Oral arguments were held on January 16, 2013, in the Natural Resources Defense Council’s (NRDC) case challenging the U.S. Environmental Protection Agency’s (EPA) conditional registration of HeiQ AGS-20, a nanosilver product. NRDC filed suit on January 26, 2012, in the U.S. Court of Appeals for the Ninth Circuit (California). During oral arguments, EPA maintained that NRDC lacks standing to challenge the conditional registration. The judges appeared to reject EPA’s argument, suggesting that NRDC could prevail because of the low bar for establishing standing to challenge a rulemaking. The judges seemed less persuaded by NRDC’s argument that EPA erred in choosing three-year-olds rather than nine-month-olds as the most vulnerable subpopulation on which to base risk values. According to NRDC, EPA should have used infants, who are more likely to chew on textiles and could have higher exposures. EPA responded that EPA has a long-standing practice of using three-year-olds in risk assessments with similar exposure patterns to AGS-20, and that its use of three-year-olds was supported by leaching studies. NRDC further argued that EPA failed to consider the risk of aggregate exposures from other nanosilver products on the market. EPA responded that it applied uncertainty factors to account for data gaps and that HeiQ must submit more data to obtain a full registration.