Dosage is THE most important aspect of using essential oils safely. This is especially true for the public or those not professionally qualified. Dosage means the amount of pure essential oil that you dilute prior to application on the skin. Essential oils should almost always be diluted prior to application on the skin. If recommended dilutions are adhered it is unlikely to experience an untoward reaction using essential oils.
For general applications dilutions should be 3% and less
In 10ml of Carrier (approx. 1 tablespoon)
1% = 5 drops
2% = 10 drops
3% = 15 drops
In 20ml of Carrier
1% = 10 drops
2% = 20 drops
3% = 30 drops
Different companies use different sized droppers, the above drops are from a small dropper insert.
BABIES: generally 0.5% or less. Camomile and Lavender oils are the only 2 essential oils recommended to use on Babies (from 8 weeks old and up).
PREGNANCY, ELDERLY, SENSITIVE SKIN: generally 1% or less.
There are few occasions where essential oils may be used undiluted (mostly first aid situations and on the feet). Repeatedly using undiluted oils greatly increases the risk of experiencing an adverse reaction. In particular irritation or allergic response of the skin.
In my experience once an allergic response has been triggered it can reoccur each time the the oil (or component in an oil) is used and this could continue for life, a good reason to generally avoid using undiluted oils on the skin. Because many oils share some of the same components, a repeated reaction of this nature could be caused by more than one oil eg different to the oil that triggered the initial allergic response.
An excellent in depth explanation may help you to understand this matter further http://tisserandinstitute.org/new-survey-reveals-dangers-of-not-diluting-essential-oils/