content: Your math is wrong. A lethal dose isn’t measured against the mass of the air in the room. It’s the mass of the person. I redid the math the way a chemist would do it, since I am one. Molar mass HCN: 27.011 g/mol 270 ppm instantly fatal dose Room size: 10 ft^3 -> 3.048 m^3 -> 28,350 L Berlin weather today: 21.11 C -> 294.26 K 30.41 inHg -> 1.016 atm Ideal gas constant 0.0821 (Latm)/(molK) (PV)/(RT) = n (1.01628,350)/(0.0821294.26) = n mols = 1,192.25 mols N = 1,192.25 mols * 6.022 molecules/mol = 7.1797 E 26 molecules ppm = 7.1797 E 26 molecules * 270 E-6 molecules = 1.9385 E 23 molecules HCN 1.9385 E 23 molecules HCN / 6.023 E 23 molecules per mol = 0.32 mols 0.32 mols * 27.011 g/mol = 8.695 g TL: DR 8.695 g of HCN in a room 10 ft cubed room would instantly kill everyone inside it.