The air in Chinese cities is getting worse, and these animations show just how severe the problem has become.

This slider shows the dramatic difference between a clear day and a smoggy one in Beijing, a reminder of how bad the air pollution can get. Use your mouse on green slider to watch the sky change.

When a ranking Chinese government official slammed the U.S. embassy and consulates in China earlier this month for measuring local air pollution data, calling it "violating diplomatic conventions," Chinese web users snapped back. "Can't you see the bad pollution yourself?" asked one typical comment.

China's censors have tremendous power in print, online, and even in public spaces such as Tiananmen Square. But when it comes to air pollution, even the Chinese government can't obscure the facts. People see and breathe it every day.

The debate over whose statistics are most "accurate" can be confusing -- how to sort out truth from spin? That's why a group of us at the Asia Society decided to launch China Air Daily, a website that provides up-to-date information on air pollution in the country's largest urban sectors, and even compares them to major cities from elsewhere in the world.

According to a 2007 report produced by the World Bank and Chinese government, up to 400,000 Chinese die prematurely every year because of air pollution. The concentration of particles in the air that are smaller than 2.5 micrometers, the size at which they can penetrate the lungs, is on average 10 times higher in Beijing than in New York, according to our past three months of data collection. But such figures alone don't tell the story. A set images at the top of this page show Beijing on a clear day versus a smoggy; move the slider back and forth with your mouse and you might start to understand the extent to which pollution affects Chinese cities such as Beijing.

This animation shows the dramatic difference between two consecutive Beijing days from two angles (one from the ground and the other from space), when the smog clears.

In the months since we started this project, seeing any sky at all in the capital city is a luxury. Many Beijing residents are buying air purifiers or filters for their homes or cars. So when the Chinese government lashed out against the U.S. government's unilateral air monitoring, a lot of Chinese residents got furious. "Why don't you spend time fixing the problem instead of playing a blame game?" was a common post on Weibo, China's version of Twitter.

China, to be fair, has spent a lot of time and money to try to fix the problem. Over the 10 years before the 2008 Games, for instance, Beijing invested an estimated $32 billion to clean the air. They also shut down some industries for months and removed nearly half of the city's cars from roads -- authoritarian governments can do that -- the air did look better, even athlete-friendly. For a while, anyway.

Federico Rampini, once a Beijing correspondent for Italian paper La Repubblica who also loved to run, had to stop when his doctor discovered spots on an X-ray of his lungs, he told me. "Have you been smoking lately?" the doctor asked him. "No," Rampini said, "I have never smoked and I never will."

In China, grassroots environmental NGO's are using their own equipment to monitor air pollution. The Institute of Public and Environmental Affairs maps air and water pollution that's produced by industries in most Chinese provinces, increasing public pressure on these polluters. Chinese celebrities sometimes hop on the bandwagon too. Beijing real estate tycoon Pan Shiyi, with 10 million followers on his weibo account, posts pollution data online.

An animated chronicle of one of Beijing's worst weeks (Christmas week of 2007) for air quality.

Few non-political issues in China garner so much attention today. The Chinese government does seem to be listening and at least trying to act. More and more local governments, under public pressure to publish pollution data, are starting to release data on the concentration of particulates in the air, as well as on pollutants such as NO2, SO2 and Ozone. The public is also getting more engaged, as bad pollution strikes lesser-known cities. In Wuhan, a city in central China, the sky turned brown on June 11 as the air pollution index registered an astounding 478 on a scale from zero to 500.

Still, the challenges to cleaning China's air are daunting. We hope that our website will help foster ideas as well as awareness. There's a long way to go, for sure, but you don't need me to tell you: just look at the pictures.

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