One of are readers wants to know what exactly is smog, and how can smog affect human health? To answer as simply as possible smog is a kind of air pollution. Smog is usually the result of coal burning, industrial and vehicular emissions. Term "smog" was coined in 1905 by Dr. Henry Antoine Des Voeux in his paper "Fog and Smoke" after he named "smoky fog" that happens only in cities "smog". At that time term smog was used to describe so called "classic smog", namely the one that results from large amounts of coal burning in an area and is caused by a mixture of smoke and sulfur dioxide. But much has changed in the last 100 years, and smog in today's cities is no more classic smog but transformed to so called "photochemical smog".
Photochemical smog results from vehicular and industrial emissions that are acted on in the atmosphere by sunlight to form secondary pollutants: afterward these secondary pollutants combine with primary emissions causing photochemical smog. What really happens here is the collision of sunlight and various pollutants in the air. This collision often forms chemicals very dangerous for human health. Since photochemical smog needs sunlight it is more common in sunny cities, with warm, dry climate, and large number of vehicles or industry facilities.
Smog has harmful effect on human health, especially for children and senior citizens, and people suffering from heart and lung diseases. Smog has negative effect on breathing, it can decrease capacity of lungs, cause short breathing, wheezing, coughing, and sometimes even serious pain when inhaling deeply. Smog has become common phenomenon in many cities of Southeast Asia, but there are also many other cities worldwide faced with smog problem, for instance London and New York.