Hi I would assume that where he was, there wasn't enough oxygen. Remember that it takes some time for the gas to mix and diffuse throughout the room. I'm told that even for halon, one should hold ones breath so that you have some oxygen in your lungs until things have a chance to mix. As Eric noted, the gas will quickly mix with the air to become a percentage of the rest of the air. The diffusion forces on gas tend to out weigh the effects of the weigh of the gas. Once mixed, it doesn't unmix easily. It is similar to water and salt. One can carefully pour fresh water over salt water but if it is mixed, it will not separate by weight. Dwight