What air pollutants does smog consist of? How are they caused?What adverse effects do they have on human beings and the environment?

The term smog has been formed by
combination of two separate words, smoke and
fog. The term was first used in 1905 to describe the condition of
thick smoke and fog that simultaneously hung over cities in England. Smog is caused by
moisture in air condensing on smoke particles. The smog frequently contains sulphur
dioxide also which is a gas that affects lungs and makes breathing
difficult.

The term smog is now also used for conditions
created by action of sunlight on exhaust from vehicles and factories. It is called
Photochemical smog. This type of smog is created by the action of sunlight on
hydrocarbons and nitrogen oxides emitted in the air by the by the burning of petrol and
other petroleum products.  Under action of sufficient sunlight, these compounds undergo
a chemical reaction producing gases called oxidants.  These includes substances such as
Ozone, nitrogen dioxide and peroxyacetyl nitrate that irritate the eyes, nose, and
throat and damage the lungs.

Smog is caused by weather
conditions such as lack of wind and a layer of warm air settling over a layer of cool
air near the ground, which prevents smoke from rising up and scattering. Mountain ranges
near a city or a factory may also prevent smog from
scattering.

Heavy concentrations destroys plant life and
speeds deterioration of building materials.