Yeah I thought of that. Nitrates are actually fairly harmless. It's only when they're transformed into nitrites by bacteria or enzymes in the body that they become dangerous. They are safe up to a certain concentration and are present in foods such as spinach (nearly 200mg/kg) and are of course used for curing meats.
For humidity control though, significantly larger amounts would be used so that if might not be the best idea because of the risk of spillage =/