Sunday, 13 May 2012

Overview of Air Pollutants

The most common air pollutants in the urban environment include:
  • Sulfur Dioxide (SO2)
  • Oxides of Nitrogen (Nox), i.e. Nitrogen Oxide, (NO) and Nitrogen Dioxide (NO2)
  • Suspended Particulate Matter (SPM and PM10), also called Particulates
  • Carbon Monoxide (CO)
  • Lead (Pb)
  • Ozone (O3) and Peroxyacetyl Nitrate (PAN)
  • Volatile Organic Compounds (VOCs)
  • Polycyclic (or Polynuclear Aromatic Hydrocarbons (PAH)
A description of Acid Rain as a consequence of air pollution is included at the bottom.
Combustion of fossil fuels in stationary sources usually leads to the production of SO2, NOx, and Particulates. Domestic fuel use, mainly coal and wood, represents a significant source of the air pollution in cities, particularly cities in developing countries. Petrol-fueled motor vehicles are responsible for the emissions of NOx, CO, and Pb (where leaded petrol is still used), whereas diesel-fueled engines lead to significant emissions of SO2, NOx, and Particulates. VOCs are emitted from various anthropogenic sources including road traffic, production and the use of organic chemicals (e.g. solvents), transport and the use of crude oil, the use and distribution of natural gas, and from waste disposal sites and waste water treatment.
Ozone is a secondary pollutant. It is not emitted directly from combustion sources but forms in the lower atmosphere from NOx and volatile organic compounds (VOCs) in the presence of sunlight. Ozone is the main constituent of photochemical smog.
Apart from these 'traditional' air pollutants, there is an increasing number of other toxic and carcinogenic chemicals being detected in the urban atmosphere. These include certain heavy metals like beryllium (Be), cadmium (Cd) and mercury (Hg); trace organics like benzene (C6H6), polychlorodibenzo-dioxins and -furans, formaldehyde, vinyl chloride (VCl) and polycyclic aromatic hydrocarbons (PAHs); radionuclides like radon; and fibres like asbestos. These chemicals are emitted from various sources including waste incinerators, sewage treatment plants, industrial manufacturing processes, solvent use, building materials, and motor vehicles. Although the absolute concentration of these chemical pollutants is usually low in the atmosphere, the toxicity or carcinogenic effects or a combination of the two can be dangerously high for humans.
Concentrations of such chemicals in the air affect human health. Health effects vary with the intensity and the duration of exposure and with the health status of the exposed person. Certain sectors of the population like the elderly, children, and those already suffering from respiratory and cardiovascular diseases, are usually at greater risk.
Air pollutants usually affect the respiratory and cardiovascular system. SO2 and SPM bring about increased mortality, morbidity, and impaired pulmonary function. NO2 and O3 also affect the respiratory system with acute exposures causing inflammatory and permeability responses, decreased lung function, and increases airway reactivity. O3 causes headaches and eye and nose irritations. Due to its high affinity for haemoglobin, resulting in blood oxygen displacement, CO can lead to cardiovascular and neurobehavioral effects. Very high levels of CO exposure also cause death. Lead (Pb) inhibits the synthesis of haemoglobin in the red blood cells in bone marrow, impairs kidney and liver function, and causes neurological damage.
Apart from having human health impacts, air pollution also adversely affects the natural environment. The atmospheric reactions of the oxides of sulfur and nitrogen lead to their corresponding acidic transformation (into sulfuric acid and nitric acid). This leads to the acidification of soil and freshwater and adverse effects on the terrestrial and aquatic ecosystems. NO2 and O3 are phytotoxic (toxic to plants). O3 is associated with crop losses and forest damage. SO2 and O3 are also associated with damages to buildings, materials, and works of art.

Sulfur Dioxide (SO2)
Sulfur dioxide (or sulphur dioxide) has the chemical formula SO2. The gas can cause upper respiratory irritation such as nasal irritation and is frequently described as smelling of burning sulfur. It is produced by volcanoes and in various industrial processes. In particular, low-quality coal and petroleum contain sulfur compounds, and generate sulfur dioxide when burned: the gas reacts with water and atmospheric oxygen to form sulfuric acid (H2SO3) and thus acid rain (source: Wikipedia).
Description:
  • Colourless heavy gas
  • Pungent and irritating smell
  • Reacts on the surface with a variety of airborne solid particles
  • Readily soluble in water and can be oxidized within airborne water droplets. Upon reaction with water, it forms sulfuric acid (H2SO4) which reacts with organic matter, metals, and materials.
Sources:
  • Combustion of fossil fuels: Coal burning accounts for 50% of annual global SO2 emissions, making it the largest source, with oil burning as second at 25 -30%
  • Oil refineries
  • Power houses
  • Metallurgical operations: smelting of non - ferrous ores of copper, lead, nickel, and zinc
  • Manufacture of sulfuric acidConversion of wood pulp to paper
  • Refuse incinerationElement sulfur productionDomestic fuel burning
  • Natural sources include volcanoes
Human Health Effects:
  • Increased breathing rate and feeling of air starvation
  • Suffocation
  • Aggravation of asthma and chronic bronchitis
  • Impairment of pulmonary functions
  • Respiratory irritation
  • Sensory irritation
  • Irritation of the throat and eyes
  • Decline in children lung function
  • Increased mortality
Environmental Effects:
  • Concentrations of 50 -100 µg/m3 affect some plant species with various tree species exhibiting effects at concentrations of less than 50 µg/m3
  • Contributes to the formation of acid rain which may cause extensive damage to materials and terrestrial ecosystems, aquatic ecosystems, materials, and humans




Oxides of Nitrogen (NO and NO2 - NOx)
The chemical compound nitric oxide is a gas with chemical formula NO. It is an important signaling molecule in the body of mammals including humans, one of the few gaseous signaling molecules known. It is also a toxic air pollutant produced by automobile engines and power plants. The nitric oxide molecule is a free radical which makes it very reactive and unstable. In air, it quickly reacts with oxygen to form the poisonous nitrogen dioxide.
At high temperatures molecular nitrogen and oxygen can combine to form nitric oxide. A major natural source is lightning. Human activity has drastically increased the production of nitric oxide in combustion chambers. One purpose of catalytic converters in cars is to partially reverse this reaction. Nitric oxide in the air may later convert to nitric acid which has been implicated in acid rain. Furthermore, both NO and NO2 participate in the ozone layer depletion.
Nitric oxide (NO) and nitrogen dioxide (NO2) are serious air polluters. They should not be confused with the 'laughing gas' nitrous oxide (N2O).
Description:
  • Oxides of nitrogen are Nitrogen Oxide (NO) and Nitrogen Dioxide (NO2). Mostly anthropogenic (man-made), NO2 is derived from NO. Since this transformation occurs rapidly, NO2 is regarded as more important in terms of human effect. Consequently most data on health risks, ambient concentrations, and standards and guidelines are usually expressed in terms of NO2 rather than NOx.
  • NO is colourless gas and is slightly soluble in water.
  • NO2 is a reddish - brown gas. It is a strong oxidant and is soluble in water. It reacts with water to form HNO3, a powerful oxidant capable of reacting with almost all metals and many organic compounds. NO2 is also involved in the formation of Ozone (O3) in the atmosphere.
Sources:
  • Natural emission sources: Bacterial action, volcanic action, and lightning. These sources far outweigh those generated by man's activities but since they are distributed over the entire surface of the earth, their background atmospheric concentrations are very small.
  • Anthropogenic emissions sources
  • Combustion of wood and refuse
  • Non-combustion processes - fertilizer industry, manufacture of nitric acid (HNO3), welding processes, explosives industry
  • Indoor sources - tobacco smoking, use of gas fired appliances, and oil stoves
Human Health Effects:
  • Respiratory irritation, headache, pulmonary emphysema, impairment of lung defences, oedema of lungs, lachrymatory effect, loss of appetite, corrosion of teeth
  • Vulnerable groups: young children, asthmatics, individuals with chronic bronchitis and emphysema or other chronic respiratory diseases. Persons with liver cirrhosis or other liver, hormonal, and blood disorders or persons on certain types of drug therapies may also be more sensitive to NO2.
Environmental Effects:
  • Increased acidic deposition contributes to the formation of acid rain which may cause extensive damage to materials, vegetation, the terrestrial ecosystems, the aquatic ecosystems, and eventually to mankind.




Particulates (SPM and PM10)
The smaller the particle, the farther into the lungs it will be deposited and therefore the more serious the health effects.
Description:
  • Complex mixture of organic substances, present in the atmosphere both as solid particles and liquid droplets. They include fumes, smoke, dust and aerosols.
  • Coarse particles have aerodynamic diameter greater than 2.5 µm and contain earth crystal materials and fugitive dust mostly from roads and industries. Fine particles have an aerodynamic diameter less than 2.5 µm and contain secondary aerosols, combustion particles and re-condensed organic metallic vapours, and acid components.
  • May be referred to by the following terminologies:
    • Reflecting measuring methods: total suspended particulate (TSP)
    • Site of deposition in humans: inhalable, thoracic particles
    • Physical characteristics: PM10, which refers to particles with aerodynamic diameter of less than 10 microns
  • Origin:
    • Primary particulates are those emitted directly into the atmosphere.
    • Secondary particulates are those formed by reactions involving other pollutants.
Sources:
  • Burning of wood, coal, oil and gaseous fuels
  • Fly-ash emissions from power plants, smelting and mining activities, asbestos factories, metallurgical industries, ceramic industries, glass industries, cement industries, etc.
  • Vehicular traffic
  • Forest fires
  • Burning of coal refuse, agricultural refuse, municipal solid waste, etc
  • Also formed by the transformation of gaseous emissions like oxides of sulfur and nitrogen and volatile organic compounds
  • Natural sources include volcanic eruptions, wind and dust storms, salt sprays, etc.
  • Cigarette smoke
Human Health Effects:
  • Fine particles of less than 3 microns in size can penetrate the nose and throat, reach the lungs, and cause breathing problems and irritation of the lung capillaries.
  • Particulates cause, respiratory morbidity, deficiencies in pulmonary functions including decreased lung function (especially in children) and lung cancer with the consequence of increased mortality.
  • Pulmonary fibrosis among asbestos mine workers, black lung disease among coal miners, and emphysema among the urban population.
Environmental Effects:
  • Accelerate the corrosion of metals
  • Cause damage to paints and sculptures and soil exposed surfaces. The extent of damage depends on the physical and chemical properties of the particulates
  • Potential to modify the climate through the formation of clouds and snow
  • Contributes to acid deposition
  • May also absorb solar radiation and impair/reduce visibility



Carbon Monoxide (CO)
Description:

  • Colorless, odorless, tasteless, toxic gas
  • Slightly lighter than air and slightly soluble in water
  • Extremely dangerous because it has a greater affinity for hemoglobin than that of oxygen
Sources:
  • Incomplete combustion of fuels, automobile exhausts, jet engines, blast furnaces, mines, waste incinerators
  • Tobacco smoking is a significant indoor source
Human Health Effects:
  • The health effects of CO result principally from its ability to attack haemoglobin and displace oxygen thus forming carboxy-haemoglobin (COHb). This reduces the oxygen carrying capacity of the blood because the normal function of haemoglobin is to transport O2 from the lungs to all the body tissues. Consequences include:
    • Impaired learning ability
    • Reduced vigilance
    • Decreased manual dexterity
    • Impaired performance of complex tasks
    • Disturbed sleep activity
    • Toxicity and blood poisoning
    • Local myocardial ischemia (in which part of heart muscle is deprived of O2)
    • Aggravation of angina pectoris
    • Myocardial infarction (heart attack including those leading to sudden death)
    • Reduced capacity for exercise and physical work
    • Enhanced development of arteriosclerosis and coronary artery disease.
  • Elevated COHb levels reduce the availability of O2 to the central nervous system (CNS) including the brain. This can cause stroke involving unconsciousness, convulsion, brain swelling and protrusions, death to part of the brain or death to the individual, depending on the duration of O2 deficiency. Repeated episodes of this deficit can damage the blood - brain barrier and possibly cause structural damage resulting in reduced ability of the CNS to transmit information. High risk groups include individuals with existing cardiovascular or chronic respiratory problems, the elderly, young children, and fetuses.
Environmental Effects:
  • Exposure to very high levels for a period of time (115µg/m3 for 3 - 35 days) can harm plants


Lead (Pb)
Leaded petrol, responsible for the vast majority of airborne lead, is not used in most countries worldwide. Check your national regulations regarding fuel quality.
Description:
  • Bluish or silver-gray soft metal.
  • Two of its most important components in terms of air pollution are tetraethyl lead (TEL, found in leaded gasoline) and tetramethyl lead. These two are used as "anti knock" additives in petrol.
Sources:
  • Vehicle emissions
  • Car batteries
  • Production of metals like lead, copper, nickel, zinc, cadmium, iron, and steel
  • Thermal power plants and other coal combustion plants
  • Mining, cement production, refuse incineration, wood combustion, and lead-based paints
  • Secondary lead smelters (especially cottage/informal industries)
  • Zinc and silver smelters
  • Dust and soil in and around industrial sites may contain toxic metals such as lead.
Human Health Effects
  • Liver and kidney damage
  • Gastro-intestinal damage
  • Mental retardation in children
  • Abnormalities in fertility and pregnancy
  • Children are the most vulnerable group to lead poisoning. They are most sensitive to lead poisoning, which leads to behavioural problems, lower IQs, and decreased ability to concentrate.
Environmental Effects
  • Lead is generally toxic to both plants and animals.
  • Lead in dust and soil does not deteriorate. Long-distance atmospheric transport of lead particles has been well documented.

Ozone (O3) and Peroxyacetyl Nitrate (PAN)
Description O3:
  • Molecular oxygen with 3 atoms
  • Pale blue gas that is fairly soluble in water, unstable, and has a sweetish odor
  • Strong oxidant thus making it very reactive and capable of combining with many organic compounds in cells and tissues as well as rubber and other materials.
  • Considered a secondary pollutant because it is formed in the atmosphere by the reaction of other pollutants
  • Tropospheric (ground-based) ozone is harmful to human health and plant life, whereas stratospheric ozone ('the ozone layer' 20-25 km above the surface of the earth) protects against harmful ultraviolet (UV) solar radiation.
Description PAN:
  • Oxidizing agent formed by the reaction of organic compounds (e.g. aldehydes) with OH radicals, followed by the addition of O2 and NO2
Sources O3:
  • Formed in the lower atmosphere (troposphere) by the action of sunlight on nitrogen dioxide (NO2)
  • Some ozone in the troposphere (10 - 15%) is transported from the stratosphere where it is formed by action of ultraviolet (UV) radiation on O2 photochemical reaction
Sources PAN:
  • Formed through photochemical reactions in the atmosphere
Human Health Effects
  • Irritation of lungs, eyes, nose, throat, and respiratory tract
  • Accumulation of fluids in lungs
  • Damage to lung capillaries
  • Decreased pulmonary functions in young adults and children
  • Increased incidence of asthmatic attacks and respiratory symptoms in asthmatics
Environmental Effects
  • Damage to animals and vegetation



Volatile Organic Compounds (VOCs)
Description:
  • Comprised of a wide range of individual substances including hydrocarbons (alkanes, alkenes, and aromatics), halocarbons (e.g. trichloroethylene), and oxygenates (alcohols, aldehydes, and hetons). They are all organic compounds and are volatile enough to exist as vapour in the atmosphere.
Sources:
  • Vary greatly for individual compounds:
    • Hydrocarbons arise substantially from gasoline evaporation and incomplete combustion. They also arise from the leakage of natural gas from distribution systems.
    • Oxygenates arise from vehicle exhaust and are also formed in atmospheric chemical reactions.
    • Hydrocarbons, oxygenates, and halocarbons are released to the atmosphere through the evaporation of solvents used e.g. in paints or industrial decreasing processes.
Human Health Effects:
  • A number of VOCs are proven or suspected to cause cancer (e.g. benzene).
  • Depresses the nervous system and neurological functions, marked by drowsiness, dizziness, headache, nausea, loss of coordination, confusion and unconsciousness.
Environmental Effects:
  • Contributes to the formation of secondary pollutants and to the depletion of stratospheric ozone.
  • Contributes indirectly to the formation of atmospheric acidity
  • Ethylene, a major VOC component, is a plant hormone and can seriously inhibit the growth of plants.




Polycyclic (or Polynuclear) Aromatic Hydrocarbons (PAH)
Description:
  • Large group of inorganic compounds with two or more benzene rings. About 500 of these compounds including their derivatives have been detected in the air. The best known and most measured is benzo(a)pyrene (BaP).
Sources:
  • Formed as a result of pyrolytic (thermal degradation) process, especially the incomplete combustion of inorganic matter
  • Major sources include coke production, coal fired heating, motor vehicles, and to a lesser extent oil - fired power generation
Human Health Effects:
  • Substantial skin exposure to PAH causes skin cancer
  • Inhalation induces lung cancer
  • BaP is a carcinogen causing lung cancer
Effects on Animals:
  • PAH are toxic to animals, especially small mammals.


Acid Rain

Rain tends to be naturally acidic with a PH of 5.6 to 5.7. This is due to the reaction of atmospheric CO2 with water to produce carbonic acid. Other atmospheric substances from volcanic eruptions, forest fires, and other natural phenomenon also contribute to the natural acidification of rain. This natural level of acidity is sufficient enough to dissolve minerals into the earth's crust and make them available to plant and animal life and not acidic enough to inflict any damage.
Formation:
The contribution of sulfur dioxide (SO2) and Nitrogen dioxide (NO2) from anthropogenic sources disturbs the acid balance of rain and converts the natural and mildly acidic rain into precipitation with far reaching environmental consequences. The reactions of SO2 and NOx in the atmosphere yield H2SO4 (sulfuric acid) and HNO3 (nitric acid) droplets. These acids are formed in a series of photochemical and chemical reactions and are catalysed by other substances present in the atmosphere. The acidic droplets are partly neutralized by bases (salts) such as particulate lime and ammonia (NH3). These salts and the remaining H2SO4 and HNO3 droplets along with hydrochloric acid (HCl) released into the atmosphere by man-made and natural activities give rise to acidic precipitation, popularly known as acid rain. Acid rain comprises of more H2SO4 than HNO3.
Occurrence:
Acid rainfall may occur at a place far away from pollution sources (up to 1000 km) Events of acid rain in Sweden and Canada have been traced to large SOx emissions from densely populated areas of the United Kingdom and United States, respectively.
Damage:
Acid rain is a manifest of major consequences of air pollution because of the large amounts of SOx and NOx. It may cause extensive damage to materials and ecosystems. Following are some of the damages caused by acid rain:
  • Damage to buildings, structural material, and valuable ancient sculptures carved from marble, limestone, sandstone etc.
  • Damage to crops and forests, leaching of nutrients from leaves, and alteration of seed germination characteristics. Damage to young growing plant tissues and the process of photosynthesis, hence hindering the development of plants and threatening their very survival.
  • Acidification of soils with consequent effects on microbial and soil fauna and nitrogen fixation
  • Alterations of soil chemistry leading to reduced forest productivity· Potential effects on aquatic systems such as acidification, decreased alkalinity, and mobilization of metals like aluminium
  • Other biological effects on aquatic biota such as altered species composition among plankton, vegetation, and invertebrates; decline in productivity of fish and amphibians; skeletal deformity; and increased fish mortality
  • Corrosive damage to steel, zinc, oil-based paints and automobile coatings
  • Possible effects on human: lungs, skin, and hair may be affected; acidification of drinking water reservoirs and concurrent increases in heavy metals may exceed public health limits

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