A 1:1 mixture of propylene glycol and water (preferably distilled) is commonly used as the 'activator solution' in cigar humidors. The vapor pressure of the propylene glycol is such that it keeps the relative humidity inside the humidor near 70%, which is generally recognized to be ideal for cigar storage and aging.
The solution of PG and water is applied to a piece of highly porous material to provide a large surface area from which water vapor can be released or absorbed. This substance is frequently referred to as ‘oasis’, and is identical to wet floral foam. The foam is normally enclosed in an aesthetically pleasing vented container and stored in the humidor.
’Activator solution’ is normally sold at cigar shops for a highly inflated price. Pure propylene glycol can be obtained from chemical suppliers for much more reasonable prices.