Sounds like you might be mixing two bits of advice. There are many materials you never cut with a laser cutter, such as PVC or vinyl (makes chlorine gas) or ABS plastic (makes cyanide and melts). Thick Lexan shouldn't be cut with an infrared laser, not necessarily because it produces harmful materials but because it will scorch and burn rather than cut. You might be able to cut it with a different type of laser cutter. And some other forms of polycarbonate may cut better (materials informally called "Lexan" sometimes aren't). You would need some detailed knowledge of the exact material and laser cutter, and thus some expertise.

See http://atxhackerspace.org/wiki/Laser_Cutter_Materials for some details.