There is disclosed a telemetry monitoring apparatus 10 for remotely monitoring a plurality of users, such as firefighters, working in a hazardous environment. The apparatus comprises a plurality of portable devices 16, each arranged to be associated with breathing apparatus 18 worn by a user, and at least one central monitoring station 12 onto which portable devices can be logged on and monitored. The portable devices 16 are arranged to transmit a logon message including an identifier identifying either the user or the portable device. The central monitoring station 12 comprises a receiver 48 arranged to receive logon messages; priority data 54 containing the identifiers of users or portable devices which should be logged onto the central monitoring station; a logon determination unit 52 arranged to determine whether the identifier included in a received logon message from a portable device 16 is contained within the priority data; and a logon unit 50 arranged to perform a logon procedure to logon the said portable device onto the central monitoring station 12. If the logon determination unit determines that the said identifier is contained within the priority data, the logon procedure is commenced. There is also disclosed a central monitoring station onto which a plurality of portable devices can be logged on and monitored, and a method of logging a portable device onto a central monitoring station.