Patent ID: 7566355

Claim:
A device for protecting an existing, intermittently acting, external vent pipe of a building from the ingress of foreign airborne substances present in external atmospheric air, which device comprises (a) a length of pipe having a slightly larger circumference than said vent pipe so that it fits over the upper end thereof, to which upper end it is stably connected at its own bottom end, (b) and at its own upper end is provided with a hinged cap, which cap automatically (i) opens fully when air is being exhausted through said vent pipe and automatically closes fully upon cessation of such air exhaustion, and (ii) is of the same geometric shape as the cross section of said length of pipe, but is slightly larger than said cross-section in all its dimensions, whereby its rim overhangs the rim of said length of pipe when said hinged cap is in its fully closed position.