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.
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.
- 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
- 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.'
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.