I've had this question come up over and over again lately about using Essential Oils around cats. Robert Tisserand explains this very well. See Article Below.
Some quite strong feelings and opinions have been expressed about essential oils and cats, and a frequently asked question is “Is it safe to diffuse essential oils around cats?”. Diffusing essential oils can be toxic to humans if it’s over-done, causing neurological symptoms such as headache or fatigue. In general, making long lists of specific oils that are allegedly “safe” or “toxic” to cats doesn’t make a lot of sense to me – it’s more about overall exposure. You can diffuse essential oils around cats safely, so long as there’s good ventilation, you only diffuse small amounts for limited periods of time, and your cat has the freedom to leave the room if it wants. A few parts per million of aromatic vapor in the air is not likely to be harmful, but be careful to avoid build-up of vapors over many hours.
Full Article on Cats and Essential Oil Safety by Robert Tisserand