxides dissolve in water vapors, which fall to earth as rain, fog, snow, or
dry particles that commonly called acid rain.
4. Suspended particulates

The solid particles suspended in air, including smoke, soot, dust and dirt.

The particles are in a wide range of sizes: “fine” particles are less than 2.5 micrometers
(micrometer = 10-6 m) in diameter and “coarser-size” particles are larger than 2.5
micrometers. {Reference: United States Environmental Protection Agency (USEPA).
USEPA web site: http://www.epa.gov/ebtpages/airairpoparticulatematterpm.html}.
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
Particles can be suspended in the air for long periods of time because of their lightweight
and driving forces from air current.

Sources include:
— Industrial plants, factories emissions, incinerators, construction sites and motorcar
exhausts.

Burning of leaded petrol will generate lead and leaded particles.

May cause anemia and damage the central nervous system especially in children.

High concentration of solid particles (especially the respirable suspended particulates,
RSP*) affects the respiration system, causing coughing and excess mucus secretion.

Toxic particulate matters can cause lung cancer.
*RSP means suspended particles in air with a nominal aerodynamic diameter of 10
micrometres
or
smaller.
(Environmental
Protection
Department
(EPD),
HKSAR.
http://www.epd.gov.hk/epd/english/environmentinhk/air/air_quality/aq_obj.html)
Suggested Student Activities：
Information search and report on the air pollutants and their sources.
Experimental study on the car exhaust gas.
Experimental survey on the particulate matter on various locations in Hong Kong.
C. Photochemical Smog
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
Nitrogen dioxide and hydrocarbons are responsible for the formation of photochemical
smog (Figure 2).

A mixture of air pollutants (include nitrogen oxides, VOCs, ozone and Peroxyacetyl
nitrates (PANs)), one of the major air pollution problems in the modern cities such as
London and New York.

Caused by accumulation of the photochemical smog precursors (Figure 2) and the
presence of heat and sunlight.
Figure 2. A simplified scheme of formation of photochemical smog.
1. Ground Level Ozone

One of the constituents of photochemical smog

Oxides of nitrogen (NOx) and volatile organic compounds (VOCs) are the ground level
precursors

In the presence of heat and sunlight, oxygen free radical formed from the precursors and
react with molecular oxygen. For example:
Sunlight (UV light)
NO2 (g)
NO (g) +
O (g) (oxygen free radical*)
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O (g)
+
O2 (g)
O3 (g)
*Free radical: A molecule with an unpaired electron. Because they have a free electron, such
molecules are highly reactive (Reference web site: www.chemistry.about.com).

Another group of pollutants in the photochemical smog is Peroxyacetyl nitrates (PANs).
For example:
Sunlight
NO2 (g)

+
O2 (g)
+
hydrocarbons
CH3CO-OO-NO2 (g)
Photochemical smog irritates lung airways and triggers a variety of health problems
including:
— Aggravated asthma,
— Reduced lung capacity, and
— Increased susceptibility to respiratory illnesses like pneumonia and bronchitis.
D. Acid Rain

Unpolluted rainwater is acidic with pH about 5.6.
— It is because atmospheric carbon dioxide dissolves into water droplets to form
carbonic acid.
H2O (l)

+
CO2 (g)
H2CO3 (aq)
"Acid rain" is a broad term used to describe several ways that acids fall, including acidic
rain, fog and snow.
— With pH lower than 5.6 is called acid rain.

Sulphur dioxide and nitrogen dioxide are the primary causative chemicals of acid rain.
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
For example, sulphur dioxide dissolves in rainwater to form sulphurous acid:
SO2 (g)
+
H2O (l)
H2SO3 (aq)
E. Effects of Acid Rain
Building materials

Acid rain destroys the common building materials include marble, limestone, cement and
concrete.
— Calcium carbonate in those building materials reacts with acids. The calcium ions
were washed away and materials were destroyed:
CaCO3 (s)

+
2H+ (aq)
Ca2+ (aq)
+
CO2 (g)
+
H2O (l)
Not only causes great damages of monuments and statues, but also attacks metal objects
in construction materials that shorten the life of the buildings and structures.
Water bodies

Acid rain flows into water bodies such ad streams, lakes, and marshes.
— Changes their pH to become acidic (pH value goes down) when the water itself and
its surrounding soil cannot resist the increase in acidity.

Leads to release of aluminum ion from soil, which is highly toxic to many species of
aquatic organisms.

Causes the death of aquatic organisms, as they cannot withstand the acidic living
environment.
Vegetation

Acid rain weakens trees by damaging their leaves.
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
Limiting the nutrients (e.g. calcium ion) available to them, or exposing them to toxic
substances (aluminum and copper ions are the examples) slowly released from the soil.

The injury or death of trees is a result of these effects of acid rain.
Suggested Student Activity：
Experimental investigation: effect of acidic solution on embryo development of brine
shrimp.
Simulation experiment on effect of acid rain or acidic solution on seed germination.
References:
1.
Hocking, C., J. Barber and J. Coonrod (1994) Acid Rain: Teacher's Guide. Berkeley,
California: University of California at Berkeley.
2.
Miller, G. T. Jr. (2003) Environmental Science: Working with the Earth. Belmont,
California: Wadsworth Publication Inc.
3. U.S. Environmental Protection Agency (USEPA) USEPA web site, Acid Rain. Web
address: http://www.epa.gov/ebtpages/airairpoacidrain.html
4.
U.S. Environmental Protection Agency (USEPA) USEPA web site, Air. Web address:
http://www.epa.gov/ebtpages/air.html
5. Wellburn, A. (1994) Air Pollution and Climate Change: the Biological Iimpact. Harlow,
Essex, England: Longman Scientific & Technical.
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