It just can't be reiterated enough that maintaining the environment within your humidor is critical for optimal storage and aging of your cigars. Towards this end, it is absolutely necessary that you have a hygrometer to gauge the humidity levels within the humidor and a humidifier or humidification device to create the proper relative humidity levels so that your cigars thrive. You've also likely heard over and over that only distilled water should be used to keep the humidifier "charged" in efforts to maintain the proper humidity, or your cigars will eventually dry out and diminish their quality. However, a mixture of distilled water AND propylene glycol solution is actually better.
Before we go any further, you should understand the followiing - propylene glycol solution, PG solution, humidor activation solution, humidor solution, and "cigar juice" are all synonymous; so if you see any of these names on the label of a bottle on the shelf of your local tobacco shop, they all actually refer to a solution that is a 50/50 mixture of distilled water and propylene glycol.
So why should you use PG solution rather than just pure distilled water? And why should you really care if both serve the same purpose when added to your humidor's humidifier? Before addressing these questions and explaining how this solution works, let's first explain what PG solution is...
For the science geeks, propylene glycol is also known chemically as 1,2-propanediol and is a colorless, odorless, viscous, and highly hygroscopic (the ability to absorb moisture from the air) liquid with a slightly sweetish taste (though confirmation of its taste is not recommended, especially in its raw form, even though its oral toxicity is relatively low in small quantities). It is considerably less toxic than ethylene glycol (the primary ingredient in antifreeze), and is often used as a food additive. In addition, it has multiple commercial uses across various industries as:
a moisturizer for medicines, cosmetics, and food products;
a flavoring agent in Angostura and orange bitters;
a solvent for food colors and flavorings;
a carrier in fragrance oils;
an ingredient in smoke machines to create artificial smoke (for theatrical productions or firefighter training);
an ingredient in hand sanitizers, antibacterial lotions, and saline solutions
a main ingredient in many cosmetic amd hygiene products such as baby wipes, bubble baths, and shampoos;
a primary ingredient in the paint within paintballs; and
a base ingredient in aircraft deicing fluid, as it lowers the freezing point of water
PG solution has two advantages over pure distilled water. First, it absorbs moisture at humidity levels over 70%, but also releases moisture at humidity levels below 70%. Therefore, it adjusts automatically in efforts to stabilize and equilibrate the relative humidity at the optimal level of 70% (the so-called magic number in terms of humidor humidification for cigars). In other words...
Humidor environment becomes too dry, PG solution will release absorbed moisture into the surrounding area.
Humidor environment becomes too moist (or too humid), PG solution will absorb the excess moisture to stabilize the surrounding area.
Secondly, in addition to discouraging the growth of mold from an overhumidified environment, the inherent properties of PG solution will actually inhibit the growth of mold, which will absolutely destroy your cigars and have the potential to grow on the surface of your humidor's cedar lining.
It is not necessary to use PG solution each and every time to fill and refill your humidifier; however, it's recommended that you use it to recharge the humidifier at least every three to six months. Whether you are using a foam/sponge humidifier, crystal humidifier, or gel beads, PG solution will optimize your cigar humidification and enhance the performance of your humidor.