This article analyzes the ways in which identical and divergent perceptions of the carcinogenicity of diesel-engine emissions emerged in the United States and West Germany between 1970 and 1990. Therefore, it reviews how automakers, policymakers, experts, and the media negotiated which exhaust emissions were either eco-friendly or hazardous. Communicating complex technological and scientific facts to the public, these actors translated the information into easily comprehensible terms. In the United States, this simplification forged the perception that diesel cars’ particulate emissions were cancer-causing between 1977 and 1981; a similar change occurred in Germany between 1984 and 1987. Consumers reacted and shied away from purchasing diesel vehicles. While Americans continued to stigmatize diesel cars, Germans re-embraced the technology by the late 1980s. Consequently, new “clean” diesels were introduced, and the discourse on exhaust emissions shifted from health-hazardous particulates to the ecological problem of the greenhouse effect, casting the fuel-efficient diesel in an entirely new light.