Los Angeles was one of the best-known cities suffering from transportation smog for much of the 20th century, so much so that it was sometimes said that "Los Angeles" was a synonym for "smog." In 1970, when the Clean Air Act was passed, Los Angeles was the most polluted basin in the country, and California was unable to create a State Implementation Plan that would enable it to meet the new air quality standards. However, ensuing strict regulations by state and federal government agencies overseeing this problem (such as the California Air Resources Board and the United States Environmental Protection Agency), including tight restrictions on allowed emissions levels for all new cars sold in California and mandatory regular emission tests of older vehicles, resulted in significant improvements in air quality. For example, air concentrations of volatile organic compounds declined by a factor of 50 between 1962 and 2012. Concentrations of air pollutants such as nitrous oxides and ozone declined by 70% to 80% over the same period of time.