The application of photocatalysts in concrete technology is by now a well-established concept. However, despite the great opportunities for air quality improvements to be derived from the considerable concrete surfaces exposed to the atmosphere, particularly in cities where air quality is greatly affected by vehicle exhaust and industrial emissions, photocatalytic concretes are still not in mainstream application. With current levels of NOx pollution considerably exceeding EU legislative guidelines in urban centres throughout the industrialised world, it is important to consider what the issues are. The likely barriers to more widespread implementation are likely to include cost effectiveness, which needs to be related to photocatalyst impact, but the challenges in measuring impact on air quality directly are complex. This paper seeks to place photocatalytic efficiencies into context, comparing performances of the conventional photocatalyst dispersion in surface mortar coatings with that of photocatalysts supported on surface exposed aggregates. The nature and impact of catalyst binding to the aggregate is also discussed.