Introduction

Most people tend to connect air pollution difficulties through the advent of the industrial revolution whereas; these troubles have somehow plagued the human race for centuries. The initial pollutants noted in the atmosphere were almost certainly of natural origin. Smoke,  fumes, ash and gases from volcanoes and forest fires; sand and dust from windstorms in arid regions; fog in humid low-lying areas; and natural terpene hazes from pine trees in  mountainous regions were part of our atmosphere extended before anthropogenic (human-induced) troubles came on the scene.

It is true though, that anthropogenic input have extremely enhanced the levels of each pollutant in the environment. Pollution of the atmosphere enhances in approximately straight proportion to the population density and is mostly related to the products of burning from heating plants, incinerators and automobiles plus gases, fumes and smokes arising from industrial procedures. Air pollution of quite another type is of major concern nowadays. These consequences from radioactive materials that gain entrance into the atmosphere through nuclear explosions.

Except in such extreme cases as volcanic eruptions, pollution from natural processes doesn't generally pose difficulties severe enough to seriously endanger life; ultimately,  human activities are to blame for pollution  problems  that  threaten  human's expectation of long and healthy life.

Air Pollution - Past, Present and Future

The smoke from isolated wood-burning fires of early cave dwellers went to the air almost unnoticed. But when the smoke from coal-burning furnaces in heavily populated cities started, the effects of pollution became severe enough to alarm several of the inhabitants of modern cities.

In A.D. 61, the philosopher Seneca explained 'the heavy air of Rome' and 'the stink of the smoky chimneys thereof'. In the year 1273, King Edward I was bothered enough through the smoke and fog mixture that brooded over London to prohibit the burning of 'sea coal'. By the time Queen Elizabeth I ascended the throne, the city's notorious pea-soup fogs had become smog. Because of her allergy and aversion to coal smoke, the Queen moved out of the city into the cleaner air of Nottingham. Toward the end of Elizabeth's reign, a law was passed prohibiting the burning of coal.

By late 1880s, there were several equivalent evidences of interest in overcoming air pollution, including the enactment of smoke-control laws in Chicago and Cincinnati. In the year 1930, an inversion trapped smog in Meuse Valley, Belgium. Sixty three persons died and several thousand others became ill. The London smog disaster of 1952 made it impossible to ignore any longer the serious consequences of air pollution. Between 4^th and 9^th December, 1952, 4000 deaths attributed to air pollution had been recorded, enough to move Britons to pass the Clean Air Act in 1956.

From that time till now, Clean Air Acts have been passed in places like the USA, Australia, Canada, and so on and so forth. There had been Clean Air  Act  Amendments as well. Though it is likely that future changes will be made, it is probable that the move  toward air pollution control has gathered enough momentum and public support to maintain a course which ensures a cleaner, healthier atmosphere for us now and generations to come.

Major Air Pollutants and their sources

Air pollutants are substances in the air that are responsible for pollution e.g. dust, fumes, gas, mist, odour, smoke, vapour, etc. Air pollution is the presence in the outdoor atmosphere of one or more air pollutants in adequate quantities, characteristics and of such duration to reasonably interfere with the comfortable enjoyment of life or property.

Particulates

Particulates can either be natural e.g. pollen, spores, bacteria, viruses, protozoa, plant fibres, rusts and volcanic dust, or anthropogenic e.g. fly ash,  smoke,  soot  particles,  metallic  oxides  and  salts,  oily  or  tarry droplets, acid droplets, silicates and other  inorganic dusts and metallic fumes.

At high concentrations, suspended particulate matter poses health dangers to humans, mainly those susceptible to respiratory illness. The nature and extent of the ill effects that may be linked to suspended particulates depend upon the concentration of particulates, the occurrence of other atmospheric contaminants (notably sulphur oxides) and the length of exposure.

Airborne substances such as pollens and spores causing allergies in sensitive persons are called aeroallergens. Ragweed pollen is one of the worst allergens. Hay fever or asthma sufferers coming into the vicinity where ragweed pollen exists may suffer allergic reactions. Other aeroallergens of biological origin include yeast, molds and animal fur, feathers or hair. Finely powdered industrial materials can also cause allergic reactions in sensitive persons.

The most serious situations of particulate air pollution develop where local conditions favour atmospheric temperature inversions and the products of combustion and of industrial processing are contained within a confined air mass. A notable example is the situation in Los Angeles, where inversions occur frequently; they also occur, though less often, in several other metropolitan areas for example Lagos, Nigeria.

Volatile Organic Compounds (VOCs)

Volatile organic compounds are a wide range of compounds with boiling points between approximately 50 and 250^oC and which, at room temperature, produce vapours. In the indoor environments particularly, VOCs originate from a number of sources:  furnishings, furniture and carpet adhesives, glues, building materials, cosmetics, cleaning agents, fungi, and tobacco smoke and fuel combustion.

By far, the greatest peak exposure to VOCs occurs during home decorating using solvent-based paints. Some of the compounds that may occur in a typical non-industrial indoor environment are aliphatic and aromatic hydrocarbons, halogenated compounds and aldehydes. Because of the diverse range of chemical substances defined as VOCs, determination of health effects is problematic. At the levels typically found indoors however, the major effects are likely to be sensory.  

Oxides of Sulphur

The oxides of sulphur (SOx) are probably the most widespread and the most intensely studied of all anthropogenic air pollutants. They include six different gaseous species:  sulphur monoxide (SO), sulphur dioxide (SO₂), sulphur trioxide (SO₃), sulphur tetroxide (SO₄), sulphur sesquioxide (S₂O₃) and sulphur heptoxide (S₂O₇). However, only SO₂ and SO₃   are the oxides of sulphur of most interest in the study of air pollution.

The burning of solid and fossil fuel contributes more than 80percent of anthropogenic SO₂ emissions. It is estimated that SO₂ remains airborne an average of two to four days, during which time it may be transported as far as 1000 kilometres. Therefore, the problem of SO₂ can become an international one. Relatively stable in the atmosphere, SO₂ acts either as a reducing or an oxidizing agent. Reacting photochemically or catalytically through other components in the atmosphere, SO₂ can produce SO₃, H₂SO₄ droplets and salts of sulphuric acid. Sulphur dioxide can react through water to form sulphurous acid, a weak acid that can react straight through organic dyes.

Sulphur dioxide, sulphuric acid and sulphate salts tend to irritate the mucous membranes of the respiratory tract and foster the expansion of chronic respiratory diseases such as bronchitis and pulmonary emphysema. In a dusty atmosphere, SO₂ is particularly harmful since both SO₂ and H₂SO₄ molecules paralyse the hair-like cilia which line the respiratory tract. Without the regular sweeping action of the cilia, particulates are able to penetrate to the lungs and settle there. Such particulates usually carry through them concentrated amounts of SO₂, therefore carrying this irritant into direct, prolonged contact through fragile lung tissues. The SO₂-particulate amalgamation has been cited as cause of death in several air pollution tragedies.

Oxides of Nitrogen

Oxides of Nitrogen (NOx) comprise 6 recognized gaseous compounds: nitric oxide (NO), nitrogen dioxide (NO₂), nitrous oxide (N₂O), nitrogen sesquioxide (N₂O₃), nitrogen tetroxide (N₂O₄) and nitrogen pentoxide (N₂O₅). The two important oxides of nitrogen in air pollution are NO and NO2, being the only two oxides of nitrogen that are emitted in significant quantities to the atmosphere. Nitrogen dioxide is eagerly soluble in water, forming nitric acid and either nitrous acid (HNO₂) or nitric oxide (NO).

2NO₂ + H₂O    HNO₃ + HNO₂

 nitrous acid

3NO + H₂O    HNO + NO

nitric oxide

Both nitric and nitrous acid will fall out in the rain or unite through ammonia in the atmosphere to form ammonium nitrate (NH₂NO₃). A good absorber of energy in the UV range, NO₂ plays a main role in the production of secondary air contaminants such as ozone (O₃). Nitric oxide is emitted to the atmosphere in much superior quantities than NO₂. It is shaped in high-temperature combustion processes when atmospheric oxygen and nitrogen combine according to the reaction.

N₂ + O₂ 2NO

Small concentrations of the NOx produced in the upper atmosphere by solar radiation reach the lower atmosphere through downward diffusion. Small amounts of NOx are produced by lightning and forest fires.

Bacteria decomposition of organic matter liberates NOx into the atmosphere. Discover burning in stationary sources and in transportation are the primary origins of human-provoked NOx. Nitric oxide (NO) is a comparatively inert gas and only reasonably toxic. Even though NO, like CO, can combine through haemoglobin to reduce the oxygen-carrying capacity of the blood, NO concentrations are generally less than 1.22 mg/m³ in the ambient  air and are thus not considered health hazards. Though, NO is readily oxidised to NO₂, which has environmental significance.

NO + ½ O₂      NO₂    

NO₂ irritates the alveoli of the lungs. Short-term animal studies showed reduced resistance to respiratory infection at exposures of 6.6 mg/m³ for 2 hours. Experimental exposures of volunteers to 9.4 mg/m³ NO₂, considerably above the 7.0 mg/m³ peak recorded in Los Angeles, for 10 minutes produced a substantial but transient increase in the resistance of the lung's airways to air movement. Concentrations from 47 to 141 mg/m³ cause reversible pneumonia.  At high concentrations of 285 mg/m³ and above, exposures to NO₂ can be fatal to humans.

Ozone (O₃)

Ozone, the major photochemical oxidant, makes up approximately 90percent of the atmospheric oxidant pool. Other significant photochemical oxidants in air pollution monitoring comprise nascent oxygen (O), excited molecular oxygen (O₂), peroxyacetyl nitrate (PAN), peroxypropinol nitrate (PPN), peroxybutyl nitrate (PBN), nitrogen dioxide (NO₂), hydrogen peroxide (H₂O₂) and alkyl nitrates.

Ozone is generated in the upper atmosphere via solar radiation, and tiny concentrations of this gas diffuse downwards. As well, small concentrations are produced through lightning and forest fires. Ozone and other photochemical oxidants can cause coughing, shortness of breath, airway constriction, headache, chest rigidness and soreness, impaired pulmonary function, altered red blood cells, pharyngitis, laryngitis, and eye, nose and throat irritation. Exposure of laboratory animals to high levels of ozone has resulted in damage to their chromosomes. Since of this, O₃ is considered to contain consequences alike to ionizing radiation.

Chromosome breakage in human cell cultures was examined at revelations of 15673 ug/m³ for 5 to 10 minutes.

Carbon monoxide (co)

Colourless, tasteless and odourless CO gas is chemically inert under usual situations and has an approximation atmospheric mean life of about 2 and half months. At present ambient levels, it has little or no results on property, vegetation or substances. Carbon monoxide sources are together natural and anthropogenic. Oxidation of methane gas from decaying vegetation results in the production of up to three and half billion tonnes of CO per year. In human metabolism, the exhalation of a resting person encloses approximately 1ppm CO. Though, such productions are so meager compared to the amounts coming from fossil fuel in complete combustion through stationary or mobile engines, solid-waste disposal and miscellaneous anthropogenic sources.

Carbon monoxide is particularly dangerous because it has no odour, colour or taste. Its toxic action is through the disarticulation of oxygen in the haemoglobin to form carboxyhaemoglobin, thus depriving the tissues of the body of their oxygen provides. Early symptoms of exposure to CO comprise tiredness, drowsiness, headaches, dizziness, pains in the chest and stomach. Excessive exposure can guide to loss of consciousness, coma and death.

Airborne carcinogens

The substances that have been exposed in fact or potentially to reason cancers are grouped according to International Agency for Research on Cancer (IARC) into 4 dissimilar categories: depend on their ability to cause cancer.

Group 1 - Proven human carcinogens. Chemicals for that there is enough confirmation for epidemiological studies to hold a causal connection between exposure and cancer for example benzene. Benzene has a proven causal connection through acute non-lymphocytic leukaemia in humans. The main toxic consequences happen on the bone marrow, through toxic exposures producing bone-marrow suppression and decreases in red cells, white cells and  blood platelets production  (pancytopenia) that might lead to bone-marrow failure (aplastic anaemia).

Group 2 - Probable human carcinogens. Chemicals for that evidence ranges from inadequate to almost sufficient.

Group 2A: Limited evidence of carcinogenicity in humans and sufficient evidence for carcinogenicity in animals e.g. benzo (a) pyrene, benzo (a) anthracene and other polycychic aromatic hydrocarbons (PAHs).

Group  2B: Inadequate  evidence  for  carcinogenicity  in  humans  and sufficient evidence  for carcinogenicity in animals e.g. 2-nitrofluorene, 1,6 -dinitropyrene and 1-nitropyrene.

Group 3: Unclassified chemicals. Chemicals which can't be classified in humans, usually because of inadequate evidence.

Hydrocarbons

Hydrocarbons are divided into two major classes - aliphatic and aromatic. The aliphatic hydrocarbons contain alkanes, alkenes and alkynes. The alkanes  are  fairly   inert  and  generally  not  active  in atmospheric  photochemical  reactions;  they are  highly   reactive  in atmospheric photochemistry. The reactivity of alkenes such as ethylene makes them much more important in the study of air pollution than alkanes, because in the presence of UV radiation they react with NO₂ at high concentrations to form secondary pollutants such as peroxyacetyl nitrate (PAN) and ozone. The alkynes, though highly reactive, are relatively rare. Hence, they are not of major concern in air pollution studies.

Aromatic hydrocarbons do not display the reactivity characteristic of unsaturated aliphatic hydrocarbons. Nevertheless, the polynuclear group of aromatic hydrocarbons (for instance polycyclic aromatic hydrocarbons, PAHs) is of concern in any study of air pollution because a number of these compounds have been shown to be carcinogenic. Increases in lung cancer in urban areas have been blamed on the consequences of PAHs from automative exhaust emissions.  Benzo-[a]-pyrene has been exposed to be the most carcinogenic hydrocarbon for test animals. Benz-[e]- acephenanthrylene and benzo-[j]-fluroranthene follow, and benzo-[3]- pyrene, benz-[a]-anthracene and chrysene are all weakly carcinogenic.

Most natural hydrocarbons originate in the air are from biological sources, though small amounts come from geothermal areas, coal fields, natural gas from petroleum fields and natural fires. The more complex, naturally produced hydrocarbons found in the atmosphere, such as volatile terpenes and isoprene, are produced by plants and trees. Industrial sources, notably refineries, have become the major anthropogenic source of hydrocarbons. Other minor sources might  include  incomplete combustion from car engines, evaporative emissions from petrol stations and  fuel  tanks, releases from solid  waste disposal, forest  fires, agricultural burning and coal waste fires.

Cigarette smoking

Cigarette smoking is particularly dangerous for those through heart disease since combustion of tobacco in this way, through limited oxygen available, introduces CO straight into the bloodstream. The products of cigarette combustion take in nicotine, acetaldehyde, acetone, benzene, formaldehyde, N′-nitrosonornicotine, N-nitrosopyrrolidine, benzo-[a]- pyrene. Among the very many products of combustion, nitrosamines, benzo-[a]-pyrene and nicotine are highly   poisonous.   Nicotine  as well throws  an  extra  strain  on  the  heart  through  stimulating  the   basal metabolic rate. Nitrosamines are identified to cause cancer in animals.

Apart from the consequence on the heart, tobacco smoking exacerbates bronchial infection and causes 30 - 40percent of all deaths from cancer. The higher occurrence of cancer in smokers arises from inhalation of PAHs and nitrosamines. Smoking as well enhances the danger to workers in industries that produce fine particles.

- Asbestos Fibres:

The group named asbestos is applied to a range of obviously happening fibrous magnesium silicate minerals with the approximate formula Mg₃P(Si₂O₅)(OH)₄. Three ordinary kinds are utilized: crocidolite (blue asbestos) - most hazardous; amosite (brown asbestos) - second most hazardous; chrysolite (white asbestos) - least hazardous. Inhalation of divide dry fibres in a confined air space is the main hazard from asbestos. These dry fibres can come from harmed thermal insulation, accumulated fibres from the manufacture, wear of asbestos, mineral substances these as brake shoe pad dust, ship breaking, waste disposal, and more insidiously, the exposure of workers' families from elements carried on clothing.

Asbestos fibres can reason lung and bowel cancer in addition to non- cancerous lung diseases. Approximately 50percent of the inhaled fibres are cleared from the lungs and swallowed, that then exposes the throat and digestive system to their hazardous consequences. Water from asbestos- cement pipes as well poses a further source of digestive tract exposure. Respiratory diseases include asbestosis (a pneumonia-like condition), bronchial cancer and mesothelioma that have a latency period of 20-30 years. There showed to be a synergistic reaction between cigarette smoke and asbestos in that the onset of illness is more pronounced in heavy smokers than in non-smokers.

-Toxic Metals:

Toxic metals are those metals that are damaging in comparatively tiny amounts for instance As, Hg, Cd and Pb. Very frequently a specified metal or its compounds are originate in connection through other ecological particles. As a effect, there are numerous routes via which these metals obtain into the atmosphere, for instance, Pb can be added into the air through drain emissions, re-suspension of particles, flaking of old Pb painted houses, fumes from soldering and plumbing processes, etc.

Depending on their element sizes and speciation, these metals can be inhaled along through the air and find their ways into vital organs in the human system. Many of the toxic metals negatively contact the nervous system. Anaemia, renal failure and neuropsychological impairment are also part of the health infarctions that can be attributed to toxic metals.

-Radioactive Gases:

The penetration of buildings via chemically inert radon gas presents a more serious risk to human health than the hazards posed through occasional failure of control of nuclear power stations. The decay of 238U produces 222

Rn which emanates from radioactive minerals in building foundations. Since it has a half-life of only 3.8 days, the danger is only important whenever the gas can increase rapidly into the instant sub-soil, conditions that gain above fissured granite. Radon has seven times the density of air and so basements and ground floor rooms are the most exposed.

The worst cancer risk from Rn arises from the first four solid decay products of Rn:

²²² Rn    ²¹⁸P0 ²¹⁴ Pb ²¹⁴ Bi ²¹⁴P0 ²¹⁰Pb

Such products are hazardous since they are connected through moisture and dust, and become deposited in the lung, so exposing the epithelium to α-particles from ²¹⁸Po and ²¹⁴Po. Other nuclides of Rn exist but they don't present the similar danger as that from ²²²Rn e.g. ²²⁰Rn shaped through decay of ²³²Th has also short a half-life to permit survival above ground. Raised occurrence of lung cancer in uranium miners in western USA and Czechoslovakia was featured to their inhaling ²²²Rn.

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