Nitrogen dioxide (NO2) is a product of combustion, for instance emitted by road transport, and is generally found in the atmosphere in close association with other primary pollutants. Nitrogen dioxide is toxic, and its concentrations are also often strongly correlated with those of other toxic pollutants. As it is easier to measure, it is often used as a proxy for them. There is growing concern about rising levels of NO2 in fast-growing cities with large numbers of vehicles (WHO, 2006).

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