This chapter examines the regulatory response to suspected chemical hazards in New Orleans, Louisiana, following the city’s catastrophic flooding from Hurricane Katrina in August 2005. For the U.S. Environmental Protection Agency, the year-long response represented an unprecedented mobilization of regulatory science, generating over 400,000 laboratory analyses of soil and flood sediment. Analysis of the resulting data, the policy frameworks that guided the collection and organization of that data, and the agency’s subsequent claims about the relative absence of risk to returning city residents reveal some of the ways in which risk assessment in the U.S. environmental regulatory system is deeply