In short: my friend, who is also a LARP organizer, has come up with the idea of representing breath weapons and some kinds of offensive weapons (those that are represented by a "cone template" in many games) via a CO2 fire extinguisher.
CO2 is basically just compressed air. However, I have experience of using it for cleaning purposes, and I know that it becomes extremely cold. So, I cannot be sure about its safety.
So, is one safe to use indoors? If the answer is yes, what safety measures do I need?
Best Answer
CO2 extinguishers are not safe to use on people or in a room full of people.
Exit temperatures of the CO2 is -52°C in gaseous form and -78°C in snow form, enough to cause frostbite if it touches the skin. If that happens, it is recommended to remove most of it without rubbing it in and then seek medical advice.
It can also cause respiratory problems, especially among person who already have some troubles such as asthma, because the CO2 is not “basically just compressed air”—it's compressed carbon dioxide, and the fire extinguisher works by replacing the oxygen in the area to put out the fire. A CO2 fire extinguisher aimed at people would work by replacing the oxygen nearby those people with CO2.
This gas is an asphyxiant and extremely dangerous for humans even in low concentrations for any sustained period. This would include anyone having a CO2 extinguisher used on them. Symptoms are debilitating and range up to and including death. This is even when there is otherwise sufficient oxygen present: CO2 alone will cause these symptoms. From a USA EPA report (source 2):
The same paragraph continues, but even low concentration exposure has debilitating effects (ibid.):
While the risk is low when used outside, it is recommended to always open doors and windows wide if it's been used indoors to evacuate the CO2 and bring in fresh air.
Sources: