On the subject of how much CO2 becomes hazardous:
While there is the possibility that the CO2 could displace all of the oxygen in a room or at floor
level, the ventilation in most auditoriums
etc should prevent this. Low spots where the heavy gas could collect with limited air movement (such as an
orchestra pit or a small basement storage room below a
stage) could be a greater problem, but even then it’s a matter of additional caution. Industrial situations where deaths from lack of oxygen (or poisonous gas) tend to be in confined, limited access, and limited ventilation locations, such as sub basements, tanks, and sometimes deep excavations.
Regardless, the consideration is not necessarily the amount of carbon dioxide that will displace all of the air, but rather the quantity of CO2 that will mix with the air and displace just enough oxygen such that the oxygen concentration is reduced from the normal 21 % down to 19.5 %, this being the definition of "oxygen deficient". [Please note that these are volume percentages not weight percentages; by convention gas percentages and concentrations are always on a volume basis unless otherwise noted.] It is at this percentage where one begins to suffer from lack of oxygen. Zero % oxygen is fatal – but so is 15% oxygen. (The best analogy someone gave me was that in the lungs, oxygen transfers across membranes from the air to the blood because, and at 21 %, the oxygen partial pressure is greater then the oxygen pressure in the blood. But when the oxygen percentage in the air drops below 19.5%, the oxygen pressure is lower than that in the blood and the transfer does not take place.)
So if you do this calculation, the amount of CO2 is not the volume of the room, but rather, a small percentage of it. Also, it is not a 1 to 1 displacement of oxygen by the CO2, because air is about 78 % nitrogen. So to move out the equivalent of 1.5 % oxygen (the difference between 21 % and 19.5 %), the CO2 must also move out 4.5 % of the nitrogen. So its about 6 % of the volume of the room. And all this assumes that the CO2 is completely mixed with the air.
But, and see below, according to
NIOSH (National Institute for Occupational
Safety and Health), CO2 has lethal effects at a somewhat lower concentration than that – namely 4 %:
NIOSH REL:
5,000 ppm TWA
30,000 ppm STEL
40,000 ppm IDLH
Current OSHA PEL:
5,000 ppm TWA
REL -
NIOSH recommended exposure limit.
TWA - indicates a time-weighted average concentration for up to a 10-hour workday during a 40-hour workweek.
STEL - short-term exposure limit; unless noted otherwise, the STEL is a 15-minute TWA exposure that should not be exceeded at any time during a workday.
IDLH - Immediately Dangerous to Life and Health.
PEL -
OSHA permissible exposure limit (This is enforceable).
ppm – parts per million (by volume).
The IDLH of 40,000 ppm = 40,000/1,000,000 x 100 = 4%.
I had some other posts on the subject see:
ControlBooth > Special F/X > F/X Questions?
Hazers vs. Dry Ice--Health Risk
Joe