Abstract: Portable electrically operable apparatus for monitoring and for giving warning of the presence of a predetermined environmental hazard in the neighbourhood of the apparatus and which comprises a portable housing, an electrical power source arranged in the housing, a first monitoring unit arranged in the housing and operable to detect the presence of a predetermined environmental hazard e.g. radiation or undesired gases, a second monitoring unit arranged in the housing to monitor the operation of the apparatus for fault detection, a warning indicator arrangement provided on the housing and operated by the first or the second monitoring unit so as to provide a warning indication upon the detection of a predetermined hazard or upon fault detection, and a safe-operation indicator arrangement provided on the housing and connected to the first and second monitoring units so as to provide a safe-operation indication in the absence of hazard detection or fault detection.