What is the difference between a CAZ and a congestion charge?

Congestion and air pollution are related but are not the same. Clean Air Zones are designed to improve air quality to meet legal requirements and therefore focus on tackling polluting vehicles. A congestion charge does not distinguish between vehicles based on pollution, but aims to reduce the total number of vehicles on the road or within an area. A Clean Air Zone may indeed impact on congestion, but this is likely to only be in the short-term whilst vehicles are upgraded to cleaner models or people adjust their travel choices.