The invention relates to a monitoring device (6) for monitoring the environment, in particular for monitoring the environment for radioactive radiation, comprising at least one receiver for receiving measured values of an environmental variable that potentially poses a health hazard and comprising a computing unit (10) for computing a hazard warning (4.1-4.4) dependent on the measured values. The receiver is designed to receive the measured values from a plurality of spatially distributed electronic terminals (2.1-2.4), in particular from mobile telephones with an image sensor for measuring the radioactive radiation. The invention further relates to a corresponding terminal (2.1-2.4) and to a complete monitoring system with a monitoring device and numerous terminals (2.1-2.4) for measuring the environmental variable.