Decomposition of organic matter through microbial activity is known as

1. Environmental Pollution:

Environmental pollution is the effect of undesirable changes in our surroundings that have harmful effects on plants, animals and human beings. A substance which causes pollution is called a pollutant. They can be solid, liquid or in the gaseous state.

2. Atmospheric Pollution:

The atmosphere that surrounds the earth is not of the same thickness at different heights Atmospheric pollution is generally studied as tropospheric and stratospheric pollution. The ozone layer prevents about 99.5% of the sun's UV rays.

3. Tropospheric Pollution:

Tropospheric pollution occurs due to the presence of undesirable solid or gaseous particles in the air. The following are the major gaseous and particulate pollutants present in the troposphere.

(i) Gaseous air pollutants: These are oxides of sulphur, nitrogen and carbon, hydrogen sulphide, hydrocarbons, ozone and other oxidants.

(ii) Particulate pollutants: these are dust, mist, fumes, smoke, smog etc.

Global warming, Greenhouse effect, Acid rain and Smog.

4. Global Warming and Greenhouse Effect:

About 75% of the solar energy reaching the earth is absorbed by the earth's surface, which increases its temperature. The rest of the heat radiates back to the atmosphere. Some of the heat is trapped by the gases such as carbon dioxide. methane, ozone, CFCS and water vapor. They add to the heating of the atmosphere causing Global warming in a greenhouse, visible light passes through the transparent glass and heats up the soil and the plants. The warm soli and plants emit infrared rays. It partly reflects and partly absorbs these radiations. This mechanism keeps the energy of the sun trapped in the greenhouse.

5. Acid Rain:

When the pH of the rainwater drops below 5.6, it is called acid rain. Acid rain is harmful for agriculture, trees and plants as it dissolves and washes away nutrients needed for their growth. It causes respiratory ailments in human beings and animals. When acid rain falls and flows as ground water to reach rivers, lakes etc., it affects plants and animal life in aquatic ecosystem.

6. Smog:

The word smog is derived from smoke and fog. There are two types of smog: classical and photochemical smog. Classical smog occurs in cool humid climate. It is a mixture of smoke, fog and sulphur dioxide. It is also called reducing smog. Whereas photochemical smog occurs in warm and dry sunny climate. It has high concentration of oxidizing agents and therefore it is also called as oxidizing smog.

7. Ozone Hole and Water Pollution:

(i) Ozone hole:

Depletion of ozone layer is known as ozone hole.

(ii) Effects of depletion of the ozone layer:

With the depletion of ozone layer, more UV radiation filters into troposphere. UV radiations lead to ageing of skin, cataract, sun burn, skin cancer, killing of many phytoplanktons, damage to fish productivity etc.

(iii) Water pollution:

Contamination of water by foreign substances which make it harmful for health of animals or plants or aquatic life and make it unfit for domestic, industrial and agriculture use.

(iv) Sources/causes of water pollution:

Sewage and domestic wastes Industrial effluents, Agriculture effluents, mixing of soil or rock into water, thermal pollutants, radioactive discharge.

8. Eutrophication, Soil Pollution, BOD and Green Chemistry

(i) Eutrophication

The process in which nutrient rich water bodies support a dense plant population, which kills animal life by depriving it of oxygen and results in subsequent loss of biodiversity is known as Eutrophication.

(ii) BOD

The amount of oxygen required by bacteria to break down the organic matter present in a certain volume of a sample of water, is called Biochemical Oxygen Demand $\left(\mathrm{BOD}\right)$.

(iii) Soil pollution

Insecticides, pesticides and herbicides cause soil pollution.

(iv) Green chemistry

Green chemistry us a way of thinking and is about utilizing the existing knowledge and principles of chemistry and other sciences to reduce the adverse impact on environment. Green chemistry is a production process that would bring out minimum pollution or deterioration to the environment. Utilization of existing knowledge base for reducing the chemical hazards along with the development of activities is the foundation of green chemistry.