A mineral filter covering UVB and UVA, including long-wave UVA1. Photostable, rarely allergenic, and one of only three filters the FDA currently calls safe and effective (with titanium dioxide and, since 2026, bemotrizinol).
Zinc oxide is a white mineral powder that absorbs (and to a lesser extent scatters) ultraviolet light. Alone it covers UVB through long-wave UVA1 up to about 380 nm, which few single filters manage, and it does not break down in sunlight.
The trade-off is a white cast. Smaller ("nano" or "micronised") particles reduce the cast; coated grades keep the particles from reacting with the rest of the formula. The same ingredient is the active in many nappy-rash and skin-protectant creams.
Zinc oxide's UV protection is measured directly in standard SPF and UVA tests, and sunscreens as a class reduce sunburn, squamous-cell skin cancer and photoageing in randomised trials. Volunteer studies show the particles stay on the surface of intact skin; a trace of dissolved zinc is absorbed, which is trivial next to dietary zinc. What is not shown: that mineral sunscreens protect better than a well-formulated chemical one at the same SPF and UVA rating.
Zinc oxide nanoparticles applied repeatedly for 5 days stayed on the skin surface and in skin furrows and did not enter or harm the living epidermis; a small amount of dissolved zinc ion penetrated without local toxicity.
20 adults used isotope-labelled zinc oxide sunscreen outdoors for 5 days. The overwhelming majority of zinc was not absorbed; tracer in blood was about 1/1000 of the body's normal blood zinc. Whether it crossed as particles or dissolved zinc was not known.