A device for generation of a mist of a treatment composition to impart a denaturation of allergens benefit to a surface or airspace, the device being fully hand-holdable and comprising: an atomising chamber comprising a base cavity opposite from an open end ; a mist generator means spanning the open end and comprising a vibrating member which includes a metal or ceramic plate , said plate being solid, porous or micropierced in the form of a grid or in the form of one or more segments or slots passing through the vibrating member; a piezoelectric actuator , which, when operated, causes vibrating motion in the vibrating member; a control circuit for operating the mist generator means ; a reservoir containing a fluid product to be aerosolized; a pump means for supplying the base cavity with the fluid product from the reservoir ; a housing; and at least one flow directing nozzle , flow directing implement or flow directing orifice adapted to direct the flow of a mist generated by the mist generating means out from the housing and towards a surface or into an airspace or both, wherein: the reservoir of the device is a refill unit; the refill unit does not include the piezoelectric actuator ; the refill unit includes the vibrating member; and the device is configured such that when the refill unit is properly installed in the device the piezoelectric actuator and the vibrating member interact to form the mist generator means , characterised in that the pump means is configured to control the rate of delivery of the fluid product to maintain a headspace (HS) above the fluid product in the base cavity . an atomising chamber comprising a base cavity opposite from an open end ; a mist generator means spanning the open end and comprising a vibrating member which includes a metal or ceramic plate , said plate being solid, porous or micropierced in the form of a grid or in the form of one or more segments or slots passing through the vibrating member; a piezoelectric actuator , which, when operated, causes vibrating motion in the vibrating member; a control circuit for operating the mist generator means ; a reservoir containing a fluid product to be aerosolized; a pump means for supplying the base cavity with the fluid product from the reservoir ; a housing; and at least one flow directing nozzle , flow directing implement or flow directing orifice adapted to direct the flow of a mist generated by the mist generating means out from the housing and towards a surface or into an airspace or both, wherein: the reservoir of the device is a refill unit; the refill unit does not include the piezoelectric actuator ; the refill unit includes the vibrating member; and the device is configured such that when the refill unit is properly installed in the device the piezoelectric actuator and the vibrating member interact to form the mist generator means , characterised in that the pump means is configured to control the rate of delivery of the fluid product to maintain a headspace (HS) above the fluid product in the base cavity . the reservoir of the device is a refill unit; the refill unit does not include the piezoelectric actuator ; the refill unit includes the vibrating member; and the device is configured such that when the refill unit is properly installed in the device the piezoelectric actuator and the vibrating member interact to form the mist generator means , <EOS>