Prob a CO2 laser, out at 10000nm. When discussing laser safety, the primary concern is eye damage. Burning flesh etc is bad, but that heals, and it takes way more energy. Eye damage is nearly instant, it does not heal, and lasers you can dig out of consumer electronics or buy >class 3b are hecka-capable of seriously injuring someone. Even the standard 3b lasers are no joke, and often output light outside the visible range in addition to the visible beam.