Polycarbonate doesn't have any chlorine at all in its chemical formula. Maybe the usual types of Lexan contain plasticizers/flame retardants/other additives that contain chlorine?
Re:When hardware Kickstarters ship
by JoshTriplett on Tuesday August 25, 2015 @01:42AM
(#10114782)
Which is exactly why I ended with "You would need some detailed knowledge of the exact material and laser cutter, and thus some expertise."; cutting any kind of polycarbonate sounds like a bad idea if you don't know exactly what you're doing.
Note: That page does say in the second paragraph: "It is not always obvious which materials will work - for example: Polycarbonate/Lexan produces flames and lethal chlorine gas which will rapidly corrode this $40,000 machine into uselessness and which is extremely hazardous to the health of people nearby." That sounds like its in shouting distance of the Epilog guy's warning.