A protective liner, attachable to the inner surface of a helmet, comprising an inner face and an outer face oriented towards the inner surface of the helmet, wherein the joining of faces gives rise to a plurality of inflatable chambers permeable to water vapour and connected by means of pressurised air distribution channels. The chambers and channels form a single body extending at least along the inner surface of the helmet above the Frankfurt plane. The outer face comprises a layer of rigid material while the inner face comprises a layer of elastic material, whose area expands when the chambers are filled with air, such that the expansion in volume occurs mainly by means of the deformation of the elastic layer.