Burning lilac wood is completely safe. In fact, the hollow wood is often sold as barbecuing fuel. If you are burning the wood indoors, however, make sure your chimney and fireplace are clean and unobstructed.
Lilac bushes are made of a hollow wood, according to John L. Fiala's "Lilacs: The Genus Syringa." For this reason, the wood burns quickly and is not ideal for long-term firewood.
Some people use lilac wood for barbecue fuel. It is sold as barbecue wood because it is light, subtle, and has a floral scent, according to BBQ'n Fools.
BBQ'n Fools recommends lilac wood for barbecuing seafood and lamb because of the flavor its smoke will instill in the meats.
Lilac bushes with diseases should be burnt, according to Fiala. Burning them will not be harmful, but if you merely discard them rather than burning, the disease could spread to other plants in your garden. In this case, burning lilac wood is not only safe, it is the safest thing you can do for your garden's or nursery's health.
As with all wood, if you burn lilac wood in your indoor fireplace, be sure that the chimney is clear. If you use the fireplace often, you should also have it inspected and swept by a professional somewhat frequently, according to the Chimney Safety Institute of America's website.