Journalist Chai Jing used $160,000 of her own money to produce a documentary on China's air pollution problem. Screenshot/Under the Dome hide caption

itoggle caption Screenshot/Under the Dome

Smoke rises from chimneys of coal-based power plants in the Sonbhadra District of Uttar Pradesh, India. The Washington Post/Getty Images hide caption

itoggle caption The Washington Post/Getty Images

A jogger goes for a run amid heavy smog in Shanghai on Wednesday. China has for the first time agreed to limit its carbon emissions, but critics are questioning whether the move goes far enough. AFP/Getty Images hide caption

itoggle caption AFP/Getty Images

A view of the Tonawanda Coke plant in Tonawanda, N.Y., which was found to have emitted carcinogens at levels many times higher than the state's limit. John W. Poole/NPR hide caption

itoggle caption John W. Poole/NPR

Many people like these Tibetans in Qinghai, China, rely on indoor stoves for heating and cooking. That causes serious health problems. Courtesy of One Earth Designs hide caption

itoggle caption Courtesy of One Earth Designs

Police were checking cars throughout Paris on Monday, including near the Arc de Triomphe, as the city tried to cut air pollution by instituting odd-even driving restrictions. Philippe Wojazer /Reuters /Landov hide caption

itoggle caption Philippe Wojazer /Reuters /Landov

Headlamps make cold nights cozier, but leave the fuel-burning lanterns and stoves outside. Gopal Vijayaraghavan/Flickr hide caption

itoggle caption Gopal Vijayaraghavan/Flickr

Skyscrapers are obscured by heavy haze in Beijing on Jan. 13. Air pollution remains a serious — sometimes overwhelming — problem, but researchers say environmental technology is available to solve it. Ng Han Guan/AP hide caption

itoggle caption Ng Han Guan/AP

Men walk along a railway line in Beijing on Jan. 12, as air pollution reached hazardous levels. Wang Zhao/AFP/Getty Images hide caption

itoggle caption Wang Zhao/AFP/Getty Images