there are many types of pollution around the world and thats a problem. One type of pollution in the cities its Photochemical smog. the photo in photochemical means "light". This smog is made up of sunlight, hydrocarbons and nitrogen oxides. when these chemicals react it makes a brownish mix of ozone and other pollutants. The ozone protects us from UV rays. (ultra violet rays) This becomes and can harm living things. for humans it will irritate throat and lungs and animals and plants. However, we are trying to improve air quality around the world making new power plants that run cleaner than older power plants, less polluting cars but the flip side to cars is that there are more running and more fossil fuels burning in cities. Cities are still very polluted and are trying to make that better.