Developers from open-source communities have reported high stress levels from frequent demands for features and bug fixes and the sometimes aggressive tone of these demands. Toxic conversations may demotivate and burn out developers, creating challenges for sustaining open source. We outline a path toward finding, understanding, and possibly mitigating such unhealthy interactions. We take a first step toward finding them, by developing and demonstrating a measurement instrument (an SVM classifier tailored towards the software engineering domain) to detect toxic discussions in GitHub issues. We used our classifier to analyze trends over time and in different GitHub communities, finding that toxicity varies by community and that toxicity decreased from 2012-2018.