In May of 2012 Ray Berendsohn of Popular Mechanics wrote a fascinating article on the metal hazards found in logs brought to sawmills for processing. These items include nails, railroad spikes, cables, bolts, tree steps and even civil war-era musket balls! You can read the whole article here.
While these items can be damaging to mill equipment (including to carbide-tipped blades), we would worry about the larger danger of these metals, or their fragments, being propelled out of the log in certain situations, which could cause serious injury. 
One safety solution is polycarbonate. This plastic is incredibly impact resistant and performs very well as safety glass. Polycarbonate is available in scratch-resistant options and UV-resistance is not a problem if used in indoor applications (note: UV-stabilized polycarbonate is available).
Interested in polycarbonate?