The term "smog" was first coined more than three decades ago to describe a mixture of smoke and fog in the air.
Subsequently, smog was referred to almost exclusively as ground-level ozone giving rise to ground-level ozone management programs in Canada, the United States and other countries in the 1980's and 1990's.
Over the last decade, the importance of particulate matter (PM) as a cause of adverse health effects and a primary component of smog has been increasing recognized. The term smog is now considered to include both particulate matter and Ozone and the precursor pollutants Sulphur Dioxide(SO2), Nitrogen Oxides(NOx), Volatile Organic Compounds(VOCs) and gaseous Ammonia(NH3).
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