I am a medical student currently and I would say from the statistics and case studies I have reviewed concerning lead poisoning that yes it is a serious concern when exposed as it is a veracious poison but your level of exposure in this situation would be considerably small. The vast majority of inhalation poisoning that I have seen have been from sanding lead-based paint. Now of course this risk would only be real assuming you used lead-based solder, which is becoming less frequently used do to it's contaminant properties. Most widely available solders are composed of mixtures of tin, silver, and copper. So lead poisoning is really not a viable risk factor here, however, standard precautions should always be used when sanding any materials, this includes metals, plastics, and even wood. Respirators are preferrable, but even one of those tissue masks would suffice. Enough soap boxing for me, just wanted to point out that I wasn't suggesting sanding lead and huffing the vapor, I usually sand metals wet anyway to reduce particles.