jats:p Hydraulic fracturing, or fracking, is an emerging and growing industry that is having considerable effects on environments and health. Yet fracking often lacks environmental regulations that might be understood as governmental forms of care. In some locations in the US, citizens have taken up environmental monitoring as a way to address this perceived absence of care, and to evidence harm in order to argue for new infrastructures of care. This article documents the practices of residents engaged in monitoring air pollution near fracking sites in the US, as well as the participatory and practice-based research undertaken by the Citizen Sense research project to develop monitoring kits for residents to use and test over a period of seven months. Citizen sensing practices for monitoring air pollution can constitute ways of expressing care about environments, communities and individual and public health. Yet practices for documenting and evidencing harm through the ongoing collection of air pollution data are also speculative attempts to make relevant these unrecognised and overlooked considerations of the need for care. Working with the concept of speculation, this article advances alternative notions of evidence, care and policy that attend to citizens’ experiences of living in the gas fields. How do citizen sensing practices work towards alternative ways of evidencing harm? In what ways does monitoring with environmental sensors facilitate this process? And what new speculative practices emerge to challenge the uses of environmental sensors, as well as to expand the types of data gathered, along with their political impact? </jats:p>