Background Health Impact Assessment (HIA) is a decision support method for promoting health and wellbeing used in urban spatial planning. There is a lack of evidence on the capacity of HIA to achieve health benefits in England, and no national guidance on the use of HIA. We undertook an evidence synthesis and used stakeholder consultation to understand the relevant factors and mechanisms in which HIA may lead to health outcomes in a population.

Methods We focused on UK evidence along with HIA frameworks from outside the UK. We searched academic databases, and conducted citation and reference list searches of included papers. We considered UK grey literature, identified primarily by discussion with our stakeholders. We extracted and tabulated data from included studies and conducted quality appraisals. We developed a typology of key factors to determine whether any existing frameworks for conducting HIA might be applicable in England. We involved a stakeholder group of planners, local and national government public health specialists, academics, and commercial HIA providers (n= 38 in total) in two workshops to identify grey literature sources and to interpret the potential suitability of the identified HIA frameworks for developing national guidance.

Results From 5117 unique records we considered 147 full texts from databases, and 58 grey literature sources. We include a total of 19 UK studies, plus 7 additional non-UK HIA frameworks. The evidence described qualitative factors which influence HIA effectiveness including team work between health and planning professionals, links with commercial developers, resources, national policy and guidance, evidence base, knowledge and understanding, training needs, timescales, health definitions and measuring impact. We also identified a small number of existing HIA frameworks. Our final stakeholder workshop identified the challenges in using existing frameworks to develop national guidance for England including the lack of national policy imperative.

Conclusion Evidence exists which could inform the development of national guidance for conducting HIA in spatial planning in England, but not to link the use of HIA in spatial planning directly to health outcomes. It is challenging to see how an evidence base could develop without a national policy requiring the use of HIA in the spatial planning process. Ironically, in order to generate data to demonstrate the effectiveness of HIA in terms of health improvement, it would first be necessary to make them an explicit requirement of the planning process.