There are mixed answers, but I think that it’s not safe. Many liquid potpourri products and essential oils, including oil of cinnamon, citrus, pennyroyal, peppermint, pine, sweet birch, tea tree (melaleuca), wintergreen, and ylang ylang, are poisonous to cats. Both ingestion and skin exposure can be toxic. However, in the end, there is little published information about whether tea tree oil fumes are directly toxic to cats.