Using essential oils incorrectly can be very risky. You should do thorough research or consult aromatherapists and essential oil experts to know how to use them correctly.
Another thing to remember is that pure essential oils are potent and should not be used undiluted. Undiluted, impure or adulterated oils may have adverse reactions ranging from mild (skin irritation) to severe (poisoning). Also, there are oils that must be avoided when you are pregnant or suffering from epilepsy. Some essential oils must not be used around pets as well.
It is best to get your essential oils from a reputable source, one that sells organic essential oils. There are manufacturers who use pesticides on their plants. So you must be picky on where you are getting your oils from.