Is a CO2 fire extinguisher safe to use indoors to represent offensive magic or breath weapons in LARP?

Question

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?

Answer 1

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):

At concentrations [of inhaled CO2] greater than 17 percent, such as those encountered during carbon dioxide fire suppressant use, loss of controlled and purposeful activity, unconsciousness, convulsions, coma, and death occur within 1 minute of initial inhalation of carbon dioxide

The same paragraph continues, but even low concentration exposure has debilitating effects (ibid.):

Exposures to 4 to 7 percent carbon dioxide can result in headache; hearing and visual disturbances; increased blood pressure; dyspnea, or difficulty breathing; mental depression; and tremors

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:

1. INRS file on fire extinguishers (PDF, in French). INRS = l’Institut national de recherche et de sécurité pour la prévention des accidents du travail et des maladies professionnelles². In English it is the French National Research and Safety Institute for the Prevention of Occupational Accidents and Diseases³.
2. Carbon Dioxide as a Fire Suppressant: Examining the Risks, by the US Environmental Protection Agency

Answer 2

In answer to your question on the safety of using a CO2 extinguisher as a breath weapon effect at a Live Roleplaying Event: NO, for exactly the reasons stated in the other answers - extreme cold, hypoxia, and CO2 poisoning. I don't think your insurers would be too pleased about that particular course of action, either.

From the point of view of a someone who rigs effects for a large system, what you probably want instead is a portable smoke machine. These can be hired relatively inexpensively, and use a water-based smoke-fuel to provide the effect. If you're technically inclined, you can even make one yourself.

The other way - slightly less directed, but I have seen used as poison clouds at events - is a smoke grenade such as one you might get for use at an Airsoft shoot.

Both of these should be covered under your event insurance.

Answer 3

DISCLAIMER: I am not a medical or safety professional.

They can be very dangerous

I would not recommend doing this. CO2 Fire extinguishers present 2 basic hazards.

Cold Burns

Do Not Touch the horn (the part that sprays CO2) when spraying a CO2 Fire extinguisher. The horn of a CO2 fire extinguisher becomes extremely cold during use, and can cause cold burns if skin comes into contact with it.

Additionally, while the CO2 rapidly warms once it is clear of the extinguisher, it is very hazardous at close range. You are spraying rapidly-evaporating dry ice out of this thing, which will cause cold-burns.

They displace oxygen.

This is the greater risk. The way a CO2 Fire Extinguisher works is that it pushes all of the oxygen in an area away, replacing it with CO2, thereby smothering the fire. This is obviously a problem, because humans need to breathe oxygen.

Based off this, here are two safety pointers if you do decide to point one of these at a person.

1. Only use in a large, extremely well-ventilated area. Ideally, outdoors. In a confined space, a CO2 Fire extinguisher can lower the oxygen content of the space far enough to cause asphyxiation
2. Do not sustain the spray for more than a few seconds at a time. Even in a well-ventilated area, the space that the extinguisher is spraying will quickly have too little oxygen to sustain a person. Sustained spraying can suffocate a person.

IMPORTANT: The CO2 continues traveling even after it is no longer visible as a cloud. Just because you can't see that you're spraying someone or something doesn't mean you aren't.

As an addition (with thanks to @supercat), CO2 is hazardous on its own, even in concentrations less than those required to leave a person hypoxic. If at any point, the air reaches a 10% concentration of CO2 or greater, the CDC qualifies that as 'Immediately Life Threatening.'

Conclusion

I would not, personally, recommend doing this.

But if you do, do so only in a very large and well ventilated space, stay several feet away from anyone while using it, and never spray for more than a few seconds at a time.

Answer 4

Use a water spray gun

Both CO2 and smoke have issues with displacing air and risking suffocation or frostbite. Water, on the other hand, doesn't displace air, is almost completely safe to use on both humans and the things one would normally wear at a LARP event, can easily be cleaned up and gives a good indication of who is hit by a spell effect (check who is wet). It's also environmentally safe and cheap to use, can be used without electricity and has a cheap solution for those that really don't want to get hit by it (a rainproof coat or plastic sheet).