In this paper, we describe how critical data designers have created projects that ‘push back’ against the eclipse of environmental problems by dominant orders: the pioneering pollution database Scorecard, released by the US NGO Environmental Defense Fund in 1997; the US Environmental Protection Agency’s EnviroAtlas that brings together numerous data sets and provides tools for valuing ecosystem services; and the Houston Clean Air Network’s maps of real-time ozone levels in Houston. Drawing on ethnographic observations and interviews, we analyse how critical data designers turn scientific data and findings into claims and visualisations that are meaningful in contemporary political terms. The skills of critical data designers cross scales and domains; they must identify problems calling for public consideration, and then locate, access, link, and create visualisations of data relevant to the problem. We conclude by describing hazards ahead in work to leverage Big Data to understand and address environmental problems. Critical data designers need to understand what counts as a societal problem in a particular context, what doesn’t, what is seen as connected and not, what is seen as ethically charged, and what is exonerated and discounted. Such recognition is produced through interpretive, ‘close reading’ of the historical moment in which they operate.