Respirators are what I do for a living so here is the straight scoop. The dose is the poison. The fumes from zinc (small metal particles that are vaporized when welding or torching it) will cause metal fume fever in too high a dose (no I don't know the actual dose for that). One quick whiff? Not likely going to cause anyone problems. Marinating in it all day? Ayup, that will be a problem.
Any time you are dealing with a nasty airborne hazard that you need to protect yourself from, there are 2 steps: 1. Eliminate the hazard 2. Personal protective equipment (PPE). In that order. You eliminate the hazard in this case through ventilation. As others have pointed out, a simple fan blowing the fumes away from you will do this. But the reality is that the wind is shifting (if outside) and you might move out of the fan area or block it at times, so considering the volume of torch work you are doing, you should also wear PPE. In the case of zinc fumes, a respirator with a NIOSH N95 rating (the lowest rating) or greater will do the trick. Any respirator that has that rating will be fine. There are respirators sold specifically as welding respirators, but you do not necessarily need one of them. The thing with welding fumes is that they plug up a filter very quickly, so welding-specific respirators are designed to have a surface that does not blind over with metal fumes as quickly as common masks so they last longer in this application. Important for day-in-day-out work, but not critical for an occasional use.
As for what type of mask, that is up to you. You can use the cheaper disposables just fine, and as long as you take a few moments to mold it to your face (most have a metal noseclip for this) and make sure it is well-sealed on your face it will be fine. If you have, or buy, a rubber facepiece, they are nicer to wear for a longer stretch and they are also a bit easier to ensure you have a good fit to your face. Then just buy the same brand filters for it, N95 or greater. More expensive, but can last for years depending on the type. You can get a box of 10 disposables at HD/etc for $15-20. A rubber facepiece will be $20-40 or more and you may need to buy filters separately. The other question you may have with a rubber mask is if you need the carbon cartridges for this. You do not. Those are for organic vapors (solvents like toluene, alcohol, MEK, paint thinner, etc) and are not needed for taking out zinc fumes, which are a particulate, not a gas/vapor.
Finally, metal fume fever is not typically something that is fatal or creates chronic problems. You feel like crap for a day or two, and your body clears it out, so it isn't the end of the world, but definitely worth some basic precautions to avoid it. So get a fan, do it outside, and wear at least an N95 mask of some sort, and you will be fine.
-Dave
