A one-way pressure relief valve for attachment over a vent on an exterior surface of a closed package containing a gas-producing product, the pressure relief valve having • a filter layer of a strip-type material having a periphery and first and second sides (322, 324) defining a body therebetween, the filter layer having a filter element defining aperture and barrier portions (330, 332) in the strip-type material providing passage of gas through the aperture portions and body and obstruction of particulate matter by the barrier portions; • adhesive on the filter layer second side to adhere the pressure relief valve to the exterior surface of the closed package; and • a flexible seal including a flexible cover joined selectively to the filter layer first side to form a gas flow path between the flexible cover and the filter layer first side along an unjoined region of the flexible cover and filter layer from the filter element to the filter layer periphery; characterized by • the flexible seal further including a first gas-impervious layer atop the filter layer and around the aperture portions and a second gas-impervious layer supported by the flexible cover over the aperture portions of the filter layer, the gas-impervious layers (26, 32) having a surface attraction which holds one gas-impervious layer against the other, the flexible seal having a first position in which the gas-impervious layers (26, 32) form a seal to obstruct gas flow through the body and a further position in which the second gas-impervious layer of the unjoined region is spaced at least partially from the first gas-impervious layer by gas pressure exerted through the filter element to permit one-way gas outflow through the gas flow path, whereby the pressure relief valve enables one-way pressurized gas outflow from an interior of the closed package through the filter element and the gas flow path while preventing particulate matter from interfering with closing of the valve once gas pressure inside the package is decreased. • a filter layer of a strip-type material having a periphery and first and second sides (322, 324) defining a body therebetween, the filter layer having a filter element defining aperture and barrier portions (330, 332) in the strip-type material providing passage of gas through the aperture portions and body and obstruction of particulate matter by the barrier portions; • adhesive on the filter layer second side to adhere the pressure relief valve to the exterior surface of the closed package; and • a flexible seal including a flexible cover joined selectively to the filter layer first side to form a gas flow path between the flexible cover and the filter layer first side along an unjoined region of the flexible cover and filter layer from the filter element to the filter layer periphery; • the flexible seal further including a first gas-impervious layer atop the filter layer and around the aperture portions and a second gas-impervious layer supported by the flexible cover over the aperture portions of the filter layer, the gas-impervious layers (26, 32) having a surface attraction which holds one gas-impervious layer against the other, the flexible seal having a first position in which the gas-impervious layers (26, 32) form a seal to obstruct gas flow through the body and a further position in which the second gas-impervious layer of the unjoined region is spaced at least partially from the first gas-impervious layer by gas pressure exerted through the filter element to permit one-way gas outflow through the gas flow path,