Urban air pollution
According to the World Health Organization, 2.4 million people die every year primarily because of air pollution. Urban areas such as Los Angeles, Mumbai, Cairo, Bejing and many of the world's most populated cities have the worst air quality. Air pollution has been strongly correlated with increased rates of asthma, and pollution from automobiles has a strong link to pneumonia-related deaths. One of the worst cases of urban air pollution happened in London in 1952, when about 8,000 people died over the course of a few months because of a single smog event.