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.

However, her colleague, Maria Neira, says there are effective measures to deal with the problem. Neira leads the WHO’s Public Health and Environmental Policy.

"You will see that in those cities where measures have been put in place, you can see a decrease on the levels of air pollution and, therefore, on the health risks caused by air pollution."

Neira agrees in the report that "urban air pollution continues to rise at an alarming rate” and severely affects human health. But she says the study shows improvements too. In her words, "awareness is rising and more cities are monitoring their air quality." This includes increased monitoring of particulate matter in the air.

"Particulate matter," also known as particle pollution, is a complex mixture of extremely small particles and liquid droplets. Particulate matter is made up of a number of parts, including acids, organic chemicals, metals, and soil or dust. The WHO study finds a reduction in air pollutants could lessen deaths from particulate matter by 15 percent.

Carlos Dora is Coordinator of WHO's Department of Public Health and Environmental Policy. He says there are low-cost ways to improve air quality for even the poorest cities. These methods include using renewable power sources, such as solar and wind, and sustainable public transportation.

"If you have clean transportation means, like cycling, walking or rapid transit systems -- where you have a lot of people being carried with a few vehicles -- then you have less air pollution. Or, if you have cities like New York, which have cleaned the fuel to heat and cool the buildings in a major way … then you have important improvements in air pollution."

The report says the areas with the poorest air quality are in the Eastern Mediterranean and South East Asia.