Zinc oxide and titanium dioxide sunscreens both protect the skin from harmful radiation by physically blocking sunlight, unlike chemical sunscreens that soak into the skin and absorb the light. These metallic agents both block UVA and UVB radiation, but don't provide the same level of performance. The correct zinc oxide particle size provides lower opacity and better, safer protection than titanium when used at the same concentration, though titanium is more stable and reflective. Sunscreen manufacturers often combine both ingredients in one product, and may also add chemical agents.
Also referred to as sunblock, products made with titanium dioxide or zinc oxide work by creating a physical barrier between the skin and solar radiation. Like most chemical sunscreens, they block UV-B radiation, which causes sunburn. Unlike many chemical products, they also block UV-A radiation, which has a longer wavelength and damages the skin over a longer period of time. UV-A is responsible for delayed tanning and sunburns that appear several days after exposure.
Older zinc and titanium-based sunscreen formulas used a relatively large particle size, producing an opaque white product that could be unattractive. Modern zinc oxide and titanium dioxide sunscreen uses microfine particles, between 280 and 380 nanometers in diameter, to create a more transparent look. Overall, a given quantity of zinc oxide is less opaque than the same amount of microfine titanium dioxide. For this reason, zinc oxide is also used in transparent white paints.
Both products offer benefits for the user, but there is some cause for concern if they are not prepared properly. Zinc blocks more sunlight than titanium dioxide, providing broad-spectrum protection in smaller quantities. Particles of zinc oxide cannot be absorbed by the skin, and present no health or allergy hazard. Titanium dioxide particles are safe when coated with alumina or silica, but uncoated particles create carcinogenic radicals in contact with sunlight, and must not be used in sunscreen. Both ingredients work well in products for people with sensitive skin or allergies to chemical sunscreen agents.
Many companies make a combination zinc oxide and titanium dioxide sunscreen, combining the greater protective ability of zinc with the lower cost and high refractive index of titanium. Titanium dioxide also resists discoloration after exposure to UV radiation, making it very stable. Most physical sunblocks contain both metallic compounds, and may also include chemical components, such as avobenzone, to absorb any unblocked radiation.