Phosgene is a colorless gas with an odor that resembles freshly cut hay or grass, and it is a valued industrial reagent in the synthesis of pharmaceutical compounds. Propylene oxide is a colorless volatile liquid with an odor resembling ether, and it is used to produce polyether polyols for making polyurethane plastics. Both chemicals are extremely toxic and may cause death with exposure in less than 6 minutes. Estimate and compare the exposure concentration in ppm that will result in fatalities for 75% of the exposed 150 individuals if they are exposed to a) phosgene and b) propylene oxide, respectively. Justify your answer.