The molecular weight of N2 is 28 compared to 32 for O2 so pure N2 is
slightly lighter than air, but not enough to make a difference. CO2
(mw=44) is significantly heavier than air and can pool in low lying
areas. The bigger problem is, if a large amount of N2 (or CO2) is
rapidly released in an open area, it will displace the air and reduce
the amount of oxygen in that area and cause suffocation.

Releasing N2 in a sealed space will simply increase the pressure in the
room and cause no particular breathing problems because the room will
still have enough oxygen in it to breath. You won't have a problem
until the pressure gets high enough for the toxic effects of N2 to show
up (nitrogen narcosis). This takes several atmospheres of pressure.

Releasing CO2 in a sealed space will start causing breathing problems
as soon as the CO2 pressure in the room reaches about 3/4 psi, even
though the same amount of oxygen is available for breathing. A CO2
partial pressure of more than 1 1/2 psi is considered lethal.

kd5sak wrote:
I think nitrogen and carbon dioxide are both heavier than air ( that is,
they will pool in low areas) A sufficient excess of either can suffocate
you. You can wind up drowning without getting wet. As a matter of fact, I
saw a cable channel episode recently describing the loss of a few buffalo in
a small valley in Yellowstone Park from such an emission near one of the
active geyser areas. That incident probably involved a mix of volcanic gases
rather than just CO2 but no one knows for sure, since it was a transient
unmonitored event.

Harold Burton