Environmental issues are often discussed in purity-related terms. For instance, pollution, contamination, toxicity, and degradation are all concepts that can evoke notions of (im)purity in an environmental context. In this paper, we assess the efficacy of purity-based norms as drivers of environmentally sustainable behavior. First, using a social media-based environmental cleanup campaign as a test case, we find that purity-based norms increase participation in the campaign. We then replicate and extend these findings in three behavioral experiments, finding that purity-based interventions do increase environmental behavior (Study 1), but that these effects are strongest for people who are more deeply connected with an in-group (Studies 2 and 3). Using an integrative approach to combine computational linguistics with behavioral experiments, we find that purity-based norms can be powerful motivators of environmental behavior, particularly if they emphasize the relation to one's in-group.