In human history, we can easily find the term is called photochemical smog. It has caused thousands of dollars lost in the past and also killed lots of people. Although it is a very serious environmental issue, some people do not know what it is.
Generally speaking, it is a kind of air pollution. And smog is formed by the smoke from the pollution and the fog in the air. One special kind of smog which is formed when photons of sunlight hit molecules of different kinds of pollutants in the atmosphere is called photochemical smog. Furthermore the photons make chemical reactions happen. So the pollution molecules turn into other kinds of nasty chemicals. That mixture of bad chemicals is what I am talking about.
The chemicals in photochemical smog include Volatile Organic Compounds (VOCs), nitrogen oxides, ozone, and PAN (peroxyacytyl nitrate). Nitrogen oxides mostly come from the engines of cars and trucks. VOCs are given off by gasoline paint, and pesticides. Ozone is a form of oxygen which is harmful to human when the level in the air exceeds maximum. PAN is a type of pollution that is made by chemical reactions between other kinds of pollution.
Photochemical smog can make our breath hard and it smells bad. So people will get some diseases like asthma, tracheitis. In 19th century London, there had a very serious condition dealing with the smog. And in this case, thousands of people died of this kind of air pollution.
In conclusion, we need to decline the total amount of our emission and shut down some chemical industrials around urban area. And everybody should pay attention to the local air quality so we can report the pollution to the government as soon as possible.