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Some dye colors are formed with toxic chemicals, which would be emitted along with the smoke, turning your harmless fog emitter into a dangerous poison gas emitter. Carefully read the labels of the dye you find in textiles stores and then look up all the names of the chemicals. Glycerin is fine already, it's what makes most of vaping smoke and is generally safe to breathe in the doses put out by a vaping ecig.
Camping Storage Heater from Bricks, Sand and an Ammo Tin.