Procedural justice, or the ability of people affected by decisions to participate in making them, is widely recognized as an important aspect of environmental justice (EJ). Procedural justice, moreover, requires that affected people have a substantial understanding of the hazards that a particular decision would impose. While EJ scholars and activists point out a number of obstacles to ensuring substantial understanding—including industry’s nondisclosure of relevant information and technocratic problem framings—this article shows how key insights from Science and Technology Studies (STS) about the nature of knowledge pose even more fundamental challenges for procedural justice. In particular, the knowledge necessary to inform participation in decision making is likely not to exist at the time of decision making, undermining the potential for people to give their informed consent to being exposed to an environmental hazard. In addition, much of the local knowledge important to understanding the consequences of hazards will develop only after decisions have been made, and technoscientific knowledge of environmental effects will inevitably change over the period during which people will be affected by a hazard. The changing landscape of knowledge calls into question the idea that consent or participation during one decision-making process can by itself constitute procedural justice. An STS-informed understanding of the nature of knowledge, this article argues, implies that procedural justice should include proactive knowledge production to fill in knowledge gaps, and ongoing opportunities for communities to consent to the presence of hazards as local knowledge emerges and scientific knowledge changes.