With the exception of Lavender (which can be used neat on burns and pimples and such) they all need to be diluted, and you do need to be careful with what you combine.

After all, chemistry is derived from nature….A good essential oil can have any where between 80 and 3,000 chemicals in it.

Here is a chart just to give you an idea of the complicated chemistry involved
http://www.essentialoils.co.za/isoprene.htm
See, they dont effect us simply because they smell good….