Ensure that you buy essential oils from trusted and recognized aromatherapy suppliers. This field is not regulated so there are no stipulated standards. This is why quality variations are seen when you compare different merchants. Some unscrupulous suppliers also go as far as diluting the essential oils with fragrant synthetic substitutes.
Quality is very important because essential oils can penetrate your skin and mucus membranes. If the source plants are grown with pesticides, the pesticides can be carried into your cells when you use essential oils.