Progress in research to assess the effectiveness of air quality interventions towards improving public health

This paper describes progress made in research to evaluate whether actions taken to improve air quality have resulted in reduced ambient concentrations of relevant air pollutants, exposure, and adverse health effects, also known as air quality health outcomes or accountability research. The past two decades have seen a number of accountability studies that have started to tackle the many difficulties encountered in conducting such research. Difficulties may range from a lack of good quality data on air quality and health outcomes, interventions that are part of complex programs affecting air quality in different ways and the need for advanced statistical approaches that can address confounding issues, especially when evaluating regulations that are implemented over an extended period of time. Substantial progress has been made on many fronts. Early studies of short to medium-term interventions (e.g., shutting down a steel mill) have shown dramatic, sudden improvements in air quality with clear improvements in health outcomes. However, in many cases, the changes in air quality may be more subtle (e.g., measures to reduce traffic congestion). Studies of complex regulations that are implemented in multiple-year programs (e.g., national ambient air quality standards for particulate matter) remain challenging due to the need to correct for other changes that occur over the same time frame, such as changes in smoking and demographic characteristics. We describe recent progress to study the effectiveness of air quality regulations, using examples from around the globe, and discuss the challenges inherent in this type of research. Common study design issues that remain important for future studies are the selection of appropriate control populations, counterfactual air quality scenarios, and time periods surrounding the implementation of the regulatory action, as well as appropriately adjusting for unmeasured and potentially confounding factors or regional meteorological effects. Continued exploration of alternative approaches and additional methods development, especially for evaluating complex long-term regulatory actions, is recommended.