Nearly every large city on Earth suffers from some degree of smog. This word itself is a combination of smoke and fog, though the actual substance that hangs in the atmosphere like a dirty brown blanket is usually far more complicated.
During the initial century of the industrial revolution, smog was almost always caused by the emissions from coal-fired plants. However, since the 1950s, smog is more commonly found as petrochemical smog, most often containing a mixture of nitrogen oxides and a whole collection of volatile organic compounds (VOCs) that are both emitted by the burning of fossil fuels, chiefly automobiles.
If these chemicals remained as they were, they’d be problematic enough. However, once released into the atmosphere, they react with sunlight to form many noxious compounds including carbon monoxide, particulate matter, ground-level (or tropospheric) ozone, sulfur dioxide and nitrogen dioxide. All of these substances are known to cause respiratory disorder and premature death.
