The processes which are responsible for the reduction of dissolved oxygen in water are excessive use of phosphatic and nitrate fertilisers, detergents, the discharge of human sewage and organic waste from food, paper and pulp industries. The microorganisms which oxidise organic matter also use oxygen dissolved in water. Moreover, during night, photosynthesis stops bu the aquatic plants continue to respire, resulting in reduction of dissolved oxygen.

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 (BOD). The amount of BOD in the water is a measure of the amount of organic matter in water, in terms of how much oxygen will be required to break it down biologically. Clean water would have BOD value of less than 5 ppm.

Domestic wastes are collected in small bins and carried to the disposable site. At the site garbage is sorted out and separated into biodegradable wastes. Biodegradable wastes such as vegetable and fruits waste, animal waste, etc., are deposited in land fills and are converted into manure.

Ozone prevents harmful UV radiations of the Sun from reaching to the earth's surface, thereby it protects life from adverse effects of UV radiations. If ozone is removed completely from the stratosphere, the UV rays will reach to the earth and will lead to several diseases like sunburn, skin infection etc.

Ozone in the stratosphere is a product of the action of UV radiations on dioxygen (O₂ ) molecules. The UV radiations split apart molecular oxygen into free oxygen atoms. These oxygen atoms combine with the molecular oxygen to form ozone.
\(O_{ 2 }\left( g \right) \underrightarrow { UV \ radiations } \left( O \right) \left( g \right) +\left( O \right) \left( g \right) \)
\(O_{ 2 }\left( g \right) +\left( O \right) \left( g \right) \begin{matrix} UV \\ \rightleftharpoons \\ Radiations \end{matrix}O_{ 3 }\left( g \right) \)

Pesticides These substance are used to kill or block the reproductive processes of pests. These are basically synthetic toxic chemicals with ecological repercussions. Their accumulation in higher animals causes serious metabolic and physiological disorders. DDt, aldrin, dieldrin, organophosphates and carbonates are some examples of pesticides.
Herbicides These are weed killers i.e. they kill unwanted herbs growing along with crops e.g. sodium chlorate (NaCIO₃) and sodium arsenite (Na₃AsO₃). These are toxic to mammals. Some herbicides cause birth defects. Now a days triazines, better herbicides are used for the corn fields.

As we go up the earth's surface, from 0-11 km (called troposphere), temperature decreases and from 11-50 km (called stratosphere). temperature increases. From 50-85 km (called mesosphere), temperature again decreases and family from 85-100 km (called thermosphere), temperature again increases.

In the stratosphere, ozone is continuously created and destroyed by the sun's radiations. This results in an equilibrium concentration of ozone. In 1980s atmospheric scientists working in Antartica reported about the depletion of ozone layer commonly known as ozone hole over the South Pole.

(i) Motor vehicles, like cars, emit harmful gases like carbon monoxide, oxides of nitrogen and sulphur etc.
(ii) The level of carbon monoxide gas in the atmosphere can be reduced in following ways.
(a) By using CNG as a fuel.
(b) Using electric cars, bicycles.
(c) Use of public transports like metro instead of personal vehicles.
(d) Avoid burning of dry leaves, plastic bags, etc.
(iii) Mr. Lalit Jain is contributing positively to reduce air pollution by using metro.

(i) Following ways can reduce water pollution.
(a) Municipality should dispose sewage after proper treatment
(b) Immersion of statues should be controlled.
(c) People should not take bath with soap in rivers and should not wash clothes.
(d) Do not allow industries to dispose off untreated pollutants into river.
(ii) It leads to waterborne disease in animals as well as human beings
(iii) Dissolved oxygen reduces in water due to pollution and aquatic animals cannot survive in polluted water.
(iv) They saved yamuna from pollution and showed concern for environments.

Photochemical smog consists of O₃ , NO, acrolein, formaldehyde and PAN. O₃ and NO irritate the nose and throat and their high concentration causes a headache, chest pain, dryness of throat, cough, difficulty in breathing. Aldehydes and PAN cause irritation in eyes. PAN is highly toxic substance to plants and causes bronzing of tender leaves. Ozone also affects the rubber articles and causes cracking and ageing.
Control of Photochemical smog if primary precursors of photochemical smog such as hydrocarbons and NO₂ are controlled, the secondary precursors such as O₃ and PAN will automatically be reduced.
Certain Plants such as Pinus, Junioparus etc., can metabolise nitrogen oxide and can help in reducing photochemical smog.

In the summer season, nitrogen dioxide and methane react with chlorine monoxide and chlorine free radicals forming chlorine sinks, preventing much ozone depletion, whereas in winters, special type of clouds, called the polar stratospheric clouds are formed over Antarctica.
These polar stratospheric clouds provide the surface on which chlorine nitrate gets hydrolysed to form hypochlorous acid.It also reacts with hydrogen chloride to give molecular chlorine.
\({ CIO }^{ . }\left( g \right) +N{ O }_{ 2 }\left( g \right) \longrightarrow { CIONO }_{ 2 }\left( g \right) \)
\({ Cl }^{ . }\left( g \right) +{ CH }_{ 4 }\left( g \right) \longrightarrow { CH }_{ 3 }\left( g \right) +HCI\left( g \right) \)
\( { CIONO }_{ 2 }\left( g \right) +{ H }_{ 2 }O\left( g \right) \longrightarrow HOCI\left( g \right) +HN{ O }_{ 3 }\left( g \right) \)
\( { CIONO }_{ 2 }\left( g \right) +HCI\left( g \right) \longrightarrow { CI }_{ 2 }\left( g \right) +HN{ O }_{ 3 }\left( g \right) \)
When sunlight returns to the Antarctica in the spring, the Sun's warmth breaks up the clouds and HOCI and CI₂ are photolysed by sunlight.
\( \mathrm{HOCl}(g) \stackrel{h \mathrm{v}}{\longrightarrow} \dot{\mathrm{O}} \cdot \mathrm{H}(g)+\dot{\mathrm{Cl}}(\mathrm{g}) \)
\(\mathrm{Cl}_{2}(g) \stackrel{h \mathrm{v}}{\longrightarrow} 2 \overline{\mathrm{C}} \mathrm{l}(\mathrm{g}) \)
The chlorine radicals thus formed, initiate the chain for ozone depletion.

(i) Certain plants, e.g. Pinus, Juniparus, Quercus, Pyrus and Vitis can metabolise nitrogen oxide (NO) and therefore, their plantation could help in reducing photochemical smog.
(ii) Liquefied CO₂ with a suitable detergent is used for dry cleaning and H₂O₂ is used for the better results and makes use of lesser amount of water.
(iii) Soaps are 100% biodegradable so they should be used in place of detergents. Now-a-days biodegradable detergents are available. Therefore, they should be used in place of non-biodegradable hard detergents.
(iv) CNG should be used as it causes much less pollution. Moreover, electrical vehicles should be used to reduce the consumption of petrol and diesel