Based on both archival research and two and a half years of ethnographic fieldwork in an Argentine shantytown with high levels of air, water, and ground contamination, this article examines the social production of environmental uncertainty. First, we dissect residents' perceptions of contamination, finding widespread doubts and mistakes about the polluted habitat. Second, we provide a sociologically informed account of uncertainty and the erroneous perceptions that underlie it. Along with inherent ambiguity surrounding toxic contamination, the generalized confusion about sources and effects of pollution is the result of two factors: (1) the "relational anchoring" of risk perceptions and (2) the "labor of confusion" generated by powerful outside actors. We derive two implications from this ethnographic case study: (1) Cognitive psychology and organizational sociology can travel beyond the boundaries of self-bounded communities and laboratory settings to understand and explain the collective production and reproduction of ignorance, uncertainty, and error. (2) Research on inequality and marginality in general, and in Latin America in particular, should pay close attention to the contaminated spaces where the urban poor live.