[pknight]

You naturally inhale essential oils when you walk through a forest and some of those (i.e. phytoncides) have been studied and linked to a range of positive health benefits, by for instance boosting immune system function and reducing negative stress. Those health effects have been reproduced by adding wood essential oils in the air of indoor environments. Example: https://www.ncbi.nlm.nih.gov/pubmed/20074458

However, different essential oils may have adverse effects as well and it seems like this is definitely understudied. It wasn't until I started looking into essential oils that I realized that almost every type of commercially available essential oil is toxic to cats. I would exercise extreme caution when using diffusers and stick to the few essential oils that have been well studied and considered safe for humans & pets.