Basically always wear a respirator when working with fiberglass resin, as the fumes are a health hazard... if you choose to not wear one do so at your own risk, always make sure you have ample ventilation, i.e. outside, vents, fans... also take breaks, it takes a few minutes for the resin to become hard so walk away from the area while it hardens.... wearing a surgical/dust mask doesn't really stop the fumes from entering your lungs its only there to keep particles out, not vapors... Also, refrain from taking deep breaths while near your work area if you are not wearing a respirator...
If at any time you feel light head, slowly walk away from your work area and do not return for some time... you also may wish to have supervision if you are not taking the manufacturers recommended safety precautions..
You wouldn't breath in gasoline fumes would you? You shouldn't breath in resin fumes either...