are essential oils toxic to cats?

The very basic answer is YES they are. They can be toxic to birds as well. Dogs, on the other hand,  have a system that is much similar to humans so dogs are not affected in the same way.
​The big culprits are the oils that contain PHENOLS, KETONES and D-LIMONENE. Basically all the most awesome ones for us humans!
A lot of people refer to cats as being "allergic" to essential oils which makes people think "Well they're not sneezing so I guess they're fine". Cats may not exhibit any external reaction initially, although, internally they may be suffering. 
​Always use caution when using diffusers and mists. Make sure the oils are out of reach of all animals and children. Never spray your cat directly with an aromatherapy mist containing essential oils. Most importantly, never apply or treat your cats directly with pure essential oils or products that are heavy with them(like certain natural over the counter flea medicine). Some say there are certain oils that are OK to use but personally, I'd rather be safe and just not do it.
​Long story short-keep this in mind and practice common sense. Your kitty is fine and good if you wear essential oils or use products that contain them. Just know that these products are for you and not them.
​Click here for a short article on the subject.