Plants naturally give off a natural toxin to protect itself against getting eaten. It is not uncommon for plants to have toxins on its leaves. A natural chemical process the plant goes through creates these metabolites to fend off animals or insects that may eat the plant.
For example, lemons give off a compound called Limonene which is fatal to moths. This, of course, doesn't harm humans at all.
Not all plants are safe to eat for humans though. Many of these houseplants can cause sickness if consumed. So we recommend keeping the houseplants out of your mouth.
For pet owners, toxicity is a major decision maker. Make sure your plant is nontoxic if you have pets. You can check the plant's toxicity level on the ASPCA's site.
You can see our pet-friendly plants HERE.