A type of air pollution that occurs when various types of particle elements contaminating the air have a chemical reaction with sunlight. This is why smog is a more prevalent problem in places where sunlight is strong for most parts of the year. Places such as London, Tehran and Mexico City have suffered from smog air pollution. In the US, cities such as New York and Los Angeles have experienced death tolls as a result of smog air pollution. Smog can create or exacerbate health conditions. For instance, asthmatics and people with lung conditions or allergies can suffer worsening symptoms during the height of a smog episode.