noun
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A form of air pollution produced by the reaction of sunlight with hydrocarbons, nitrogen compounds, and other gases primarily released in automobile exhaust. Smog is common in large urban areas, especially during hot, sunny weather, where it appears as a brownish haze that can irritate the eyes and lungs. Ozone, a toxic gas that is not normally produced at lower atmospheric levels, is one of the primary pollutants created in this kind of smog.
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Also called photochemical smog
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Fog that has become polluted with smoke and particulates, especially from burning coal.
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In North America, the primary cause of smog is pollution from automobile exhaust.
The Los Angeles basin, where pollutants can be trapped by inversions and the surrounding mountains, has frequent problems with smog, as do other major urban areas.
The word smog is a combination of smoke and fog.
Other Word Forms
Etymology
Origin of smog
Explanation
What do you get when you combine "smoke" and "fog"? Smog! Smog is the thick haze or smoke that's caused by polluting substances in the air. Generally, the more cars there are in a city, the more likely there is to be smog. There are other culprits for smog, though, including coal-burning factories, other chemical emissions from plants and mills, and nearby forest fires. If there's too much smog in a city, it can make it hard for people with asthma to breathe.Smog is a great example of a portmanteau, a word formed by combining two other words into one: it comes from smoke and fog.
Vocabulary lists containing smog
Weather and Climate - Introductory
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Weather and Climate - Middle School
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Earth Science - Middle School
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Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
When the sun hits the smog enveloping Indonesian Borneo, it turns everything the colour of rust.
From BBC ● Oct. 20, 2026
The smog - also known as the haze - has also drifted beyond Indonesia's borders to neighbouring countries, raising public health concerns.
From BBC ● Oct. 13, 2026
A Japanese firefighting force sent to help tackle huge blazes in Indonesia is struggling to operate because the smog is too thick to fly helicopters, a spokesman told AFP on Thursday.
From Barron's ● Sep. 17, 2026
Under current law, cars manufactured before the 1975 model year are already exempt from smog check requirements.
From Los Angeles Times ● Sep. 17, 2026
The dust cloud behind the SUV was still thick like morning smog in Phoenix.
From "Healer of the Water Monster" by Brian Young
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Get a wordDefinitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
