The Public Health Impacts of UK's Clean Air Zones (PHICAZ) project is an ambitious research programme aiming to characterise the effects of urban clean air zones (CAZ) on air quality and to establish the impact of these changes on the health of the local residents. London was the first UK city in April 2019 to implement a large-scale CAZ, the Ultra Low Emission Zone (ULEZ), covering an area of central London with 200,000 residents. The planned expansion of the ULEZ will cover over 3.8 million people and several other large UK cities are planning to implement their own CAZs. This setting provides a unique opportunity to carry out a natural experiment to evaluate the public health impacts of a large-scale intervention to reduce urban air pollution. This research will determine the impact of the ULEZ on air quality, pollutant composition, noncommunicable disease and health inequalities throughout life. These results will then be tested against data coming from other UK cities implementing CAZs. The findings from this study should provide robust evidence to underpin the roll out of CAZs in the UK and worldwide.