If something is smoggy, it's hazy with a smoky kind of fog. On a smoggy day, you can see the pollution in the air.
Smoggy air is full of smog, a word that comes from combining smoke and fog. The term was invented in the early 1900s to describe an unfortunate weather phenomenon first noted in London, where "pea soup fog" was a serious problem for much of the 19th and 20th centuries. Today's smoggy cities get their smog from different sources—instead of coal fires, for example, smoggy pollution is mainly caused by cars.