What it is
E249 is potassium nitrite, a curing salt. Synthetic.
Why it matters
Used to fix the pink colour and inhibit Clostridium botulinum in cured meats — bacon, ham, salami. The additive itself is vegan; the products it's typically used in are not.
What to do
Always vegan as an additive. The closely-related E250 sodium nitrite is the same.