Citizen sensing, or the practice of monitoring environments through low-cost and do-it-yourself (DIY) digital technologies, is often structured as an individual pursuit. The very term citizen within citizen sensing suggests that the practice of sensing is the terrain of one political subject using a digital device to monitor her or his environment to take individual action. Yet in some circumstances, citizen sensing practices are reworking the sites and distributions of environmental monitoring toward other configurations that are more multiple and collective. What are the qualities and capacities of these collective modes of sensing, and how might they shift the assumed parameters—and effectiveness—of citizen sensing? We engage with Simondon’s writing to consider how a “perceptive problematic” generates collectives for feeling and responding to events (or an “affective problematic”), here through the ongoing event of air pollution. Further drawing on writing from Stengers, we discuss how the “work” of citizen sensing involves much more than developing new technologies, and instead points to the ways in which new practices, subjects, milieus, evidence, and politics are worked through as perceptive and affective commitments to making sense of and addressing the problem of pollution.