Abstract: When fumigating a bulk particulate commodity stored in a top-vented vertical silo by slowly passing a gas containing a low concentration of a gaseous fumigant through the bulk mass, differences between the temperature, T.sub.g, in the bulk mass and the ambient temperature, T.sub.a, outside the silo cause a "chimney effect" within the silo, and possible unwanted dilution of the fumigant. The chimney effect also occurs in other top-vented storage structures containing particulate commodities. To overcome the chimney effect, the fumigant-containing gas is supplied through a duct to the base of the storage structure at a flow rate Q.sub.f (in m.sup.3 sec.sup.1) determined by the relationship ##EQU1## where g is the acceleration due to gravity (approximately 9.8 m sec .sup.2); P.sub.a is the atmospheric pressure expressed in Pascals; R.sub.a is the gas constant for air (=287 J kg.sup.-1 K.sup.-1); T.sub.a is in K; T.sub.g is also in K; R is a resistance factor (in Pascals sec m.sup.