(i) A whole variety of chemicals from industries, such as metals and solvents are poisonous to fish and other aquatic life.
(ii) Toxic pesticides can accumulate in fish and shellfish and poison the people who eat them.
(iii) Detergents and oil float spoils the water bodies.
(iv) Acids from mine drainage and salts from various sources can also contaminate water sources.

(i) Nitrogen oxides are introduced directly into the stratosphere by the supersonic jet aircraft engines in the form of exhaust gases.
(ii) These oxides are released by combustion of fossil fuels and nitrogen fertilisers. Inert nitrous oxide in the stratosphere is photochemically converted into more reactive nitric oxide.
(iii) Oxides of nitrogen catalyses the decomposition of ozone and are themselves regenerated. Ozone gets depleted as follows:
NO + O₃ ⟶ NO₂ + O₂
O₂ \(\underrightarrow { hv } \) O + O
NO₂ + O ⟶ NO + 2O^o.
Thus NO is regenerated in the chain reaction

(i) Smog is primarily responsible for acid rain.
(ii) Smog results in poor visibility and it affects air and road transport.
(iii) It also causes bronchial irritation.

(i) The great smog of London (1952) was an instance of severe air pollution that affected the London from 5th December-9th of December, 1952 and then dispersed quickly when the whether changed.
(ii) It causes major disruption by reducing the visibility and even penetrating indoor areas.
(iii) Government medical reports estimated that 4000 people had died as a direct result of smog and 100,000 were made will by the smog effect on their respiratory tract.

(i) The compounds composed of carbon and hydrogen only are called hydrocarbons. They are mainly produced naturally and also by incomplete combustion of automobile fuels.
(ii) They are potential cancer causing (carcinogenic) agents.
(iii) For example polynuclear aromatic hydrocarbons (PAH) are carcinogenic, they cause irritation in eyes and mucous membranes.