Because hydrogen cyanide is explosive, Leuchter maintained that the gas chambers could never have been operated due to their proximity to the ovens of the crematoria. It is correct that hydrogen cyanide is explosive, but only at concentrations of 56,000 ppm and above – over 186 times more than the lethal dose of 300 ppm. Critics estimate conservatively that within 5 to 15 minutes, gas chamber victims were exposed to 450 – 1810 ppmv – again considerably lower than the lower explosion limit.