Water evaporates fairly quick. We were running RainMate for a while in our shop and we had to refill it several times during the day. It is only effective while it splashes water on the sides of the bowl. How soon water evaporates also depends on the room temperature and humidity. As far as the fragrances themselves - I do not believe that eucalyptus plants or oils were anywhere close to the manufacturing facility where the fragrances were made (and if they were close to the building they were trees with live leaves playing in the wind ). This is my personal opinion though and I hope that I am mistaken. If I were you I'd look for eucalyptus oils and oil containing ointments or cough syrups. Rainmate is made to be used with water soluble fragrances but you can try putting some oil (I'd say 5-20 drops). Shop for oil as prices may vary greatly. Go to your local Whole Foods store if price is not a concern (they should have it). Price there is probably going to be double if not triple of what you can get it for online but you will have it today. Do not worry about Rainmate working with no water - It is safe.