Abstract: A fumigant applicator for releasing therefrom a volatile substance, e.g., phosphine from a metal phosphide, at a controlled rate, thereby avoiding excessive local phosphine build-up and with complete protection of the contents against liquid water, in the shape of a sachet at least one face of which is formed from a spun-bonded, polyolefin sheet or fleece of heat and pressure bonded polyolefin fibers, which is vapor- and gas-pervious but impervious to liquid water and use, naturally water-resistant and of high tensile and tear strength, which sheet or fleece is rendered heat-weldable and is heat-sealed at its edges by a high porous coating, preferably a powder sprinkle coating, of a thermoplastic heat welding bonding layer which melts lower than the polyolefin fibers so that a heat welding seam at the edges of the sheet can form before the fibers melt so that the physical properties of the polyolefin fibers and the gas permeability thereof are substantially unaffected.