<P> Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or damages the natural environment into the atmosphere . Ever since the beginning of the Industrial Revolution in the United States, America has had much trouble with environmental issues, air pollution in particular . According to a 2009 report, around "60 percent of Americans live in areas where air pollution has reached unhealthy levels that can make people sick". Pollution in the United States has plummeted in the recent decade, with pollutants such as nitrogen dioxide decreasing despite the fact the number of vehicles on the road isn't . This is due to better regulations, economic shifts, and technological innovations . With respect to nitrogen dioxide, NASA reported a 32% decrease in New York City and a 42% decrease in Atlanta between the periods of 2005 - 2007 and 2009 - 2011 . Air pollution can cause a variety of health problems including, but not limited to infections, behavioral changes, cancer, organ failure, and even premature death . These health effects are not equally distributed in terms of race, ethnicity, socioeconomic status, education, and more in the United States . California has the worst air quality of any state, and in most surveys the cities in California rank in the top 5 or top 10 of most polluted air in the United States . </P> <P> In the 1960s, 1970s, and 1990s, the United States Congress enacted a series of Clean Air Acts which significantly strengthened regulation of air pollution . Individual U.S. states, some European nations and eventually the European Union followed these initiatives . The Clean Air Act sets numerical limits on the concentrations of a basic group of air pollutants and provide reporting and enforcement mechanisms . </P>

Where does most air pollution in cities come from