There are a couple things you can do to prevent mildew in a system that require no Propylene glycol. Be careful, but if you secure some colorless Iodine and use 1 (only 1) you will effectively kill bacteria that produces the mold which is harmful to your cigars. I mix, 1 gal of distilled water to one drop-2 at the most of clear Iodine. Mix a batch and test it to see if you want to use it. I used to be a chemist in another life. Also, for those of you looking to make your own refill solution for your floral foam type humidifiers use 1 gal of distilled water, 1 drop of iodine, and 1 tablespoon of glycerin all of which can be found at your local drug store, and for the final touch add 10% Propylene Glycol which can be found at most auto stores. Remember, the maximum effective life for a floral foam humidifier is about 2 years max. The propylene glycol accumulates in the foam and eventually renders it ineffective. Once or wice a year addition is all you should need.