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.