London currently has the highest nitrogen dioxide (NO2) concentration recorded for any European city and for particulate matter (PM) it has some of the worst hot spots. Therefore overall, for these two pollutants, London is the worst in the UK and amongst the worst in Europe. Exposure to elevated concentrations of air pollutants such as PM and NO2 has well-established heath effects and most countries now have strict guidelines for air quality. London's air quality problems are driven largely by traffic. This, along with the high density of people in an urban area results in air quality guidelines being exceeded on a regular basis and large numbers of people being affected. In an attempt to combat London's air quality problems the Mayor of London introduced a series of measures to decrease traffic emissions. These included both a restriction on the number of vehicles entering central London each day--the Congestions Charging Scheme (CCS), and the discouragement of the most polluting heavy goods vehicles from entering--the London Low Emission Zone (LEZ). Together, it is hoped that these measures will lead to an improvement in air quality and provide a direct health benefit to Londoners. Research underway is charting the progress of this real world experiment.