Pollutants like sulphur dioxide which is released while burning sulphur containing fuel and particulate matter like soot present in stagnant air masses, get modified in sunlight and form a sheet called photochemical smog.
Smog is a combination of fog, smoke and fumes released by mills and factories, homes and automobiles.
When sunlight falls on stagnant air under low humid conditions in the presence of pollutants such as SO2 soot, nitrogen oxide and hydrocarbons, photochemical smog is formed. (Photochemical: chemical reactions in the presence of light). Smog stays close to the ground and reduces visibility.
Photochemical smog is also called Pan smog due to the production of peroxyacetyl nitrate (PAN) and ozone which form from hydrocarbons and nitrogen oxides in the air in presence of solar radiation. PAN and Ozone are called photochemical oxidants. Both of these are toxic irritants to human lungs.
Smog formation is accompanied by temperature inversion or Thermal inversion. Temperature inversion causes smog to settle and remain near the ground till wind sweeps it away. Normally, warm air rises up into atmosphere. When a layer of cool air at the ground level is trapped there by an overlying layer of warm stagnant air, it is called temperature or thermal inversion.
Exposure to smog causes respiratory problems, bronchitis, sore throat, cold, headache and irritation to eye (red shot eyes). Smog also destroys crops and reduces crop yield.