Environmental issues are in a constant interaction with social, economic, cultural, administrative and political variables that affect each other in a number of ways. As a result, environmental problems are complex problems and their solutions require a holistic approach. In parallel, sustainable development, as an environmental policy principle and objective promotes policy integration to deal with complexity. In this context, data, knowledge and evidence gathering activities play a major role in environmental policy process, from problem definition to selection of alternatives and policy implementation. It is impossible to develop a holistic approach and solve environmental problems without resorting to facts. However, there is not a straightforward line between the evidence, environmental policies and policy change. According to interpretative approaches scientific evidence is socially constructed, and subject to a number of challenges by competing theories and methods. Policy process is not a gentlemen’s business where scientific evidence is used to support political arguments and to legitimize a course of action, rather it is frequently disregarded, side-lined or even discredited if it challenges established practices and vested interests. However, this paper does not aim to focus on the social construction of evidence, but to the establishing a mechanism for the data collection. Public policies involve actions and inaction of governments and inaction demonstrates the concern of public authorities’. By focusing on problems in the development of air quality monitoring systems in Turkey, the paper argues that lack of a sufficient data gathering system is itself a sign of inaction or non-decision. In this case, lack of evidence also counts as an instrument of power, because scientific ambiguity or lack of evidence is used as an instrument of power to preserve status quo and contributes to sustenance of environmental problems, like air pollution, with an extra effect on every stage of policy process.