Air pollution has always been a hot topic in the science community, but NASA reports that there has been a steady decline in air pollution throughout the Eastern part of the United States, including Washington D.C., since 2005. A similar decline was also seen in Western parts of the United States, although the study was focused more towards the East coast nitrogen levels.
According to NASA, Philadelphia experienced a 26 percent decrease in nitrogen dioxide between 2005 and 2007, and 2009 and 2011. "Nitrogen dioxide is one of the six common pollutants regulated by the U.S. Environmental Protection Agency to protect human health," explains NASA in a statement to Science World Report. "Alone it can impact the respiratory system, but it also contributes to the formation of other pollutants including ground-level ozone and particulates, which also carry adverse health effects."
It appears that the primary reasons for decreased rates of air pollution was primarily due to better regulations, technological improvements, and economic shifts. Almost 19.4 million metric tons of Nitrogen oxides are released by man-made sources in the United States every year - these can primarily be found from vehicles and coal-powered plants, reports CNN. Nitrogen oxide is just one of six common air pollutants that the Environmental Protection Agency has set limits on in our atmosphere, and the EPA reports that over 140 million Americans may still live in areas that contain unhealthy levels of air pollution, such as Nitrogen oxide.
"While our air quality has certainly improved over the last few decades, there is still work to do - ozone and particulate matter are still problems," explains Bryan Duncan, an atmospheric scientist at NASA's Goddard Space Flight Center.
More information can be found at the NASA official website.