We evaluate methods to calculate the economic value of protected areas derived from the improved mental health of visitors. A conservative global estimate using quality-adjusted life years, a standard measure in health economics, is US$6 trillion p.a. This is an order of magnitude greater than the global value of protected area tourism, and two to three orders greater than global aggregate protected area management agency budgets. Future research should: refine this estimate using more precise methods; consider interactions between health and conservation policies and budgets at national scales; and examine links between personalities and protected area experiences at individual scale. C onservation is key to sustainability 1-3 , but biodiversity continues to decrease worldwide 4-6. Protected areas remain the core of global conservation strategies 7,8 , but are under increasing pressure 9-11 from political and economic factors 12,13 as well as climate change 14. Conservation relies on political advocacy, influenced by economic arguments 2,15 , based on ecosystem services 16 or tourism 17. Nature exposure improves human mental health and wellbeing 18-21. Poor mental health imposes major costs on human economies 22-24. Therefore, parks have an additional economic value through the mental health of visitors 25. We refer to this as a health services value. This may be considered as a component of ecosystem services value 26. Here we consider how to calculate health services value. Research on nature exposure and mental health falls into four main categories 20 : spatial correlations between nature access and mental health 27-29 ; joint patterns across populations 30,31 ; experimental tests linking nature exposure to specific psychological parameters 18-22,32-34 ; and qualitative analyses examining the psychological processes underlying these links 35,36. Healthrelated benefits include improved attention 32 , cognition 33 , sleep 37 , and stress recovery 38 , and apply across demographic and socioeconomic population sectors 18,19,25. Research on economic costs of poor mental health recognises four main categories: treatments 39,40 , both consultations and pharmaceuticals; caregivers, both paid and unpaid (e.g. family members); lost workplace productivity, through absenteeism 39 or poor performance (presenteeism) 41 ; and antisocial behaviour 42-44 , both public (e.g. vandalism) and private (e.g. domestic violence). Human economies have underinvested severely in nature conservation, despite the high value of ecosystem services 16 , because these services have been provided free of charge. The same applies for health services, but we suggest that there may be one key difference. In agrarian and manufacturing economies, the relationship between individual mental health and society-scale economic performance is a step function: irrelevant, until it is severe enough to generate crime or workplace absenteeism. In professional and service economies, however, the relationship is gradual: poor mental health decreases contribut...
