Smog is a portmanteau (blended) word formed from Smoke + Fog. The term is believed to have been coined in the early 20th century (often attributed to Henry Antoine Des Voeux, around 1905) to describe the thick, polluted air hanging over industrial cities like London, where coal smoke combined with natural fog to create a dense, harmful haze.

Smog is essentially a severe form of air pollution in which fine particulate matter, gases, and moisture combine to drastically reduce visibility and make the air irritating or hazardous to breathe. Two major types are recognised:

• Sulphurous / Classical (London-type) smog: Formed from smoke (soot, unburnt carbon particles) and sulphur dioxide (SO₂, from burning coal) reacting with fog in cold, humid, windless conditions. The infamous London smog episodes of the mid-20th century caused serious respiratory illness and mortality, eventually leading to clean-air legislation.
• Photochemical (Los Angeles-type) smog: Forms in warm, sunny, high-traffic conditions when nitrogen oxides (NOx) and volatile organic compounds (VOCs) from vehicle exhaust react under sunlight to produce ground-level ozone and other secondary pollutants, giving a brownish haze rather than a grey one.

Effects of smog include aggravated asthma and bronchitis, eye and throat irritation, reduced visibility (a driving and aviation hazard), damage to crops and vegetation, and corrosion of buildings/monuments.

The distractor options fail on definition: pairing smog with mist is circular and does not explain the word's origin; fog and clouds is incorrect because clouds are elevated water-droplet formations high in the atmosphere and are not a pollution phenomenon; and smoke and haze is close in spirit but not the actual etymology — haze is typically caused by dry dust/pollutant particles without the moisture component that defines true fog-based smog.