Essential oils are pretty safe but sometimes essential oils can cause allergic reactions in some people.
It's rare though that people will have allergic reactions to some essential oils but it is possible.
If you use essential oils and notice that you get rashes on your skin then you might be allergic to the essential oils.
But then again it could be that you do not dilute the essential oils before applying them to the skin.
You must always dilute essential oils using a carrier oil before applying the essential oils to your skin because it can cause rashes and burns on your skin.
Another way to tell if you're allergic to some essential oils is if you have some itching after applying the essential oils.
I've never had any allergic reactions when using essential oils properly but it is possible for some people to be allergic to the essential oils.