Gases diffuse and mix given enough time. There's not enough time when you have a massive indoor gas leak. Even outdoors, large amounts of CO2 tend to sink to t

Gases diffuse and mix given enough time. There's not enough time when you have a massive indoor gas leak. Even outdoors, large amounts of CO2 tend to sink to the ground and suffocate people in low-lying areas. Look up Lake Nyos for a particularly grizzly example of that.