Community exchange and time currencies: a systematic and in-depth thematic review of impact on public health outcomes

•Few published studies of time currencies robustly measure health or well-being, and none report costs.•Studies contain rich narratives of positive impact on health, well-being and broader community outcomes.•A theory-driven approach to investigating time currencies could reveal underpinning mechanisms and pathways to impact.•There is an urgent need to address outcome measurement in community-based initiatives to support public health practice.