Yes, they look cool when they float off and light up the night. Yes, they're lots of fun. Yes, they're very popular and used in connection with many cultural events. And yes, they also pose an unacceptable fire risk. For this reason, the 2015 edition of NFPA 1: Fire Code, prohibits the use of unmanned, free-floating sky lanterns. A recent fire involving a cell tower in North Carolina underscores the potential danger - fortunately there were no injuries. An excerpt from the upcoming 2015 NFPA 1 Fire Code Handbook explains:
The potential hazard posed by sky lanterns should be obvious — once ignited and released, the device becomes an uncontrolled, flying ignition source, whose direction of travel is dependent on the wind direction, which can change unpredictably. Although the combustible fuel load of the device itself might be small, the potential exists for the device to ignite vegetation or other combustibles in the area and cause a significant fire if it is not quickly extinguished.