PG solution (propylene glycol solution, also sold as humidor or "50/50" solution) is used in foam and some crystal humidifiers. The propylene glycol slows evaporation and pulls the humidity toward about 70%. Because the glycol barely evaporates, refilling repeatedly with solution makes the mix stronger until it starts drying the air, so many makers say to top up with distilled water between charges. Never use it in humidity beads or electronic humidifiers. Follow the instructions for your unit.
Also called: propylene glycol solution, 50/50 solution, humidor solution