Essential oils can be very poisonous to cats. Some are safer than others, and they can carry a wide range of toxic effects. Most of the most toxic oils to cats are the ones that contain "phenols". These include (but are not limited to): peppermint, lavender, citrus, tea tree, cinnamon, wintergreen, thyme, and many others.
Essential oils are a concern for many reasons. Many will cause rashes and inflammation if applied directly to the skin. They are often also very irritating to the eyes and mucous membranes. Some may cause signs if ingested of absorbed through the skin, including seizures, liver failure, anemia, and death.
Less serious signs are related to the effects of the oils on the lungs and skin- these include development or exacerbation of asthma or bronchitis, allergic reactions, and others. Remember that the sense of smell of cats is much stronger than in humans, meaning that what we find a mild, pleasant smell they may find intolerable or overpowering.
Most of the data in cats involves direct contact or ingestion of essential oils. It is not always clear what the effect of inhalation of oils diffused at a distance, or diluted in a vaporizer may be. It is likely that even toxic oils may be safe if very dilute and far from a cat, but they may also remain toxic.
If you want to be safe and sure you will not cause issues, I recommend avoiding essential oils with cats in the house. If you must use them try to ensure they do not contain phenols, make sure they are dilute, far from the cat, and that there is no close physical contact between the cat and the oil.
Written by Dr.Matt Kornya