A poorly maintained laser system can be a safety concern. Remember that the laser beam is invisible energy being reflected from mirror to mirror before being focused into a small spot by the lens. A mirror or a lens that is coated with smoke residue will not transmit the laser energy fully, resulting in the buildup of intense heat.
The high temperature that results from not allowing the laser energy to pass through the lens or reflect off the mirror can result in permanent damage to optic components. A damaged or compromised lens or mirror can lead to expensive downtime.
-Mike Fruciano, Coherent