Harmful effects from LEDs exploding
Not an expert, but my guess is that most, if not all, of the smoke was created by the burning plastic lens, and not from the semiconductor materials (the "rock") that constitutes the LED. I mean, consider the vaporization temperatures for the semiconductor materials versus the smoke point temperature of the plastic lens. It seems unlikely to me that the semiconductor materials got so hot that they not only melted but actually boiled to the gaseous state or burned. The plastic lens, on the other hand, could easily burn and produce smoke due to the flash of heat produced by a catastrophic failure of this sort. Specifically, the heat probably melted the plastic above the die, boiled/burned the plastic, and the rapid expansion of trapped gasses above the die caused the plastic lens to explode. (If any experts out there can refute this assertion, please do.) FWIW, the manufacturer might publish a material safety data sheet for the LED that identifies known health hazzards.