The answer is yes, essential oils can set your sauna on fire. Members of the Maroubra Seals Sports Club in Sydney Australia found this out firsthand this past weekend.
According to the Sydney Telegraph:
General manager Peter Reid believed the fire was the result of sauna patrons who had previously been warned against the practice. “The fire started in the sauna. I’m of the opinion it started by people putting things on the sauna – eucalyptus oil – which they have been told not to do,” he said.
Eucalyptus and other essential oils are flammable. In fact, eucalyptus oil has a flash point about the same as kerosene. A sane person would never pour kerosene onto hot sauna rocks. It would burst into flame.
Eucalyptus oil and other essential oils do the same thing. They will burst into flame when poured onto hot sauna rocks. As the members of the Maroubra Seals Sports Club can tell you, flames inside a wood-lined room are a very bad thing.
If you plan to use eucalyptus or any other essential oils in your sauna, remember to never pour oils directly on your sauna stove. Always mix just a few drops of oil into a bucket of water first. This way you get the scent of the oils without the potential flare-ups.
A final note: be extra careful with oil of wintergreen in the sauna. Not only is wintergreen oil flammable, but it is the active ingredient in aspirin. Just 5 ml (one teaspoon) of oil of wintergreen can be the same as 23 aspirin tablets. Not only will it burn, you can also overdose on it.