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Air quality in South Africa. Map of South Africa showing atmospheric nitrogen dioxide concentrations, averaged over 2014. Blue and green denote lower concentrations and red and red areas indicate higher concentrations. Concentrations are highest in the region of Johannesburg and Pretoria. 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. Data obtained by the Ozone Monitoring Instrument aboard NASA's Aura satellite.