DAVID NUNUK / SCIENCE PHOTO LIBRARY DAVID NUNUK / SCIENCE PHOTO LIBRARY
Smog. A hazy cloud of smog hangs over a city sky- line. Smog (smoky fog) occurs when atmospheric pollution builds up in city air, often reaching toxic levels. Acid smogs from domestic fires in London killed over 4000 people in 1952 and 1962. Modern smogs are usually caused by exhaust fumes from traffic, and although less severe can cause lung disease among children and the elderly. In hot countries, a toxic "photochemical smog" some- times occurs when sunlight reacts with atmospheric pollution, producing a hazy shroud that can be seen from a distance. This photograph was taken in Vancouver in British Columbia, Canada.
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