propylene glycol

C₃H₈0₂, a clear, colorless, slightly syrupy liquid at room temperature, practically odorless and tasteless and must be heated or shaken briskly to produce a vapor. Often used in making antifreeze and other deicing solutions, polyester compounds and a solvent for paints and plastics. Propylene glycol has been approved by the FDA as an additive that is "generally recognized as safe" for use in food, used to absorb extra water and maintain moisture in certain medicines, cosmetics, or food products. It is also used as a solvent for food colors and flavors as well as to create artificial smoke or fog used in fire-fighting training and in theatrical productions.

Propylene glycol increases the amound of acid in the body, resulting in metabolic problems. Large amounts of it are needed for this effect, unlike its more toxic cousin, ethylene glycol. Propylene glycol is not carcinogenic and breaks down quickly inside the body and the environment.

made with help from the CDC's toxFAQs - http://www.atsdr.cdc.gov/

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.