investigating underlying principles to guide health impact assessment

background many countries conduct health impact assessment (hia) of their projects and policies to predict their positive and negative health impacts. in recent years many guides have been developed to inform hia practice, largely reflecting local developments in hia. these guides have often been designed for specific contexts and specific need, making the choice between guides difficult. the objective of the current study is to identify underlying principles in order to guide hia practice in iran.   methods this study was conducted in three stages: 1) studies comparing hia guidelines were reviewed to identify criteria used for comparison seeking emphasized principles. 2) the hia characteristics extracted from published papers were categorized in order to determine the principles that could guide hia practice. 3) finally, these principles were agreed by experts using nominal group technique.   results the review of the studies comparing hia guides demonstrated there are no clear comparison criteria for reviewing hia guides and no study mentioned hia principles. investigating the hia principles from peer-reviewed papers, we found 14 issues. these were, considering of general features in planning and conducting hias such as hia stream, level, timing and type, considering of the wider socio-political and economic context, considering of economic, technical and legal aspects of hia and capacities for hia, rationality and comprehensiveness, using appropriate evidence, elaborating on hia relation to other forms of impact assessment, considering of equity, and encouraging intersectoral and interdisciplinary cooperation, involvement of stakeholders and transparency as underlying principles to guide hia practice. the results emphasize how critical these technical as well as tactical considerations are in the early scoping step of an hia which plans the conduct of the hia in reponse to local contextual issues.   conclusion determining the principles of hia from peer-reviewed papers provides an opportunity for guiding hia practice comprehensively. it seems to be feasible to develop a universal guide that covers all principles required.