Issues of environment and health form complex networks of natural and societal phenomena. Their due assessment and policy making upon them necessarily requires consideration of both knowledge and values from multiple sources representing plural views regarding the issues at stake. Important decisions relevant to environment and health are also made by several actors with differing societal roles, not only policy makers in administrative bodies. In scientific literature, environment and health assessments are usually considered as tightly interlinked with policy making and importance of stakeholder and public participation is emphasized. The practice, however, most often is that assessment and policy making are clearly distinct processes, participation is treated rather as a compulsory add-on to these fundamentally closed processes, more often in relation to policy making than assessment, and the use of assessment output is rarely considered during design and making of assessments. Encouraged by the examples of successful mass collaboration, e.g. Linux, Wikipedia and the human genome project, and recent developments in theories of collaborative learning, we would like to introduce a new approach to environment and health policy-support: open collaboration. In this approach assessments engage policy makers, experts, stakeholders, and citizens, i.e. anyone interested, to co-generate sound solutions to practical societal problems by means of science. They are open processes of collective knowledge creation through co-development of shared information objects. Openness may be limited only based on cogent and well-argued explicit reasons. Open assessment is a method for open collaboration in environment and health policy support. Opasnet (http://en.opasnet.org) is a web-based workspace for carrying out open assessments.