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 this paper we present open assessment and Opasnet. Open assessment is a trialogical method for creating and communicating knowledge in open collaboration for the purpose of supporting societal decision making in the field of environmental health. Opasnet (http://en.opasnet.org) is an open ontological web-based workspace for facilitating the practice of open assessment.

Environmental health assessment (EHA) is an endeavour of analysing relations between environmental phenomena and human health for the purpose of informing decision making upon actions to reduce adverse health effects and enhance beneficial effects. EHA is by nature multi-disciplinary and it addresses issues of potential interest to a great number of people with different kinds of perspectives; scientific experts appointed to the task and policy makers with an obligation to deal with the issue, but also e.g. representatives of industry and commerce, NGO's and the general public. EHA should thus be perceived as a science-based activity taking place on the interface between science and society aiming for enlightenment and increased awareness upon important matters.

Disability Adjusted Life Years (DALYs) combine the number of people affected by disease or mortality in a population and the duration and severity of their condition into one number. The environmental burden of disease is the number of DALYs that can be attributed to environmental factors. Environmental burden of disease estimates enable policy makers to evaluate, compare and prioritize dissimilar environmental health problems or interventions. These estimates often have various uncertaintiesi and assumptions which are not always made explicit.