#22 Graz, Austria

Unfortunately, small particles that interact with light to enhance sunsets are also the most damaging to human health. They penetrate deeply into the lungs and blood, shuttling toxic chemicals to vital organs. These tiny particles are blamed for increases in cancer, brain damage, heart attacks, premature births, pneumonia, asthma and bronchitis.

China, India and Iran -- some of the world's top polluters -- don't appear on this top 25 list. That's because they don't widely measure dangerous small-particle pollution, the gold standard for gauging pollution's health risks. The different testing methods can lead to major discrepancies. Last month, for instance, Beijing's environmental agency reported "light pollution" and "breathable air" on the same hazy night that a $20,000 device atop the U.S. embassy just blocks away reported pollution of 434 on a 500-point index. The Beijing embassy's machine generated this caution to thousands of people who subscribe to updates on Twitter or an iPhone: "Emergency conditions. The entire population is more likely to be affected."??By comparison, a monitor in New York read 24 that day.

The view from the top of Schoeckl mountain, near Graz.