Assuming it isn't engineered or laminated lumber of some sort, it should be perfectly safe.
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Thanks for taking the time to answer my question. Most box springs that are available for upcycling are old. I am leaning towards going ahead and using it to make a raised bed for non edible plants. Thanks, again.
You can always lacquer the wood if you're concerned, but there's probably nothing to worry about.
. It may contain some flame-retardant(s) (eg, TBBA), but not enough to do much harm.