Fiberglass insulation: how safe is it the the health of an installer not wearing some respitory protection?

Not cancer, at least: All i can say is that there does not seem to be an increased risk for cancer. (fiber glass exposure and human respiratory system cancer risk: lack of evidence persists since 2001 iarc re-evaluation.Regul toxicol pharmacol. 2011; 60(1):84-92) there may be other effects such as pulmonary fibrosis (scarring of the lungs) or copd, but I am unaware of studies evaluating for this.