Many countries have toxic waste disposal capabilities and may take old flares. For those who decide to ignite old handheld flares in their backyards or elsewhere, wear an oven mitt or similar protection when doing so.
Give some thought to where the ash or slag is going to come down. A few years ago, while anchored in a tightly packed Annapolis Harbor to witness 4th of July fireworks, a fool on a nearby boat set off a handheld flare. Some hot ash came down on the Bimini of a nearby boat and set it on fire. A real mess ensued as boats scrambled to get out of the way as fire boats approached.