In March 2019, the European Space Agency (ESA) published images took from their satellites. These images show a big stain, made of nitrogen dioxide and fine particles, situated above the Po Valley area. The Lombardy region is the geographic and economic epicenter of this area, with more than 10 million residents and the highest gross domestic product per inhabitant of the country. Most of its major cities are located in the basin of the Po River, which crosses the entire region. The big stain analyzed by ESA is the main reason why Po valley air pollution levels are so high it is considered nowadays the worst area in Europe for air quality. Milan has high levels of ozone and nitrogen oxides, which are mainly produced by cars diesel and petrol engines.