At the heart of the VW revelations are tests for exhaust emissions of nitrogen oxides. These are a family of pollutants that become dominated by nitrogen dioxide as they mix in the air, causing most UK cities and towns to exceed World Health Organisation guidelines.

Defra estimates that the UK death rate is 4% higher due to nitrogen dioxide pollution – around 23,500 extra deaths per year. This has a massive cost to society – around £13bn per year or 0.7% of our gross domestic product.

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Source: Guardian Environment