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.

Scientific literature regarding EHA has emphasized the importance of dialogue with stakeholders and the public for some time. However far too often the practice of EHA remains in the form of monologue of the assessors, where contact with the users of the assessment are reduced to minimum, collaboration within the community of experts is rare, and public participation is allowed only where obligatory. At the same time several examples, facilitated by the nearly ubiquitous web-access in many parts of the world, have shown the power of mass collaboration in creating scientifically sound knowledge; Linux, Human Genome Project, Wikipedia etc. A new approach to EHA is needed to make the shift towards the desired state of EHA become materialized.

In recent discussions on knowledge creation, a trialogical approach is developed to meet the demands of collaboration and distributed expertise of knowledge-intensive societies. The research on expertise has shifted from monological processes of knowledge acquisition to dialogical processes of participation, and the advanced theories of learning now approach their subjects of study as trialogical processes of mediation through epistemic artifacts. In this paper we consider EHA as a trialogical process and present Opasnet; a web-based workspace for conducting EHA's in unrestricted mass collaboration.

EHA progresses iteratively from problem identification and formulation to information collection and synthesis, finally ending in dissemination of results to users. In this process, the assessment product mediates the actions of contributors involved at different phases of the assessment. Opasnet facilitates the evolution of the assessment product in situations where access to contribute to the assessment is unlimited. It provides a mediumi for the assessment product, but also an ontological information structure for effectively collecting and representing knowledge. Collaboration is organized by means of web-aided pragma-dialectical argumentation. In addition to formal information objects, Opasnet also supports freely structured content and a database for storing results and datai. Opasnet is designed to mediate knowledge creation among experts, policy makers and public primarily in EHA, but it is adaptable to scientific decision support in principle on any field.

Trialogical approach appears to work well for EHA and decision support in general, but some questions still require further attention, e.g.:
* What system functionalities are essential for supporting collaborative knowledge creation?
* How to manage the knowledge practices of the collaborators?