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.