Smog is a thick, hazy mixture of air pollution that hangs over cities and makes the air look dirty and hard to breathe. The word combines “smoke” and “fog,” which describes what it looks like: a gray or brownish cloud that blurs distant buildings and can make your eyes water.

Smog forms when pollution from cars, factories, and power plants mixes with sunlight and gets trapped near the ground, especially in valleys or on hot, still days. Los Angeles became famous for its smog in the 1950s and 1960s, when the pollution got so thick that people couldn't see the mountains surrounding the city. Beijing and Delhi still struggle with dangerous smog levels that force schools to close and people to wear masks outdoors.

The word can also be used as a verb: when pollution smogs up an area, it creates that characteristic haze. Scientists sometimes distinguish between different types: photochemical smog forms from car exhaust reacting with sunlight, while industrial smog comes mainly from burning coal.