Smog

Smog is a thick, smoky fog that forms when pollution mixes with sunlight and still air. It can make it hard to breathe and see clearly.

The word smog combines smoke and fog.

Smog is most common in busy cities on hot, calm days, when pollution cannot blow away.

People with asthma are especially affected. When smog warnings are issued, it is wise to stay indoors and avoid heavy exercise outside.