Magnesium Carbonate

Magnesium carbonate is an inert, insoluble mineral powder used primarily as an absorbent/opacifier or anti-caking agent, typically at low percentages in cosmetics, and it is not a biologically active irritant in standard patch-testing contexts. However, as a fine particulate it can be mildly drying or mechanically irritating on compromised eczematous skin (especially in leave-on powders or high-load formulations), so I score it as very gentle rather than completely inert for patient safety. Safety Notes: In cosmetics, magnesium carbonate is used mainly as an absorbent/opacifying and slip/feel modifier in powders and some creams; at the low end it appears as a minor auxiliary (often with other fillers) in leave-on lotions/creams and occasional cleansers around ~0.05–0.5%. The highest consumer-available levels are found in anhydrous body powders, foot powders, and dry shampoos where magnesium carbonate can be a primary absorbent/filler, commonly 10–25% and observed up to ~35% in powder formats. There is no widely cited EU/FDA maximum specific to magnesium carbonate in cosmetics, so practical use is driven by sensory, dusting/whitening, and stability constraints, with much higher feasible loadings in dry (rinse-off/leave-on powder) products than in emulsions.