His first response was the standard idea that the blast doesn't necessarily lose energy by doing work (causing destruction), since the debris will pick up some of the energy and carry it outward as flying bricks, panels and glass.  However it is clear that the blast loses energy by the work done in breaking walls, which is irreversibly lost in warming up the rubble.  If each house destroyed takes 1 % of the blast energy, then the energy after destroying 200 houses on a radial line outward from the explosion is down to just 100(0.99^200) = 13 % of what it would be over desert.  This is valid for wood-frame houses.  Brick and concrete buildings absorb far more energy per building destroyed, so in a modern city the blast pressure would fall very rapidly indeed.  This is non-scalable, so it is most pronounced at high yields with large destruction radii computed for open terrain.  Brode did concede, when presented with Penney's data, that this effect is not taken into account in American blast calculations at present.  See http://nigelcook0.tripod.com/ for further data.   