Different machines use different fluids. The show I currently work on uses LeMaitre fog machines, that can be used for either for or haze, that use a glycol and water based fluid, though different formulations are used for fog or haze. To keep the fog low it is cooled using liquid CO2. It’s safer for the actors as there is no water left on the stage as with the old dry ice fog machines. For haze we use MDG machines which use mineral oil, much like the old Mole Richardson fog machines. The haze is there to accentuate the lighting, especially the moving lights. I remember doing shoots where smoke cookies were used, till it was found out how toxic they were.