Some oils are extracted from the plant by means of pure alcohol and are therefore properly called an "essence" and in the strict sense of the word can not be considered an Essential Oil.  (See also FAQ - What are Essential Oils.  Nevertheless, they have the same healing property as pure Essential Oils, although their concentration is severely diminished because of the alcohol.  Nevertheless if high quality plants are used, the healing properties are the same, although it will take longer than with a pure therapeutic oil. 

Some oils are extracted by other chemicals or solvents, such as DDT, hexane, taluene or di-ethelene glycol.  The reason for this extraction method is purely economical.  The yield per harvest is much higher.  Although these oils should never be declared as essential oils, many companies do so.  These oils are used primarily in the perfume industry, where they still create havoc with the human body, for the reasons below.

There are three negative aspects to this extraction method. 
1.  All the above mentioned solvents are carcinogenic. 
2.  These solvents do not extract all of the plant constituents and therefore leave the oil incomplete for healing purposes.  (A high quality Essential Oil must have all the constituents in order to be a proper healing agent, I will explain this in a future article, so please watch out for it.) 
3.  The solvents react chemically with the constituents and form new chemical bonds foreign to the human or animal metabolism.  Thus causing all kinds of ailments, skin rashes or other diseases.  Many companies claim that they "flash off" the solvents after extraction, but once a chemical bond has been created, flashing off is not possible anymore. 

So definitely stay away from the above oils.  Although you will not be able to recognize the difference of such an oil from a real pure Essential Oil just by smell, its extremely low price is usually a give-away.