Medical sensor networks allow for pervasive health monitoring in hospitals, at home, or on the way. Each user carries a set of wireless medical sensors that allows caregivers to monitor his health state. The privacy and confidentiality of medical data need to be guaranteed at any moment to make sure that third parties cannot access confidential information. Additionally, the system also has to be aware of context changes, such as emergencies, to ensure fast access to the medical data, and thus, the safety of the users. In this context, this paper presents a modular context-aware access control model to allow for context-awareness and pro-active decision making during acute, emergency, and normal medical situations. We analyze its structure, features, and operation in detail and further describe how our system can be applied to existing security architectures for medical sensor networks.