less than about 0 04 g cm 3 a tap density less than about 0 03 g cm 3 tap density can be measured by using instruments known to those skilled in the art such as the dual platform microprocessor controlled tap density tester vankel n c or a geopyc u2122 instrument micrometrics instrument corp norcross ga 30093 tap density is a standard measure of the envelope mass density tap density can be determined using the method of usp bulk density and tapped density united states pharmacopia convention rockville md 10 th supplement 4950 4951 1999 features which can contribute to low tap density include irregular surface texture and porous structure the envelope mass density of an isotropic particle is defined as the mass of the particle divided by the minimum sphere envelope volume within which it can be enclosed in one embodiment of the invention the particles have an envelope mass density of less than about 0 4 g cm 3 the inhalable powder of the invention has a preferred particle size e g a volume median geometric diameter vmgd of at least about 1 micron u03bcm in one embodiment the vmgd is greater than 5 u03bcm in other embodiments the vmgd is between about.