Beyond direct impact: evidence synthesis towards a better understanding of effectiveness of environmental health interventions.

Systematic reviews are a cornerstone of evidence-based public health, and there is much discussion on how this method may need to be modified to do justice to complex interventions, such as environmental health interventions. This paper asserts that intervention effectiveness is influenced by variability in five distinct layers--direct (intrinsic) impact, user compliance, delivery, programming and policy measures--which are embedded in the broader geographical, socio-economic, political and cultural context. The multi-component, multi-sectoral nature of most environmental health interventions results in a complex relationship between these layers of influence, involving systemic interactions. As illustrated with examples, understanding environmental health interventions critically relies on considering all of these layers. These distinct layers of influence can serve as a framework towards the comprehensive analysis of environmental health interventions in systematic reviews, drawing on quantitative and qualitative methods and a variety of disciplines.