Objective
To synthesize evidence on mental health outcomes among volunteers in natural disasters, examining differences across volunteer groups and identifying risk and protective factors.

Methods
A systematic search was conducted in PubMed, Scopus, Web of Science, and Embase using terms on mental health, natural disasters, and volunteers. Eligible studies included cross-sectional, longitudinal, and randomized controlled trials with quantitative or mixed methods, published in English or Portuguese (2014-2024), involving adult volunteers in natural disasters. Two independent reviewers selected studies, methodological quality was assessed using the Joanna Briggs Institute checklist, and findings were narratively synthesized due to study heterogeneity.

Results
Seventeen studies (4,083 participants) were included. Post-traumatic stress symptoms were the most investigated, followed by depression, anxiety, and stress, whereas resilience and post-traumatic growth were less frequently reported. Community and spontaneous volunteers more frequently reported adverse outcomes. Adaptive psychological outcomes were frequently reported among students and observed among members of humanitarian organizations, professionals, and volunteer professionals, although distress-related outcomes occurred across all volunteer categories. Risk factors included trauma exposure and disaster-related losses, whereas protective factors included training, social and organizational support, and individual resources.

Conclusion
Mental health outcomes among disaster volunteers appear to vary by volunteer role, disaster exposure, training, and organizational support. Community and spontaneous volunteers showed greater vulnerability, although adaptive and distress-related outcomes occurred across groups. Findings support integrating training and mental health support into disaster response, although evidence is limited by study heterogeneity and predominantly cross-sectional designs.