Air pollution is an unwanted change in the quality of earth’s atmosphere caused by emission of gases due to burning of fossil fuels, outpouring of ashes and gases from the particulate matter due to soil erosion. Pollen and organic compounds from vegetation and lightening strokes also pollute the atmosphere.
Now, we will study about the different agents that pollute the air. For convenience, air pollutants can be classified into the following three types:
The pollutants which come out from natural sources such as forest fires started by lightening, dispersal of pollen, soil erosion, volcanic eruptions, volatile organic compounds from leaves and trees, decomposition of organic matter and natural radioactivity, etc. are natural pollutants. This type of atmospheric pollution is not a new phenomena. It is perhaps as old as the earth itself. Nature has its own mechanisms of dealing with such pollution. In any case, the concentration of pollutants from the natural sources is often quite low and rarely causes any serious damage.
A primary pollutant can be defined as a harmful chemical that directly enters the air as a result of a natural event or human activities. For example, when coal, oil, natural gas or wood is burnt, carbon dioxide and carbon monoxide are formed, automobiles contributing a large share of carbon monoxide. All these gases enter the atmosphere. Another important pollutant is sulphur dioxide (SO2) which is added to atmosphere by burning of coal and oil containing sulphur as impurity in electric power plants. Other primary pollutants are oxides of nitrogen, hydrocarbons and suspended particulate matter.
Secondary pollutants result from harmful chemical reaction between two or more air components. For example, sulphur dioxide, the primary pollutant reacts with oxygen in the atmosphere to form the secondary pollutant, sulphur trioxide (SO3) (2SO2+O2 — >2SO2). The sulphur trioxide can then react with water vapour in air to form droplets of sulphuric acid (H2SO4) (SO3+H2O — >H2SO4), another secondary pollutants.
SOME MAJOR AIR POLLUTANTS
The major air pollutants are those which are produced in significant amounts and have health and other environmental hazard. We will describe here some of the major air pollutants.
Oxides of Sulphur
The oxides of sulphur have deleterious environmental effects. Electrical power plants which use coal are largely blamed for sulphur dioxide emissions into the atmosphere.
Sulphur dioxide is itself injurious to plant and animal life. It can also react with ozone, hydrogen peroxide, .or water vapour in the atmosphere to form sulphuric acid (H2SO4). Sulphuric acid is one of the strongest acids known. It corrodes lime stone and metals and destroys clothing. It also has injurious effects on respiratory tissue. Sulphuric acid is considered as one of the most toxic and dangerous air pollutants, being a major component of acid rain. You will read more about acid rain in the later sections of this Unit.
Since most of the sulphur dioxide comes from burning of ‘coal in the power plants, the control of sulphur dioxide emissions into the atmosphere will depend upon the production and use of energy as well as on the quality of the coal used.
Oxides of Nitrogen
Oxides of nitrogen are produced when fuel is burned, at high temperatures. Nitrogen, which is, ordinarily inert, combines with oxygen in high temperature flames and tends to stay combined if the exhaust gases are cooled quickly.
Oxides of nitrogen are relatively harmless at ordinary concentrations. They are released into the air mainly from vehicular smoke, electric power plants, industrial establishments, commercial institutions and residential units. The automobile emission is the major contributor of nitrogen oxide, accounting for about 50% of the total. Electric power plants contribute about 33% and the percentage share of the industrial establishments, commercial institutions and residential units is 12%, 14% and 1% respectively. In Indian cities diesel operated vehicles contribute about 90% of the oxides of nitrogen to the air.
Nitric oxide (NO) is the first product of the combination of atmospheric nitrogen with oxygen at high temperatures. At high concentrations, nitric oxide causes asphyxiation.
Further, nitric oxide combines easily with the atmospheric oxygen to form nitrogen dioxide (NO2). Nitrogen dioxide may create a variety of human ailments, from mere gum inflammation to internal bleeding, pneumonia, lung cancer, etc.
Hydrocarbons are compounds made up of carbon and hydrogen. Some hydrocarbons have direct effect on human beings and are carcinogenic in nature. They are produced during the production of coke and smoldering of refuse piles near coal mines or during improper burning of coal.
Carbon monoxide (CO) is one of the major pollutants from automobiles comprising 80% of all automobile exhausts. Small amounts are also emitted from volcanoes and
forest fires Carbon monoxide is a product of incomplete combustion of fuels. Carbon monoxide reduces the oxygen carrying capacity of the blood. It can be fatal at concentrations exceeding 1000 ppm. Therefore, it is not advisable to work in closed rooms with open coal fire.
Carbon dioxide (CO2) is one of the basic end-products of the burning of fossil fuels, paper, leaves, tobacco and other carbon containing materials. Carbon dioxide is considered relatively innocuous because it has no direct effect on health. But it has several important side effects. It contributes towards acid rain and greenhouse effect.
Lead is a chemical pollutant which enters the atmosphere from automobile exhaust. A compound of lead, tetraethyl lead (TEL) is used as an anti-knock agent in petrol or gasoline for smooth and easy running of vehicles. If the octane rating of petrol is low, the fuel and air mixture can ignite prematurely and disturb the smooth running of the engine and cause knocking. Therefore, the octane rating of gasoline is raised by the addition of TEL in concentrations between 0.3 and 0.6 grams per litre. Developed countries have by and large discontinued the use of TEL as an antiknock agent and have increasingly gone over to the use of other methods for raising the octane number of gasoline. It is a matter of concern that we, in India, still use leaded gasoline.
The lead mixed air, if inhaled, may produce injurious effects on the kidneys, blood and liver. It can get mixed up with water and food and can create cumulative poisoning. The effects are far-reaching in case of growing children. The increase in level of lead in children may cause lowering of intelligence. Hence, it forms an important parameter of air pollution.
Suspended Particulate Matter
Any small solid particles and liquid droplets suspended in the air are known as particulate matter. The size and weight, in particular, determine their suspension from a few seconds to months in the atmosphere. Most of such particulate matter is emitted by establishments which use coal as fuel. These air pollutants are also known as aerosols.
The aerosols remain constantly under the influence of gravitational force of earth. The aerosols in the atmosphere are of two kinds: one is the natural, such as fog, bacteria, plant spores, pollen, etc. These usually do not cause any atmospheric pollution. The second type of aerosols, such as cement powder, flue dust from coal dust combustion, quartz and asbestos powder, oil smokes, tobacco smokes and radioactive aerosols are air pollutants due mainly to man’s activities and cause constant damage and threat to the atmosphere.
The term refers to the smoking of tobacco, but many other substances can also be smoked, for example, opium, dhatura and other herbs. Cigarette smoking is responsible for many diseases due to the presence of carcinogenic tars in cigarette smoke. The effectiveness of carcinogenic tars is lesser ir case of cigar and pipe smoking due to lower temperature at which tobacco is smoked. It is because of the harmful effects that the Government of India has made statutory provisions for cigarette manufacturers in the country to print on every packet of cigarettes a warning saying that “smoking is injurious to health”
Besides, it has also been established that non-smokers whose work conditions lie in smoke-filled rooms are also vulnerable to health problems.
In spite of warnings, cigarette smoking continues, and more and more problems are predicted in future years to come. The most common health problem is the occurrence of cancer of the lungs and larynx. The other health problem associated with smoking is coronary (heart) disease. The well known ‘smokers’ cough’ is of local nature which may bring irritation and bronchitis. Finally, the death rate in middle aged people, who smoke more than 20 cigarettes a day, is more than twice that of non-smokers, taking all diseases into account.
The smoking habit, however, can be controlled only by ‘will power’ and in some cases by substituting disagreeable lozenge for the cigarette.
You must be aware that radioactive substances disintegrate with emission of radiation. Three kinds of radiation namely alpha, beta and gamma have been identified. These radiations interact with living tissues and damage them. The sources of radiation can be natural, which include both—cosmic and terrestrial, and artificial or man-made. Cosmic rays are the primary source of radiation which enter the atmosphere from outer space. The primary radiation on entering the atmosphere produces secondary radiation through interactions with atoms in the atmosphere.
Terrestrial radioactivity originates from radioactive materials of the earth. A large proportion of this radiation is from radioactive materials, such as uranium, thorium, actinium, potassium and carbon. However, strontium-90 which is a major long-term hazard gets incorporated into vegetation, dairy products and even building materials like bricks, concrete, etc. These materials are believed to have been present in the earth since its inception.
In recent decades radioactive materials have been increasingly used for generation of power, research and medicinal purposes. The radiation from industrial and research units is believed to be absolutely negligible, as these are so shielded by fool-proof measures that the radiation level outside the reactor’s core seems to be very much lower than the danger dose. Furthermore, these reactors are so designed that they cannot explode like atomic bombs. The real danger comes from the use of radioactive material for destructive purposes. The world has not yet forgotten the destruction caused by two fission bombs which were dropped in Hiroshima and Nagasaki towards the end of Second World War.
The harmful effects of radiation, are cancer, gene mutation, damage to the central nervous system, blood forming tissues, eyes, skin, and a host of other organs and systems. Although, at present, the total radiation from all of the man-made sources is considerably low, it may, however, increase in future years with expanded use of radioactive materials. Therefore, the adverse biological effects of the exposure must be evaluated carefully.