Understanding air pollution

Environmental pollution is the effect of undesirable changesin our surroundings that have harmful effects on plants, animals and human beings. A substance, which causespollution, is known as pollutant. Pollutants can be solid, liquid or gaseous substances present in greaterconcentration than in natural abundance and areproduced due to human activities or due to naturalhappenings. Do you know an average human beingrequires nearly 12-15 times more air than the food? So, even small amounts of pollutants in the air becomesignificant compared to similar levels present in the food.

Pollutants can be degradable, like discarded vegetables which rapidly break down by natural processes. On the other hand, pollutants which are slowly degradable remain in the environment in an unchanged form for many decades. For example, substances such as dichlorodiphenyltrichloroethane (DDT), plastic materials, heavy metals, many chemicals, and nuclear wastes etc., once released into the environment are difficult to remove.

Thesepollutants cannot be degraded by natural processes and are harmful to living organisms. In the process of environmental pollution, pollutants originate from a source and get transported by air or water or are dumped into the soil by human beings.

Atmospheric pollution

The atmosphere that surrounds the earth isnot of the same thickness at all heights. There are concentric layers of air or regions and each layer has different density. The lowest region of atmosphere in which the human beings along with other organisms live is called troposphere. It extends up to the height of ~ 10 km from sea level.

Above the troposphere, between 10 and 50 km above sea level lies stratosphere. Troposphere is a turbulent, dusty zone containing air, much water vapour and clouds. This is the region of strong air movement and cloud formation. The stratosphere, on the other hand, contains dinitrogen, dioxygen, ozone and little water vapour.

Atmospheric pollution is generally studied as tropospheric and stratospheric pollution. The presence of ozone in the stratosphere prevents about 99.5 per cent of the sun’s harmful ultraviolet (UV) radiations from reaching the earth’s surface and thereby protecting humans and other animals from its effects.

Tropospheric Pollution

Tropospheric pollution occurs due to thepresence of undesirable solid or gaseousparticles in the air. The following are the majorgaseous and particulate pollutants present inthe troposphere:

1. Gaseous air pollutants: These are oxidesof sulphur, nitrogen and carbon, hydrogensulphide, hydrocarbons, ozone and otheroxidants.

2. Particulate pollutants: These are dust, mist, fumes, smoke, smog etc.

Gaseous air pollutants

Oxides of Sulphur

Oxides of sulphurare produced when sulphur containing fossilfuel is burnt. The most common species,sulphur dioxide, is a gas that is poisonous to both animals and plants. It has been reported that even a low concentration of sulphur dioxide causes respiratory diseases e.g., asthma, bronchitis, emphysema in humanbeings.

Sulphur dioxide causes irritation to the eyes, resulting in tears and redness. High concentration of SO leads to stiffness of flower buds which eventually fall off from plants. Un-catalysed oxidation of sulphur dioxide is slow. However, the presence of particulate matter in polluted air catalyses the oxidation of sulphur dioxide to sulphur trioxide.

Oxides of Nitrogen:

Di-nitrogen and di-oxygen are the main constituents of air. These gases do not react with each other at a normal temperature. At high altitudes when lightning strikes, they combine to form oxides of nitrogen. NO2is oxidised to nitrate ion, which is washed into soil, where it serves as a fertilizer.

In an automobile engine, (at high temperature) when fossil fuel is burnt, di-nitrogen and di-oxygen combine to yield significant quantities of nitric oxide (NO) The irritant red haze in the traffic andcongested places is due to oxides of nitrogen. Higher concentrations of NOdamage theleaves of plants and retard the rate ofphotosynthesis.

Nitrogen dioxide is a lungirritant that can lead to an acute respiratorydisease in children. It is toxic to living tissuesalso. Nitrogen dioxide is also harmful tovarious textile fibres and metals.

Hydrocarbons

Hydrocarbons are composed of hydrogen and carbon only and are formed by incomplete combustion of fuel used in automobiles. Hydrocarbons are carcinogenic, i.e., they cause cancer. They harm plants by causing ageing, breakdown of tissues and shedding of leaves, flowers and twigs.

Oxides of Carbon

Carbon monoxide

Carbon monoxide (CO) is one of the most serious air pollutants. It is acolourless and odourless gas; highlypoisonous to living beings because of its abilityto block the delivery of oxygen to the organsand tissues. It is produced as result ofincomplete combustion of carbon. Carbonmonoxide is mainly released into the air byautomobile exhaust.

Other sources, whichproduce CO, involve incomplete combustionof coal, firewood, petrol, etc. The number ofvehicles has been increasing over the years allover the world. Many vehicles are poorlymaintained and several have inadequate pollution control equipments resulting in the release of greater amount of carbon monoxideand other polluting gases.

Do you know whycarbon monoxide is poisonous? It binds to haemoglobin to form carboxyl haemoglobin, which is about 300 times more stable than theoxygen-haemoglobin complex. In blood, when the concentration of carboxyl-haemoglobin reaches about 3–4 per cent, the oxygencarrying capacity of blood is greatlyreduced.

This oxygen deficiency, results intoheadache, weak eyesight, nervousness andcardiovascular disorder. This is the reason whypeople are advised not to smoke. In pregnantwomen who have the habit of smoking theincreased CO level in blood may inducepremature birth, spontaneous abortions anddeformed babies.

Carbon dioxide:

Carbon dioxide (CO) is released into the atmosphere by respiration, burning of fossil fuels for energy, and bydecomposition of limestone during themanufacture of cement. It is also emittedduring volcanic eruptions. Carbon dioxide gasis confined to troposphere only.

Normally it forms about 0.03 per cent by volume of theatmosphere. With the increased use of fossil fuels, a large amount of carbon dioxide gets released into the atmosphere. Excess of CO2in the air is removed by green plants and this maintains an appropriate level of CO in the atmosphere.

Green plants require CO2 for photosynthesis and they, in turn, emit oxygen, thus maintaining the delicate balance. As you know, deforestation and burning of fossil fuel increases the CO2level and disturb the balance in the atmosphere. The increased amount of CO2in the air is mainly responsible for globalwarming.

Global Warming and Greenhouse Effect

About 75 per cent of the solar energy reaching theearth is absorbed by the earth’s surface, whichincreases its temperature. The rest of the heat radiates back to the atmosphere. Some of the heat is trapped by gases such as carbondioxide, methane, ozone, chlorofluorocarboncompounds (CFCs) and water vapour in theatmosphere.

Thus, they add to the heating ofthe atmosphere. This causes global warming.We all know that in cold places flowers, vegetables and fruits are grown in glasscovered areas called greenhouse. Do you knowthat we humans also live in a greenhouse? Ofcourse, we are not surrounded by glass but ablanket of air called the atmosphere, which haskept the temperature on earth constant forcenturies. But it is now undergoing change, though slowly.

Just as the glass in agreenhouse holds the sun’s warmth inside, atmosphere traps the sun’s heat near theearth’s surface and keeps it warm. This iscalled natural greenhouse effect because itmaintains the temperature and makes theearth perfect for life. In a greenhouse, visiblelight passes through the transparent glass andheats up the soil and the plants.

The warmsoil and plants emit infrared radiations. Sinceglass is opaque to infrared (heat) radiations; itpartly reflects and partly absorbs theseradiations. This mechanism keeps the energyof the sun trapped in the greenhouse. Similarly, carbon dioxide molecules also trap heat as they are transparent to sunlight but not to the heat radiation. If the amount ofcarbon dioxide crosses the delicate proportion of 0.03 per cent, the natural greenhousebalance may get disturbed. Carbon dioxide is the major contributor to global warming.

Besides carbon dioxide, other greenhouse gases are methane, water vapour, nitrous oxide, CFCs and ozone. Methane is produced naturally when vegetation is burnt, digested or rotted in the absence of oxygen. Large amounts of methane are released in paddy fields, coal mines, from rotting garbage dumps and by fossil fuels.

Chlorofluorocarbons (CFCs) are man-made industrial chemicals used in air conditioning etc. CFCs are also damaging the ozone layer. Nitrous oxide occurs naturally in the environment. In recent years, their quantities have increased significantly due to the use of chemical fertilsers and the burning of fossil fuels.

If these trends continue, the average global temperature will increase to a level which may lead to melting of polar ice caps and flooding of low lying areas all over the earth. Increase in the global temperature increases the incidence of infectious diseases like dengue, malaria, yellow fever, sleeping sickness etc.