Volkswagen and pollution: How do you test a car's emissions?
Volkswagen chief executive Martin Winterkorn has resigned following the revelation that the firm manipulated US diesel car emissions tests. It has put the issue of testing car pollution in the spotlight.
But how do you go about testing a car for its capacity to pollute? BBC News visited Emission Analytics in London to find out about one common method of doing a detailed measurement.
You can learn more about the Volkswagen scandal here.
Video Journalist: Dougal Shaw