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Air quality trends in East Asia. Map of East Asia, showing changes in atmospheric nitrogen dioxide concentrations from 2005 to 2014. Blue indicates a decrease in nitrogen dioxide, and red indicates an increase. Nitrogen dioxide gas is a common emission from vehicles, power plants and industry, and is used as an indicator of general air quality. It can cause respiratory problems, and can transform into ozone, another major respiratory pollutant. Observations indicate that Japan and Hong Kong have improved air quality while Eastern China's expanding industrial regions show increased air pollution. Data obtained by the Ozone Monitoring Instrument aboard NASA's Aura satellite.