Himalayan Salt Lamps purify air through the power of HYGROSCOPY, meaning that they attract water molecules from the surrounding environment then absorb those molecules - as well as any foreign particles they may be carrying - into the salt crystals.
As the globe lamp warms up from the heat produced by the oven light bulb inside, that same water then evaporates back into the air and the trapped particles of dust, pollen, smoke, etc remain locked in the salt lamp.