Therapeutic grade essential oils have been scientifically tested to meet a specific standard and include very specific levels of botanical elements within them. The origin of the harvest plant material, how it is harvested, the way it is distilled, the temperature it was created with all have a factor. 

Essential oils are the most important group of chemical molecules of … carbon- and hydrogen-based compounds called terpenes or hydrocarbons.

There are some oils on the market that are not rated as therapeutic oils. These oils may or may not have the botanical elements within them that bring about therapeutic or metabolic change in the body. 

Because the oils are volatile, the terpenes or hydrocarbons can be lost in the process leaving a lower grade of essential oil that still smells nice, but doesn’t have quite the therapeutic properties. This distillate is usually sold off cheaper by some companies instead of discarded. 

Additionally some companies sink so low as to actually add other oils to their “essential oils” without telling the consumer in an effort to stretch out the original batch and make more money. 

Hope this answers your questions. So long as the company you are buying from can show you scientific proof of the therapeutic grade of their essential oil, they are probably ok. But if they aren’t willing to share this info with you, or if they don’t even have it… I’d stay away from their oils.