A new study finds that more than 80 percent of people living in cities are breathing unsafe air. The World Health Organization study on urban air quality says those most affected live in the world's poorest cities. The study finds urban air pollution has nearly doubled in 3,000 cities over the past two years. The cities are in 103 countries. The study also shows that almost all cities with populations over 100,000, and in developing countries, have air pollution levels that do not meet WHO guidelines. The WHO warns that as air quality worsens the risk increases for many diseases. These include stroke, heart disease, lung cancer, and breathing diseases such as asthma. Poor air quality is also responsible for an estimated seven million premature deaths every year. Flavia Bustreo is WHO Assistant-Director General of Family, Women and Children's Health. In the report, she says that dirty air in cities most affects the youngest, oldest, and poorest people.