Abstract: A nozzle unit for discharging and atomizing a fire or explosion suppressant comprises a rigid-walled cylindrical container having a nozzle portion at one end with radially directed discharge orifices. A rupturable barrier normally blocks the nozzle portion from the interior of the container. At the other end of the container, a piston is positioned so as to be sealingly slidable along the container in response to pressure generated by a gas pressure generator. The moving piston pressurizes the suppressant agent within the hollow interior until the rupturable barrier bursts. The suppresessant agent is accelerated substantially instantaneously and discharged in atomised form through the discharge orifices. The gas pressure generator may be a pyrotechnic gas generator. The arrangement is such that the pressurizing gas does not come into contact with the suppressant agent.