Wearable devices give new opportunities to monitor the safety of human behaviour in critical situations. Strategies used for monitoring device swarms can be naturally ported to human monitoring, by running the monitors on wearable devices connected to their human owners. In most wearable architectures, a crucial role is played by a smartphone, which usually coordinates multiple wearables of a single owner. Thus, for the time being, the smartphone itself is the natural candidate for running such a monitor. In this paper, we present a prototype implementation of a monitoring application of human behaviour in critical situations. The application is run on Android smartphones, does not require an internet connection by relying on Bluetooth advertisements, and is able to monitor space-time properties expressed as logical formulas, then translated into aggregate computing (AC) programs. The monitors are continuously updated and interact with the users through a simple user interface (UI). A preliminary experiment illustrates potential uses and shows the effectiveness of the approach in a small-scale real-world scenario.