Dr. Safdar Khan,  DVM, MS, PhD, DABVT                                         Certified by the American Board of Veterinary Toxicology
National Animal Poison Control Center
College of Veterinary Medicine, University of Illinois

Why the Opinion: Cats and Essential Oils Are Not Suitable?
 
Dr. Khan:  My response to why cats may be more sensitive to toxicity/adverse effects from essential oils: Generally essential oils and hydrosols* have terpenes (along with other things) in them(terpenes are hydrocarbons of plant origin). Terpenes are rapidly absorbed orally and dermally and are metabolized in the liver. The metabolites of terpenes are conjugated with glucuronic acid (glucuronidation) and glycine depending on the type of terpene and animal species involved. The conjugated metabolites are usually more water soluble and are easily excreted through the kidney and feces.

Why Cats Can't Metabolize Essential Oils:

Dr. Khan: Cats are known to be deficient in their ability to eliminate compounds through hepatic glucuronidation (they lack enzyme glucuronyl tranferases).  Glucuronidation is an important detoxification mechanism present in most animals except cats.  Lack of this important detoxification mechanism in cats may result in slower elimination and thus build up of the toxic metabolites in the body causing toxicity.

Webguest interview with Dr Khan is sole copyright of The Lavender Cat and is not to be reproduced without expressed permission from The Lavender Cat/Sue Martin. NAPCC/SM-6/2000©

The ASPCA has many safety articles, the following is one of many PDF documents found on their site.

* Though hydrosol suppliers are not required to test their product for inert compounds, most hydrosols tested do have some terpenes at a lesser degree in them than essentail oils. When you buy a hydrosol, you along with your supplier do not know the contents of the hydrosol. Is it safe for cats, no one knows!