Over 80% of the world’s cities have pollution levels exceeding WHO’s guidelines for safe air
From smog hanging over cities to smoke inside the home, air pollution poses a major threat to health and climate. The combined effects of ambient (outdoor) and household air pollution cause about 6.5 million premature deaths every year, largely as a result of increased mortality from stroke, heart disease, chronic obstructive pulmonary disease, lung cancer and acute respiratory infections. More than 80% of people living in urban areas that monitor air pollution are exposed to air quality levels that exceed the WHO guideline level of 10µg/m3, with low- and middle-income countries suffering from the highest exposures.
http://www.who.int/airpollution/ambient/about/en/
photo credit: Sunset Noir