The purpose of this paper is to explore the possibility of using crew member vital sign changes as an indicator of changes in the environment and breathable atmosphere within space habitats. As such, we have compiled a list of findings describing how different environmental conditions aboard isolated orbiting or landed space habitats can be assessed through monitoring changes in crew member vital signs via noninvasive techniques, such as electrocardiogram (ECG) readings, blood oxygenation measurements, ocular structure observation, and visual performance assessment. Akin to the 'canary in the coal mine' principle, the crew members act as biosensors that, when integrated into other subsystems, help monitor space habitat health as a whole as part of an overarching Prognostics and Health Management (PHM) approach. The availability and compactness of ECG, blood oxygenation, ocular structure, and vision performance monitoring equipment - ranging from devices having a form-factor of a wristwatch to smartphone-based examination equipment - lend themselves to be ideal in environments where self-diagnostics and payload considerations are vital to mission success. We provide a list of results on how carbon monoxide (CO), carbon dioxide (CO2), oxygen (O2), and radiation exposure impact ECG measurements, blood oxygenation, ocular structure, and vision performance to quantify the health of the space habitat environment. The findings reported in this paper may lay the foundation for subsequent, e.g., deep learning based, anomaly detection frameworks that, in conjunction with other subsystems, may help determine more accurately cross correlations between different environmental factors aboard space habitats and corresponding vital sign changes of crew members.