The World Health Organization (WHO) has classified air pollution as a leading cause of cancer in humans.
Kurt Straif, the head of the WHO's International Agency for Research on Cancer (IARC), said "the air we breathe has become polluted with a mixture of cancer-causing substances" and is "not only a major risk to health in general but also a leading cause of cancer.
Straif said the sources of air pollution are widespread and include transportation, power plants, and industrial and agricultural emissions.
Straif said the most polluted cities are in China and India.
A report released by European environmental regulators earlier this week said the most-polluted cities in Europe are in Bulgaria and Poland.
Based on reporting by AP and AFP